Low noise amplifier microcircuits. Highly sensitive microphones with low-noise bass amplifiers

Hi all.

When assembling low-noise, high-quality microphone amplifiers, radio amateurs most often use circuit solutions based on discrete bipolar or field-effect transistors, or low-noise operational amplifiers. High-quality amplifiers for microphones using transistors are often quite complex and do not guarantee stable repeatability of parameters, and to assemble an amplifier using low-noise op-amps, you may not have the necessary microcircuits on hand or their prices will be more than acceptable.

A high-quality amplifier for a stereo microphone can be made not only using special low-noise transistors (Fig. 1, 2), integrated operational amplifiers (op-amps) or specialized ICs, but also using something that radio amateurs most often have in abundance, but few people think of potential of some “uncommon” microcircuits. This refers to integrated circuits - specialized low-noise playback amplifiers for cassette and reel-to-reel tape recorders of analogue sound recording. Household magnetic recording of sound is quickly becoming a thing of the past; many imported radios and car radios have already served their time, and when they are disassembled for spare parts, integrated playback amplifier chips most often remain unnecessary.

Based on one of these LA3161 chips

you can make a simple, single-supply stereo microphone amplifier that requires no setup in just two hours. The schematic diagram of this amplifier is presented below.

This device is a low noise stereo amplifier that has a voltage gain of approximately 100. The nominal supply voltage for this amplifier is 9 Volts, the quiescent current is approximately 6 mA, the nominal input voltage is 5 mV, and the nominal output voltage is 500 mV at THD. distortion 0.05%. The output resistance is approximately 100 kOhm. The microcircuit can operate on a power supply of 2.5 - 16 Volts. But with a power supply of less than 7 Volts, its main characteristics deteriorate.

The microcircuit is powered by a stable voltage source passing through an LC filter C1L1C2C3. In a particular case, a galvanic battery “Krona” or its equivalent can be used as a power source.

The amplifier's transmission coefficient depends on the ratio of the resistance of resistors R5/R3 and R6/R4. If there is a need for a large voltage gain, the resistance of resistors R3 and R4 can be reduced by 10 - 20 times. You can use both dynamic and condenser microphones as microphones VM1 and VM2. If there is no source follower in a condenser or electret microphone, you can introduce it into the amplifier, for example, by installing a K513UE1 microcircuit in each channel. Capacitors C4 and C5 prevent various radio interference from entering the input. Resistors R9 and R10 eliminate the possible appearance of a “click” when a microphone amplifier is connected to sound reproduction equipment, and are also needed for the correct polarization of the plates of oxide capacitors C10 and C11. The functional diagram of the LA3161 chip is shown in the figure below. If you use only one of the two amplifiers of the microcircuit, the corresponding non-inverting input (pin 1 or 8) must be connected to a common wire.

You can assemble the amplifier on a board measuring 70×27 mm (see photo). There needs to be some space left on the left side of the board to accommodate any additional components that may be needed to match some dynamic microphones to the amplifier input.

Resistors can be used like MLT, S2-23 or their analogues. It is better to take into account that the higher the power of resistors of the same type, the lower their own noise level will be. If the gain is more than 500, it is better to install resistors R1 - R6 with a power of 0.5 - 1 Watt. Non-polar capacitors - imported small-sized film or ceramic. Oxide capacitors C6, C7 should have the lowest leakage current. If you cannot find high-quality capacitors among ordinary aluminum ones, you can use ceramic or film capacitors with a capacity of 4.7 μF. Choke L1 can be any small-sized, low-power one with an inductance greater than 100 μH. If the supply voltage is 12 Volts or more, then it would be better to connect a 1 kOhm resistor in series with it. You can replace the LA3161 chip with LA3160.

These two microcircuits are produced by Sanyo in the SIP-8 package, they have the same pinouts and similar parameters. Microcircuits of low-noise amplifiers for playing magnetic sound recordings with disabled correction circuits can be used not only as microphone amplifiers, but also in units of preliminary normalizing amplifiers, passive tone and volume controls or as signal amplifiers from piezoelectric sensors and pyrodetectors.

All the best.

There are many amplifiers for which one of the main required parameters is the requirement to ensure minimal noise at the output. Typically, such circuits are used to amplify signals from various sensors, as well as in direct conversion receivers, where the main amplification is carried out at low frequencies.

An increase in noise makes it impossible to distinguish weak signals against a background of noise.
Internal noise in an amplifier occurs when current passes through the passive and active elements of the circuit.

The noise characteristics also largely depend on the design of the circuit (circuitry).

When developing an amplifier with a high signal-to-noise ratio, in addition to the optimal choice of circuit type, it is important to correctly select the element base and optimize the operating mode of the cascades.
Selecting Circuit Components
In a real amplifier, the source of internal noise is:
1) thermal and current noise of resistors;
2) flicker noise of capacitors, diodes and zener diodes;

3) fluctuation noise of active elements (transistors);

4) vibration and contact noise.

Resistors

The intrinsic noise of resistors consists of thermal and current noise.
Thermal noise is caused by the movement of electrons in the conductive substance from which the resistor is made (this noise increases with increasing temperature). If there is no voltage acting on the resistor, then the noise emf across it (in μV) is determined from the relationship:

Esh=0.0125 x f x R,

The frequency spectrum of both types of noise is continuous (“white noise”). And if for thermal noise it is evenly distributed up to very high frequencies, then for current noise it begins to decrease from about 10 MHz.

The total amount of noise is proportional to the square root of the resistance, so to reduce it, the amount of resistance in the circuit must also be reduced.
Sometimes, in order to reduce the noise caused by resistors, they resort to their parallel (or series) connection, and also install more power than is required for operation. In addition, you can use those types in which, due to the manufacturing technology, this parameter is smaller.

In non-wire resistors, current noise is much greater than thermal noise. The overall noise level for different types of resistors can range from 0.1 to 100 µV/V.

To compare different resistors (fixed and tuning from the SP group), the maximum noise values are given in Table 1

Type of resistors Technological design Noise level, μV/V BLT brown carbon 0.5 S2-13 S2-29V metal-dielectric 1.0 S2-50 metal-dielectric 1.5 MLT OMLT S2-23S2-33 metal-dielectric 1...5 S2-26 metal oxide 0 .5 SP3-4
SP3-19
SP3-23 film composite 47...100
25...47
25...47
Table 1 - Noise properties of resistors

As can be seen from the table, adjusted resistors are much more noisy. For this reason, it is better to use them with small denominations or to exclude them from the circuit altogether.
The noise properties of resistors can be used to make a wideband noise generator.

As recommendations for choosing resistors for assembling a low-noise amplifier, it can be noted that it is most convenient to use the following types: C2-26, C2-29V, C2-33 and C1-4 (unpackaged chip design). Recently, low-noise imported metal-dielectric resistors have appeared on sale, similar in design to C2-23, but with a lower noise figure (0.2 µV/V).

It is possible to significantly reduce the noise of resistors by strongly cooling them, but this method is too expensive and is used very rarely.

Capacitors

In capacitors, the source of flicker noise is leakage current. High-capacity oxide capacitors have the highest leakage currents. Moreover, leakage increases with increasing capacitance and decreases with increasing permissible rated operating voltage.

Reference data for the most common oxide capacitors is given in Table 29.
The lowest leakage currents among polar capacitors are: K53-1A, K53-18, K53-16, K52-18, K53-4 and others.
Oxide capacitors installed at the input as isolation capacitors can significantly increase the noise of the amplifier. Therefore, it is advisable to avoid their use, replacing them with film ones (K10-17, K73-9, K73-17, KM-6, etc.), although this will lead to a significant increase in the size of the structure.

Capacitor type Manufacturing technology Operating temperature, C Leakage current, µA K50-6
K50-16
K50-24
aluminum oxide-electrolytic -10...+85
-20...+70
-25...+70 4...5000
4...5000
18...3200 K52-1
K52-2
K52-18 tantalum oxide volumetric porous -60...+85
-50...+155
-60...+155 1,2...8,5
2...30
1...30 K53-1
K53-1A
K53-18 tantalum oxide semiconductor -80...+85
-60...+125
-60...+125 2...5
1...8
1...63
Table 2 - Reference parameters of capacitors

Diodes and Zener diodes

When current passes directly, the noise of the diodes is minimal. The greatest noise is provided by the leakage current (under the action of reverse voltage), and the smaller it is, the better.

Zener diodes are quite noisy. This property is even sometimes used to make the simplest noise generators for children's toys (simulators of surf noise, sounds of a fire, etc. - L16, L17). To obtain maximum noise in such circuits, zener diodes operate at low currents (with a large additional resistor).

Transistors

In the transistor itself, the main types of noise are thermal and generation-recombination, the spectral power density of which does not depend on frequency.

To reduce the noise level, low-noise bipolar transistors with a standardized noise figure (Ksh) are usually used in input stages in our country. These are: (p-n-p) KT3102D(E), KT342V and (p-n-p) KT3107E(Zh, L) and a number of others. It should be noted here that the use of low-noise high-frequency bipolar transistors in the low frequency range, as a rule, may be inappropriate.

For such transistors, the noise figure is rated only in the high-frequency region, and in the range below 100 kHz they can make no less noise than any other. In addition, such transistors may exhibit a tendency to excitation (self-generation).

occur due to poor-quality soldering (with violation of the temperature regime) or at the junction of connectors. For this reason, it is not recommended to connect the input circuits of the low noise amplifier through plug connections. I have also encountered a situation where transistors made more noise in the same circuit after re-soldering.

Vibration noises

may occur when operating the device on moving objects or in places with increased vibration from operating equipment. They arise due to the transfer of mechanical vibrations to the capacitor plates, between which there is a potential difference (the so-called “piezo-microphone effect”). This is observed even in small-sized ceramic capacitors (K10, K15, etc.) with high capacity (more than 0.01 μF). This interference can be especially pronounced in coupling capacitors installed at the input of the amplifier. The interference signal from mechanical vibrations takes the form of short, sharp-edged pulses, the spectrum of which is in the low frequency range. To combat this type of interference, depreciation of the entire structure can be used. This interference does not occur in oxide capacitors.

When selecting parts for assembling a low-noise circuit, it is necessary to take into account their production time. The manufacturer guarantees parameters only for a certain storage period. This is usually no more than 8... 15 years. Over time, aging processes occur, manifested in a decrease in insulation resistance, the capacitance of capacitors decreases and leakage currents increase. Oxide capacitors especially change their characteristics over time. For this reason, it is best to avoid their use in signal paths if possible.

Moshe Gerstenhaber, Rayal Johnson and Scott Hunt, Analog Devices

Analogue Dialogue

Introduction

Creating a measurement system with sensitivity in the nanovolt range is a very difficult engineering task. The best available operational amplifiers (op amps), such as ultra low noise, at 1 kHz can achieve noise voltages of less than 1 nV/√Hz, but from 0.1 Hz to 10 Hz the nature of low frequency noise limits the best achievable values to 50 nV peak. -peak. Oversampling and sample averaging can reduce the RMS contribution from flat-spectrum noise at the cost of higher data rates and additional power consumption, but oversampling will not reduce the spectral density of the noise and will have no effect on flicker noise (1/f). In addition, the high gain of the input signal pre-processing circuit, necessary to eliminate the noise contribution of subsequent stages, reduces the system bandwidth. Without isolation, any noise on the ground bus will show up at the output, where it can cancel out both the amplifier's weak internal noise and its input signal. A good low-noise instrumentation amplifier simplifies the design and construction of such systems and reduces residual errors caused by common-mode voltage, power supply fluctuations, and temperature drift.

The low noise instrumentation amplifier provides 2000 precision gain and has everything you need to solve these problems. With a gain temperature drift of no more than 5 ppm/°C, a maximum offset voltage drift of 0.3 μV/°C, a minimum common-mode voltage rejection ratio of 140 dB at 60 Hz (no more than 120 dB at 50 kHz), a power supply ripple rejection ratio of 130 dB and 3.5 MHz bandwidth, the AD8428 is ideal for low-end measurement systems. But most importantly, the amplifier's self-noise voltage spectral density of just 1.3 nV/√Hz at 1 kHz and industry-leading 40 nV peak-to-peak noise from 0.1 to 10 Hz provide a high signal-to-noise ratio for very weak signals. Two additional pins (+FIL, -FIL) give designers the ability to narrow the noise bandwidth by changing the gain or adding a filter. Additionally, these filter pins provide a unique means of improving signal-to-noise ratio.

Using the AD8428 Instrumentation Amplifier to Reduce Noise

Figure 1 shows a circuit configuration that can further reduce noise. Parallel connection of the amplifier inputs and filter outputs of four AD8428 chips reduces noise by half.

The output impedance of the circuit will be low no matter which instrumentation amplifier the signal is taken from. This circuit can be extended to reduce noise by the square root of the amplifiers.

How the circuit reduces noise

The typical 1.3 nV/√Hz input-referenced noise voltage generated by each AD8428 amplifier is uncorrelated with the noise generated by the other amplifiers. Noise from uncorrelated sources is added at the filter terminals as the root of the sum of squares. At the same time, the input signal has a positive correlation. The voltages that appear at the filter pins of each chip due to the input signal are the same, so connecting several AD8428s in parallel does not change the voltage at these points, and the gain remains equal to 2000.

Noise Analysis

The following analysis of the simplified circuit in Figure 2 shows that two AD8428 amplifiers connected in this way reduce noise by a factor of √2. Each amplifier's noise can be modeled by the voltage at its +IN input. To determine the total noise, ground the inputs and use a superposition method to combine the noise sources.

The noise of the source e n1 comes to the output of the preamplifier of the A1 chip, differentially amplified by 200 times. For this part of the analysis, we consider the outputs of the preamplifier of chip A2 to be noise-free, and its inputs to be grounded. The 6 kΩ/6 kΩ resistive divider between each preamp output of IC A1 and the corresponding preamp output of IC A2 can be replaced by its Thevenin equivalent: half the noise voltage of preamp A1 with a series resistance of 3 kΩ. This division is the mechanism that reduces noise. A full analysis by the nodal potential method shows that the noise e n1 is amplified at the output to a level of 1000 × e n1 . Based on the symmetry of the circuit, it is natural to conclude that the contribution from e n2 will be equal to 1000 × e n2 . Equal and equal en levels e n1 and e n2 are added as the root sum of squares, resulting in a total noise output of 1414 × e n .

In order to bring it back to the input, it is necessary to determine the magnitude of the gain. Let's assume that a differential signal V IN is applied between the +INPUT and -INPUT pins. The differential voltage at the output of the first stage A1 will be equal to V IN × 200. The same voltages also appear at the outputs of the pre-amplifier of the A2 chip, therefore the 6 kOhm/6 kOhm divider does not affect the signal in any way, and analysis by the node potential method shows that the output voltage is equal to V IN × 2000. Thus, the total voltage of the noise referred to the input is equal to e n × 1414/2000, or, which is the same, e n /√2. Substituting here the typical AD8428 noise density value of 1.3 nV/√Hz, we find that the configuration of two amplifiers gives a noise density of about 0.92 nV/√Hz.

As amplifiers are added, the impedance of the filter output changes, which also reduces the noise level. For example, when using four AD8428s in the configuration shown in Figure 1, there are three 6 kΩ resistors connected between the filter pin and each of the preamp's non-noise outputs. This effectively forms a 6k/2k resistive divider, attenuating the noise voltage by a factor of four. Then the total noise of the four amplifiers, as predicted, becomes equal to e n /2.

Trade-off between noise and power

From a noise-to-power standpoint, the AD8428 is very efficient. With an input noise density of 1.3 nV/√Hz, its current consumption does not exceed 6.8 mA. For comparison, the AD797 low noise op amp requires a maximum current of 10.5 mA to achieve 0.9 nV/√Hz. A discrete instrumentation amplifier with two AD797 op amps and one low-power differential amplifier with a gain of 2000 may require more than 21 mA to produce an input noise voltage of 1.45 nV/√Hz, which will be consumed primarily by two op amps and a 30.15 ohm resistor. In addition to the total current consumed by a group of parallel-connected amplifiers, the designer must also take into account their thermal conditions. The power dissipated within a single AD8428 chassis when powered at ±5V increases its temperature by approximately 8°C. If several devices are arranged in a compact group on the board or are located in a confined space of the case, they can heat each other, which will require taking into account thermal aspects when designing the circuit.

SPICE modeling

SPICE modeling, while not meant to replace prototyping, can be useful as a first step to test an idea itself. To test and simulate the operation of a circuit consisting of two devices connected in parallel, the ADIsimPE simulator with the AD8428 SPICE macro model was used. The results shown in Figure 3 demonstrate the expected behavior of the circuit: gain of 2000 and noise reduced by 30%.

Measurement results

The complete four-chip AD8428 design has been tested in the lab. The measured input-referred noise had a spectral density of 0.7 nV/√Hz at 1 kHz and a level of 25 nV peak-to-peak from 0.1 Hz to 10 Hz. This is less noise than many nanovoltmeters. The results of the spectral density and peak noise voltage measurements are presented in Figures 4 and 5, respectively.

Conclusion

Creating devices with nanovolt-level sensitivity is a very difficult task, creating many design challenges. The AD8428 instrumentation amplifier has all the features needed to implement high-quality systems requiring low noise and high gain. Moreover, its unique structure allows designers to add this unusual circuit to their arsenal of nanovolt solutions.

Links

MT-047 Tutorial. Op Amp Noise.
MT-048 Tutorial. Op Amp Noise Relationships: 1/f Noise, RMS Noise, and Equivalent Noise Bandwidth.
MT-049 Tutorial. Op Amp Total Output Noise Calculations for Single-Pole System.
MT-050 Tutorial. Op Amp Total Output Noise Calculations for Second-Order System.
MT-065 Tutorial. In-Amp Noise.

The circuits and designs of highly sensitive microphones in combination with homemade low-noise low-frequency amplifiers (LNF) are considered.

The design of a sensitive and low-noise amplifier (ULA) has its own characteristics. The greatest influence on the quality of sound reproduction and speech intelligibility is exerted by the amplitude-frequency response (AFC) of the amplifier, its noise level, microphone parameters (AFC, polar pattern, sensitivity, etc.) or sensors replacing it, as well as their mutual consistency with the amplifier . The amplifier must have sufficient gain.

When using a microphone, it is 60db-80db, i.e. 1000-10000 times. Taking into account the peculiarities of receiving a useful signal and its low value in conditions of a relatively significant level of interference, which always exists, it is advisable in the design of the amplifier to provide for the possibility of correcting the frequency response, i.e. frequency selection of the processed signal.

It should be taken into account that the most informative part of the audio range is concentrated in the band from 300 Hz to 3-3.5 kHz. True, sometimes in order to reduce interference this band is reduced even more. The use of a bandpass filter as part of an amplifier can significantly increase the listening range (2 times or more).

An even greater range can be achieved by using high-Q selective filters in the ULF, which make it possible to isolate or suppress a signal at certain frequencies. This makes it possible to significantly increase the signal-to-noise ratio.

Elementary base

Modern element base allows you to create high-quality ULF based on low-noise operational amplifiers(OU), for example, K548UN1, K548UN2, K548UNZ, KR140UD12, KR140UD20, etc.

However, despite the wide range of specialized microcircuits and op-amps, and their high parameters, ULF on transistors have not lost their significance at present. The use of modern, low-noise transistors, especially in the first stage, makes it possible to create amplifiers with optimal parameters and complexity: low-noise, compact, economical, designed for low-voltage power supply. Therefore, transistor ULFs often turn out to be a good alternative to integrated circuit amplifiers.

To minimize the noise level in amplifiers, especially in the first stages, it is advisable to use high-quality elements. Such elements include low-noise bipolar transistors with high gain, for example, KT3102, KT3107. However, depending on the purpose of the ULF, field-effect transistors are also used.

The parameters of other elements are also of great importance. In low-noise cascades of electronic circuits, oxide capacitors K53-1, K53-14, K50-35, etc. are used, non-polar ones - KM6, MBM, etc., resistors - no worse than traditional 5% MLT-0.25 and ML T- 0.125, the best type of resistors is wirewound, non-inductive resistors.

The input resistance of the ULF must match the resistance of the signal source - a microphone or a sensor replacing it. Typically, they try to make the input impedance of the ULF equal (or slightly greater) to the resistance of the signal source-converter at fundamental frequencies.

To minimize electrical interference, it is advisable to use shielded wires of a minimum length to connect the microphone to the ULF. It is recommended to mount the IEC-3 electret microphone directly on the board of the first stage of the microphone amplifier.

If it is necessary to significantly distance the microphone from the ULF, you should use an amplifier with a differential input, and the connection should be made using a twisted pair of wires in the screen. The screen is connected to the circuit at one point of the common wire as close as possible to the first op-amp. This ensures that the level of electrical noise induced in the wires is minimized.

Low-noise ULF for microphone on K548UN1A

Figure 1 shows an example of a ULF based on a specialized microcircuit - IC K548UN1A, containing 2 low-noise op-amps. The op amp and ULF created on the basis of these op amps (IC K548UN1A) are designed for a unipolar supply voltage of 9V - ZOV. In the above ULF circuit, the first op-amp is included in a version that ensures the minimum noise level of the op-amp.

Rice. 1. ULF circuit on the K548UN1A op-amp and microphone connection options: a - ULF on the K548UN1A op-amp, b - connection of a dynamic microphone, c - connection of an electret microphone, d - connection of a remote microphone.

Elements for the circuit in Figure 1:

R1 =240-510, R2=2.4k, R3=24k-51k (gain adjustment),
R4=3k-10k, R5=1k-3k, R6=240k, R7=20k-100k (gain adjustment), R8=10; R9=820-1.6k (for 9V);
C1 =0.2-0.47, C2=10µF-50µF, C3=0.1, C4=4.7µF-50µF,
C5=4.7uF-50uF, C6=10uF-50uF, C7=10uF-50uF, C8=0.1-0.47, C9=100uF-500uF;
Op-amps 1 and 2 - IS K548UN1A (B), two op-amps in one IC package;
T1, T2 - KT315, KT361 or KT3102, KT3107 or similar;
T - TM-2A.

The output transistors of this ULF circuit operate without an initial bias (with Irest = 0). “Step” type distortion is practically absent due to the deep negative feedback covering the second op-amp of the microcircuit and the output transistors. If it is necessary to change the mode of the output transistors (Iquiescent = 0), the circuit must be adjusted accordingly: include a resistor or diodes in the circuit between the bases T1 and T2, two 3-5k resistors from the bases of the transistors to the common wire and the power wire.

By the way, outdated germanium transistors work well in ULF in push-pull output stages without an initial bias. This allows the use of op-amps with a relatively low slew rate of the output voltage with this output stage structure without the risk of distortion associated with zero quiescent current. To eliminate the danger of excitation of the amplifier at high frequencies, a capacitor SZ is used, connected next to the op-amp, and the R8C8 chain at the ULF output (quite often RC at the amplifier output can be eliminated).

Low-noise microphone ULF using transistors

Figure 2 shows an example ULF circuits on transistors. In the first stages, the transistors operate in microcurrent mode, which minimizes internal ULF noise. Here it is advisable to use transistors with a high gain but low reverse current.

This could be, for example, 159NT1V (Ik0=20nA) or KT3102 (Ik0=50nA), or similar.

Rice. 2. ULF circuit with transistors and options for connecting microphones: a ULF with transistors, b - connection of a dynamic microphone, c - connection of an electret microphone, d - connection of a remote microphone.

Elements for the circuit in Figure 2:

R3=5.6k-6.8k (volume control), R4=3k, R5=750,
R6=150k, R7=150k, R8=33k; R9=820-1.2k, R10=200-330,
R11=100k (adjustment, Uet5=Uet6=1.5V),
R12=1 k (adjustment of the quiescent current T5 and T6, 1-2 mA);
C1=10uF-50uF, C2=0.15uF-1uF, C3=1800,
C4=10µF-20µF, C5=1µF, C6=10µF-50µF, C7=100µF-500µF;
T1, T2, T3 -159NT1 V, KT3102E or similar,
T4, T5 - KT315 or similar, but MP38A is also possible,
T6 - KT361 or similar, but MP42B is also possible;
M - MD64, MD200 (b), IEC-3 or similar (c),
T - TM-2A.

The use of such transistors allows not only to ensure stable operation of the transistors at low collector currents, but also to achieve good amplification characteristics with a low noise level.

Output transistors can be used either silicon (KT315 and KT361, KT3102 and KT3107, etc.) or germanium (MP38A and MP42B, etc.). Setting up the circuit comes down to setting resistor R2 and resistor RЗ the corresponding voltages on the transistors: 1.5V on the collector T2 and 1.5V on the emitters T5 and T6.

Op-amp microphone amplifier with differential input

Figure 3 shows an example of ULF on Differential input op amp. A properly assembled and tuned ULF provides significant suppression of common-mode interference (60 dB or more). This ensures that the useful signal is isolated with a significant level of common-mode interference.

It should be recalled that common-mode interference is interference arriving in equal phases at both inputs of the ULF op-amp, for example, interference induced on both signal wires from a microphone. To ensure correct operation of the differential cascade, it is necessary to precisely fulfill the condition: R1 = R2, R3 = R4.

Fig.3. ULF circuit on an op-amp with a differential input and options for connecting microphones: a - ULF with a differential input, b - connection of a dynamic microphone, c - connection of an electret microphone, d - connection of a remote microphone.

Elements for the circuit in Figure 3:

R7=47k-300k (gain adjustment, K=1+R7/R6), R8=10, R9=1.2k-2.4k;
C1=0.1-0.22, C2=0.1-0.22, SZ=4.7uF-20uF, C4=0.1;
Op-amp - KR1407UD2, KR140UD20, KR1401UD2B, K140UD8 or other op-amps in a standard configuration, preferably with internal correction;
D1 - zener diode, for example, KS133, you can use an LED in normal switching, for example, AL307;
M - MD64, MD200 (b), IEC-3 or similar (c),
T - TM-2A.

It is advisable to select resistors using an ohmmeter among 1% resistors with good temperature stability. To ensure the necessary balance, it is recommended that one of the four resistors (for example, R2 or R4) be made variable. This can be a high-precision variable resistor trimmer with an internal gearbox.

To minimize noise, the input impedance of the ULF (the values of resistors R1 and R2) must match the resistance of the microphone or the sensor replacing it. The ULF output transistors operate without an initial bias (from 1 rest = 0). Step-type distortion is practically absent due to the deep negative feedback covering the second op-amp and the output transistors. If necessary, the transistor connection circuit can be changed.

Setting up the differential cascade: apply a 50 Hz sinusoidal signal to both inputs of the differential channel simultaneously, selecting the value of RЗ or R4 to ensure a zero signal level of 50 Hz at the output of op amp 1. For tuning, a 50 Hz signal is used, because A power supply with a frequency of 50 Hz makes the maximum contribution to the total value of the interference voltage. Good resistors and careful tuning can achieve common-mode noise suppression of 60dB-80dB or more.

To increase the stability of the ULF operation, it is advisable to bypass the power supply pins of the op-amp with capacitors and turn on an RC integer at the amplifier output (as in the amplifier circuit in Figure 1). For this purpose, you can use KM6 capacitors.

To connect the microphone, a twisted pair of wires in the screen is used. The screen is connected to the ULF (only at one point!!) as close as possible to the op-amp input.

Improved amplifiers for sensitive microphones

The use of low-speed op-amps in ULF output stages and the operation of silicon transistors in power amplifiers in a mode without an initial bias (quiescent current is zero - mode B) can, as noted above, lead to transient distortions of the “step” type. In this case, to eliminate these distortions, it is advisable to change the structure of the output stage so that the output transistors operate with a small initial current (AB mode).

Figure 4 shows an example of such a modernization of the above amplifier circuit with a differential input (Figure 3).

Fig.4. ULF circuit using an op-amp with a differential input and a low-distortion output stage.

Elements for the circuit in Figure 4:

R1=R2=20k (equal to or slightly higher than the maximum source resistance in the operating frequency range),
RЗ=R4=1m-2m; R5=2k-10k, R6=1k-Zk,
R7=47k-300k (gain adjustment, K=1+R7/R6),
R8=10, R10=10k-20k, R11=10k-20k;
C1 =0.1-0.22, C2=0.1-0.22, SZ=4.7uF-20uF, C4=0.1;
OU - K140UD8, KR1407UD2, KR140UD12, KR140UD20, KR1401UD2B or other op amps in standard configuration and preferably with internal correction;
T1, T2 - KT3102, KT3107 or KT315, KT361, or similar;
D2, D3 - KD523 or similar;
M - MD64, MD200, IEC-3 or similar (c),
T - TM-2A.

Figure 5 shows an example ULF on transistors. In the first stages, transistors operate in microcurrent mode, which minimizes ULF noise. The circuit is in many ways similar to the circuit in Figure 2. To increase the share of the useful low-level signal against the background of inevitable interference, a bandpass filter is included in the ULF circuit, which ensures the selection of frequencies in the 300 Hz -3.5 kHz band.

Fig.5. ULF circuit using transistors with a band-pass filter and options for connecting microphones: a - ULF with a band-pass filter, b - connecting a dynamic microphone, c - connecting an electret microphone.

Elements for the circuit in Figure 5:

R1=43k-51k, R2=510k (adjustment, Ukt2=1.2V-1.8V),
R3=5.6k-6.8k (volume control), R4=3k, R5=8.2k,
R6=8.2k, R7=180, R8=750; R9=150k, R10=150k, R11=33k,
R12=620, R13=820-1.2k, R14=200-330,
R15=100k (adjustment, Uet5=Uet6=1.5V), R16=1k (adjustment of the quiescent current T5 and T6, 1-2mA);
C1=10uF-50uF, C2=0.15-0.33, C3=1800,
C4=10uF-20uF, C5=0.022, C6=0.022,
C7=0.022, C8=1uF, C9=10uF-20uF, C10=100uF-500uF;
T1, T2, T3 -159NT1 V, KT3102E or similar;
T4, T5 - KT3102, KT315 or similar, but you can also use outdated germanium transistors, for example, MP38A,
T6 - KT3107 (if T5 - KT3102), KT361 (if T5 - KT315) or similar, but obsolete germanium transistors can also be used, for example, MP42B (if T5 - MP38A);
M - MD64, MD200 (b), IEC-3 or similar (c),
T - TM-2A.

In this circuit, it is also advisable to use transistors with a high gain, but a small reverse collector current (Ik0), for example, 159NT1V (Ik0=20nA) or KT3102 (Ik0=50nA), or similar. Output transistors can be used either silicon (KT315 and KT361, KT3102 and KT3107, etc.) or germanium (outdated transistors MP38A and MP42B, etc.).

Setting up the circuit, as in the case of the ULF circuit in Fig. 11.2, comes down to setting resistor R2 and resistor RЗ the corresponding voltages on transistors T2 and T5, T6: 1.5V - on the collector of T2 and 1.5V - on the emitters T5 and T6.

Microphone design

A pipe with a diameter of 10-15 cm and a length of 1.5-2 m is made from a large sheet of thick paper with a pile, like velvet. The pile, as you might guess, of course, should not be on the outside, but on the inside. A sensitive microphone is inserted into one end of this pipe. It would be better if it was a good dynamic or condenser microphone.

However, you can also use a regular household microphone. This could be, for example, a dynamic microphone such as MD64, MD200, or even a miniature MKE-3.

True, with a household microphone the result will be somewhat worse. Of course, the microphone must be connected using a shielded cable to a sensitive amplifier with a low self-noise level (Fig. 1 and 2). If the cable length exceeds 0.5 m, then it is better to use a microphone amplifier that has a differential input, for example, a VLF to an op-amp (Fig.

This will reduce the common-mode component of interference - various types of interference from nearby electromagnetic devices, 50 Hz background from a 220 V network, etc. Now about the second end of this paper pipe. If this free end of the pipe is directed at a sound source, for example, at a group of talking people, then speech can be heard. It would seem nothing special.

That's what microphones are for. And you don't need a pipe for this at all. However, what is surprising is that the distance to the people talking can be significant, for example, 100 meters or more. Both the amplifier and the microphone equipped with such a tube make it possible to hear everything quite well at such a considerable distance.

The distance can even be increased by using special selective filters that allow the signal to be isolated or suppressed in narrow frequency bands.

This makes it possible to increase the level of the useful signal in conditions of inevitable interference. In a simplified version, instead of special filters, you can use a bandpass filter in the ULF (Fig. 4) or use a conventional equalizer - a multi-band tone control, or, in extreme cases, a traditional one, i.e. conventional, two-band, bass and treble tone control.

Literature: Rudomedov E.A., Rudometov V.E - Electronics and spy passions-3.

graduate work

2.1 Selecting a low noise amplifier circuit

In accordance with the above considerations, it is necessary that the low noise amplifier meets the following technical requirements:

gain not less than 20 dB;

noise figure no more than 3 dB;

dynamic range of at least 90 dB,

central frequency 808 MHz.

In addition, it had high stability of characteristics, high operational reliability, small dimensions and weight.

Taking into account the requirements for a low-noise amplifier, we will consider possible options for solving the problem. When considering possible options, we will take into account the conditions in which the transceiver module will be operated (placement on board an aircraft and the influence of external factors, such as temperature changes, vibration, pressure, etc.). Let's analyze low-noise amplifiers made using different element bases.

The lowest-noise microwave amplifiers currently are quantum paramagnetic amplifiers (masers), which are characterized by extremely low noise temperatures (less than 20 °K) and, as a result, very high sensitivity. However, the quantum amplifier includes a cryogenic cooling system (up to a liquid helium temperature of 4.2 o K), which has large dimensions and weight, high cost, as well as a bulky magnetic system for creating a strong constant magnetic field. All this limits the scope of application of quantum amplifiers to unique radio systems - space communications, long-range radar, etc.

The need to miniaturize microwave radio receiving devices, increase their efficiency, and reduce cost has led to the intensive use of low-noise amplifiers based on semiconductor devices, which include semiconductor parametric ones, tunnel diodes, and transistor microwave amplifiers.

Semiconductor parametric amplifiers (SPA) operate in a wide frequency range (0.3...35 GHz), have bandwidths from fractions to several percent of the central frequency (typical values 0.5...7%, but bandwidths up to 40% can be obtained); the transmission coefficient of one stage reaches 17...30dB, the dynamic range of input signals is 70...80dB. Generators based on avalanche diodes and Gunn diodes, as well as microwave transistors (with and without frequency multiplication) are used as pump generators. Semiconductor parametric amplifiers are the lowest noise of semiconductor and, in general, of all uncooled microwave amplifiers. Their noise temperature ranges from tens (at decimeter waves) to hundreds (at centimeter waves) degrees Kelvin. When deeply cooled (up to 20 °K and below), their noise properties are comparable to quantum amplifiers. However, the cooling system increases the dimensions, weight, power consumption and cost of the PPU. Therefore, cooled PPUs are used mainly in terrestrial radio systems, where highly sensitive radio receiving devices are required, and dimensions, weight, and power consumption are not so significant.

The advantages of PPU compared to amplifiers based on tunnel diodes and microwave transistors, in addition to better noise properties, include the ability to operate in the higher frequency range, greater gain of one stage, and the possibility of quick and simple electronic frequency tuning (within 2...30%). The disadvantages of PPU are the presence of a microwave pump generator, lower bandwidth, large dimensions and weight, and significantly higher cost, in contrast to transistor microwave amplifiers.

Amplifiers based on tunnel diodes have smaller dimensions and weight compared to other semiconductor amplifiers, determined mainly by the dimensions and weight of ferrite circulators and valves, lower power consumption and a wide bandwidth. They operate in the frequency range 1...20 GHz, have a relative bandwidth of 1.7...65% (typical values 3.5...18%), a transmission coefficient of one stage of 6...20dB, a noise figure of 3.5...4.5dB at decimeter waves and 4...7dB on centimeter, the dynamic range of input signals is 50...90dB. Tunnel diode amplifiers are used mainly in devices where it is necessary to place a large number of light and small-sized amplifiers in a small area, for example, in active phased array antennas. However, recently, due to their inherent disadvantages (relatively high noise figure, insufficient dynamic range, low electrical strength of the tunnel diode, difficulty in ensuring stability, the need for decoupling devices), amplifiers based on tunnel diodes have been intensively replaced by transistor microwave amplifiers.

The main advantages of semiconductor low-noise amplifiers - small dimensions and weight, low power consumption, long service life, the ability to build microwave integrated circuits - allow them to be used in active phased array antennas and on-board equipment. Moreover, transistor microwave amplifiers have the greatest prospects.

Advances in the development of physics and semiconductor technology have made it possible to create transistors with good noise and amplification properties and capable of operating in the microwave range. Microwave low-noise amplifiers were developed based on these transistors.

Transistor amplifiers, unlike amplifiers based on semiconductor parametric and tunnel diodes, are not regenerative, so ensuring their stable operation is much easier than, for example, amplifiers based on tunnel diodes.

Microwave LNAs use low-noise transistors, both bipolar (germanium and silicon) and field-effect transistors with a Schottky barrier (silicon and gallium arsenide). Germanium bipolar transistors provide a lower noise figure than silicon ones, but the latter are higher frequency. Schottky barrier field effect transistors have superior amplification properties to bipolar transistors and can operate at higher frequencies, especially gallium arsenide transistors. The noise characteristics at relatively low frequencies are better for bipolar transistors, and at higher frequencies - for field-effect transistors. The disadvantage of field-effect transistors is their high input and output resistance, which makes broadband matching difficult.

The above considerations allow us to outline a strategy for the synthesis of a low-noise amplifier based on a field-effect transistor, in a monolithic integrated design.

As was chosen earlier, we will build the LNA based on the MGA-86563 module. The electrical circuit diagram is shown in Figure 2.1. A typical connection diagram is shown in Figure 2.2: Figure 2.1 Electrical circuit diagram MGA-86563. Figure 2...

High frequency receiving path

As a result of the work carried out, the MGA86563 low-noise amplifier was investigated. The study of the frequency response of the LNA was carried out using the SNPU-135 stand, a device for studying the frequency response X1-42. The connection diagram for measuring the frequency response is shown in Figure 4...

AC to DC voltage measuring converter

To implement the rectifier circuit, we use a dual high-speed op-amp with field-effect transistors at the input of the KR140UD282 type. Its parameters are given in Table 5, and the connection diagram is shown in Fig. 8...

Low noise integrated amplifier

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Based on the building, it is necessary to construct a three-wire circuit (2 options) for measuring temperature using a RTD using a current source (see Fig. 6.2.1). No. Circuit Voltage at the input of the DUT at 2 Fig.6.2.1...

Design of the amplifier part of the device

Let's use the diagram presented in Fig. 5, to calculate the power amplifier. When calculating the UM, the given values are: a). Rated load power Рн = 0.4 W; b). Load resistance Rн = 100 Ohm...

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Transmitter development

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Calculation of a switching amplifier

A switching voltage amplifier is a signal pre-amplifier that ensures normal operation of the PA...

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Microphone amplifier circuit

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effective rated output voltage; effective rated input voltage...

Wideband amplifier

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