Microphone amplifier with single-wire power supply. Scheme, description

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ENCYCLOPEDIA OF RADIO ELECTRONICS AND ELECTRICAL ENGINEERING
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Microphone amplifier with single-wire power supply. Encyclopedia of radio electronics and electrical engineering

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Encyclopedia of radio electronics and electrical engineering / Preamplifiers

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Microphones with preamps placed in their housings require power wires to be connected to the transceiver (in addition to a shielded signal wire). From a constructive point of view, this is not very convenient. The number of connecting wires can be reduced by applying the supply voltage through the same wire that carries the signal, i.e. the center conductor of the cable. It is this method of power supply that is used in the amplifier brought to the attention of readers. Its schematic diagram is shown in the figure.

Single-wire powered microphone amplifier

The amplifier is designed to operate from an electret microphone of any type (for example, MKE-3). Power is supplied to the microphone through resistor R1. The sound signal from the microphone is fed to the base of the transistor VT1 through the coupling capacitor C1. The required bias on the basis of this transistor (about 0,5 V) is set by the voltage divider R2R3. The amplified audio frequency voltage is allocated to the load resistor R5 and then fed to the base of the transistor VT2, which is included in the composite emitter follower, assembled on transistors VT2 and VT3. The emitter of the latter is connected to the upper contact of the XP1 connector (amplifier output), to which the central conductor of the connecting shielded cable is connected, the braid of which is connected to a common wire. Note that the presence of an emitter follower at the output of the preamplifier significantly reduces the level of interference at the microphone input of the transceiver.

Two more parts are mounted near the input connector of the device to which the microphone is connected: a load resistor R6, through which power is supplied, and an isolation capacitor C3, which serves to separate the audio signal from the DC component of the supply voltage.

The circuitry solution used in this amplifier provides automatic installation and stabilization of its operation mode. Let's see how this happens. After turning on the power, the voltage at the upper terminal of the XP1 connector increases by approximately

up to 6 V. In this case, the voltage at the base of the transistor VT1 reaches its opening threshold of 0,5 V and current begins to flow through the transistor. The voltage drop that occurs in this case across the resistor R5 causes the compound emitter follower transistor to open. As a result, the total current of the amplifier increases, and with it the voltage drop across the resistor R6 increases, after which the mode stabilizes.

Since the current gain of the composite emitter follower (it is equal to the product of the current gain of transistors VT2 and VT3) can reach several thousand, the stabilization of the mode is very tough. The amplifier as a whole works like a zener diode, fixing the output voltage at 6 V, regardless of the supply voltage. However, when using a power source with a different voltage, it is necessary to select the divider resistors R2R3 so that the voltage on the upper pin of the XP1 connector is equal to half the supply voltage. Curiously, the mode is practically impossible to change by adjusting the resistance of the load resistor R5. The voltage drop across it is always equal to the total opening voltage of the transistors of the composite emitter follower (about 1 V), and changes in its resistance only lead to a change in the current through the transistor VT1. The same applies to the resistor R6.

Even more interesting is the operation of the amplifier in AC amplification mode. The audio frequency voltage from the lower output of the resistor R5 is transmitted by an emitter follower with very little attenuation to the upper output - the output of the amplifier. In this case, the current through the resistor is constant and almost does not fluctuate with sound frequency. In other words, the only amplifying stage is loaded on the current generator, i.e. for very high resistance. The input impedance of the follower is also very high, and as a result the gain is very high. With a quiet conversation in front of a microphone, the amplitude of the output voltage can reach several volts. The R4C2 chain does not pass the variable component of the audio frequency signal to the power supply circuit of the microphone and voltage divider.

A single-stage amplifier is not at all prone to self-excitation, so the location of the parts on the board does not really matter, it is desirable to place the input and output at different ends of the board.

Establishment is reduced to the selection of divider resistors R2R3 until half the supply voltage is obtained at the output. It is also useful to choose the resistor R1, focusing on the best sound of the signal taken from the microphone. If the input impedance of the radio with which this amplifier is used is less than 100 kOhm, the capacitance of the capacitor C3 should be increased accordingly.

Author: V. Polyakov, Moscow; Publication: cxem.net

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