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Microphone amplifier. Encyclopedia of radio electronics and electrical engineering

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Placing a microphone amplifier in close proximity to the microphone dramatically reduces the requirements for shielding connecting wires and improves the signal-to-hum ratio. However, this raises a new problem related to the power supply of the microphone amplifier: the built-in battery requires frequent replacement, and using an additional power cable is not always convenient.

The figure shows a diagram of a two-stage microphone amplifier, which is powered by a signal wire. In this case, only one resistor R4, which serves as the load of the microphone amplifier, and an isolation capacitor C2 must be added to the main amplifier.

microphone amplifier

Bias based on transistor T1 and temperature stabilization of the entire amplifier is provided by the R2R3 divider in the emitter circuit of transistor T2. The second stage of the amplifier is covered by negative feedback through the resistor RI, which is also the load of the first stage. Feedback reduces non-linear distortion to a negligible value and reduces the output impedance of the amplifier to a standard value of 600 ohms.

The amplitude-frequency characteristic of the amplifier in the region of lower audio frequencies is determined by the capacitances of capacitors C1 and C2. The capacitance of the capacitor C2 is calculated by the formula: C2=160/(fnRin), uF, where fn is the lowest operating frequency of the amplifier, Hz; Rin - input resistance of the main amplifier, kOhm. With the capacitance of capacitor C1 indicated in the diagram, the lowest operating frequency is 16 Hz.

The gain of the microphone amplifier turns out to be about 150-250 and depends on the values ​​of the Vst coefficient of the applied transistors and on the supply voltage. The amplifier works well with low-impedance dynamic microphones that have a DC resistance of 100-600 ohms. It can use any low-frequency transistors.

Setting up a microphone amplifier comes down to checking the collector voltage of the transistor T2, it should be equal to half the supply voltage. If necessary, within a small range, the resistance of the resistor R3 is selected, which determines the current of the second stage of the amplifier.

When using an amplifier for telephone communication or speech reporting, it is advisable to reduce the capacitance of capacitor C1 to 0,5-1 μF, which will cause a blockage of the lower audio frequencies, respectively, to 320 and 160 Hz.

Author: V. Polyakov, Moscow; Publication: N. Bolshakov, rf.atnn.ru

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Denis
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