Circuit

A microphone preamplifier circuit is shown in Figure 60. The purpose of the circuit is to take a signal from a microphone and prepare it for further amplification in an audio system.

**Figure 60:** Microphone preamplifier circuit.
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The emitter resistor R_E is split into two: R_E= R_E1+R_E2. This is done to reduce sensitivity to parameter variations ( $\beta$ , r_e), but without reducing the gain too much.

The BC109 data sheets specify that the current gain $\beta$ ranges between 200 and 800, with a typical value of 400.

ANU Engineering - ENGN2211