The circuit as shown by SPARKY256 in his contribution contains TWO feedback elements. Simulate this circuit – Schematic created using CircuitLab Low noise pre-amp stages maybe biased for a CE current of 50 uA to 100 uA, to keep schott noise to a minimum. Theses combined feed backs give the circuit a good 'stability factor'. This also helps cancel out CE current drift with temperature. Thanks to reminder that the emitter resistor, and 1 K is a substantial value, helps out by combining with Q1 and its fixed base voltage to act as a constant-current sink. Too low of a current and you lack drive power for the next stage, and all transistors have a collector cut-off current. Any higher current and schott noise and self-heating begins to be an issue if more stages are after this one. It may have to be 500 K or so, as long as the CE current is 50 uA to 1.5 mA. R4 could be omitted if R2 had a high enough value to match the gain of Q1. This would not affect audio, just block DC from a previous stage. In this case it is -3dB at about 0.153 HZ. You must choose resistors based on the transistors Beta and the input loading of the previous stage.Ĭ1 and R4 form a time constant, a high pass filter, so their values affect mostly the bandwidth of the signal at the low end. If the collector voltage drops, so does the base current, which stops the collector from dropping. Study the feedback loop and you can see where it is self-stabilizing, even with some temperature change. An ancient transistor with a gain of 10 would need much lower resistor values for proper bias. This depend greatly on the transistors Beta. The base then would have about 1.3 volts on it, not including any injected waveform.Īs it is you should have about 6.5 volts at the collector, about 1/2 the supply voltage. R2/R4 form a 5:1 divider but the transistors feedback loop means at least 0.65 volts on the base, but with Q1 having a Beta of 100 the voltage at the emitter is more like 0.65 volts, for a CE current of 650 uA.

The gain is controlled by the ratio of R1/R3, so even if this transistor had a gain of 100, R1/R3 limits the gain as a circuit to 10. It was nothing more than feeding some collector voltage back to the base.

Self bias was important back in the days when stages were connected by capacitors. β is equal to Beta, hFE, or the DC gain of the transistor. The stability factor (S) for a fixed bias circuit is (1+β).

Importance of transistor biasing