Cascaded BJT Amplifier

Question

I found this circuit in here. I know that it's a multi-stage cascaded BJT Amplifier and I know how to calculate it.

My question is, what R8 is used for? And If I remove it, is it cause any problem or bad effect. Thank you guys so much

Answer 1

That resistor is to set the AC gain of the second stage.

The AC gain of a common emitter amplifier is roughly the impedance in the collector circuit divided by the impedance in the emitter circuit. When using an emitter resistor alone as in the first stage the gain can be approximated by \$R_C/R_E\$, so around 4.7 for the first stage.

To increase the gain without changing the DC bias it's common to put a capacitor to bypass \$R_E\$, with the capacitor having around 1/10th the impedance of \$R_E\$ at the frequency of interest. This makes the emitter circuit impedance low and so the gain will be high, sometimes too high. To get higher gain than a resistor alone, but not as much as a bypassed resistor, you can add a second resistor in series with the capacitor. Now the AC gain will be roughly \$R_C\$ divided by the two emitter resistors in parallel. In the second stage the gain would be roughly 47. Stage loading and some other factors will affect the exact gain, so these are just ballpark figures.

Removing it (like cutting it out of the circuit leaving C4 disconnected) will reduce the gain of your amplifier by around a factor of 10.

Removing it (like replacing it with a short so C4 is across R7) will increase the gain to whatever the transistor will do based on it's \$H_{FE}\$.