Electronic – x100 BJT Amplifier


I need to design an amplifier providing voltage gain of 100.

Capacitor calculations are done based on 14 Hz high pass.

Here's the design :

enter image description here

Vcc = 9V
Vin = 50mV, 20 Hz

To calculate RC and RE, I derived KVL equation from the CE loop (right hand side of the circuit) by drawing DC equivalent. Then assuming that RE is about 5 times greater than RC, I gave the values.

R1 and R2 calculations are done by using this formula below. enter image description here

I do not get the amplification rate I desire. What could be the problem ?

If someone explains how to and where to start a design, how to use datasheet values, I really appreciate it.


Best Answer

100 is too high of a voltage gain to attempt to get from a single transistor. Remember that for amplifier gain to be predictable, the transistor must be used with negative feedback so that the gain it produces is at least several times less than what it could do open loop.

Start with at least two stages. A voltage gain of 10 per stage in the final closed loop circuit is quite achievable.

The reason you aren't getting the gain the equations are indicating is because those equations effectively assume infinite gain from the transistor. That's a valid approximation when the actual closed loop gain is significantly less than what the transistor could provide open loop. However, that approximation is not valid when you are asking for 100x closed loop voltage gain from a single stage.