- Joined Jun 10, 2015
I hope this is the right section!
As a final project, a group of us are designing a Three Stage BJT amplifier using the 2N3904 transistor with an expected non-inverting gain of 15 V/V at a midband frequency of 15 Hz. The source is a 100mV 1Hz pulse with a Thevenin resistance of 200k. The two common emitter stages contribute the 15 V/V and the final stage is a simple emitter follower (voltage buffer). Each stage is separated by a coupling capacitor, which strips any DC component that may have become amplified during the previous stage. This allows the output to only realize AC amplication. The amplifier will output a peak voltage of 1.5 V to a load resistance of 100 ohms. Note, the capacitor crossed-out in red is considered removed. Since we want to realize a high input impedance at stage one, the capacitor would only work against this goal. However, an alternative solution is to include a small emitter resistance in series with R4//C2.
We are having a hard time trying to find a "baseline" or a place to start for finding component values. So far, we have come up with expressions for Rin, Rout, gain, vpi, and Vc for each common-emitter stage (hopefully they are right). Obviously, we're not looking for any responses that solve the problem for us, but simply nudge us in the right direction. All we know is: (a) There is a high input impedance that we want to resolve with the first stage, (b) Due to (a), the gain will be fairly small compared to stage 2 where stage 1 will roughly contribute only 1.5 V/V, (c) we are willing to suggest that 50mV (50% of Vin) will drop across the 200k.
Any thoughts on how to approach the resistor values? I feel like we're almost at the point where all the values will fall into place, but we are missing that important piece of information that will allow us to really pick up speed.