Created by
Created September 24, 2013
Last modified September 30, 2013
Tags amplifier   emitter-follower   gain   transistor  


Two transistor amplifier. Gain of about 30 times (inverted) and low impedence output.


This was just an experiment to try to get my head around proper transistor biasing. I've always used op-amps but thought I really ought to at least UNDERSTAND transistors! This is a start...

Transistor Q3 is biased within its linear activation region and the Collector-Emitter path is geared to provide 30 times inverted voltage gain at the Collector.

Transistor Q1 is connected as an Emitter-follower with unity Gain. It's job is to act as a buffer to allow a lower impedence output.

Capacitor C7 filters out a proportion of the AC voltage changes across Emitter resister R17. By varying the position of the variable resistor wiper (trimpot) it can be made to filter enough of the AC voltage out to prevent negative feedback from reducing the gain. In this circuit it's set to about 90% position which is just about right.

Without the capacitor, as the current out of the Emitter grows with the output voltage, it has the effect of reducing the voltage drop across the base-emitter junction and so effectively reduces the gain. This has a significant effect.


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