Home > Circuit Bricks > Audio Transistor Amplifier
UPDATED:.20:55 21 October 2013


  • An audio amplifier will amplify an low voltage AC signal. The output will be a larger version of the input signal.
  • The difference between these 2 signals is called the Gain.
  • This is a general purpose amplifier that will work at most voltages and gives a Gain of about 75.
The gain of the amplifier can be calculated as

G = (R1/R3) x hfe

You can alter the values to adjust the gain but do not exceed more than 150 as this can cause the circuit to become unstable. The values chosen in this case are to give an all round performance at most voltage levels.


Circuit Diagram

Signal In
This is the input signal and should not really be greater than about 100mV (0.1v), otherwise the output could be distorted
C1 decouples the input from any DC signal. It should be removed if the circuit you are connecting to also has a decoupling capacitor on its output.


The output will be an amplified version of the input and inverted (ie. when the input goes positive, the output will go negative - and vice versa).


Shown right is a graph comparing input and output signals. The gain is set at -10 for better illustration, (the minus means it's inverted)

If the signal sounds distorted, it is likely that the wave is clipping. This is when the top and bottom of the sound wave are lost, clipped. This is because the amplifier cannot produce large enough output voltages due to the supply voltage being too low.

If viewed on an oscilloscope it might look something like the lower graph.

There are a few ways to get around a clipping output. Try the following:

  1. Increase the power supply voltage - but do not go above 18 volts.
  2. Reduce the input signal using a volume control (preset or potentiometer).

Illustration of 'clipping' with supply of 10v and gain of -20
NPN Transistor
This can be any general purpose type such as one of the following:
  • BC184
  • BC108
  • BC109
  • BC549
Pin connections


Symbol connections

For more transistor options, go to the NPN Transistor Specifications page
Written by Phil Townshend - 2008
www.edutek.ltd.uk - Working Electronics For Students & Teachers