Home > Circuit Bricks > Audio Amplifier - Op-Amp
UPDATED:.20:04 24 October 2011
FUNCTION BLOCK
DESCRIPTION


  • This audio amplifier will amplify a low voltage AC signal. The output will be a larger inverted version of the input signal.
  • The difference between these 2 signals is called the Gain.
  • This can be used when a high amplification (gain) is needed, up to 1000 times. Also see Transistor Amplifier
  • Good for microphone applications.
CIRCUIT DETAILS
The gain of the amplifier can be calculated as

G = -Rf/Rin

You can alter the values to adjust the gain but do not exceed more than 1000. The values chosen in this example set the gain to 100.
Input

Circuit Diagram

Signal In
This is the input signal and should not really be greater than about 10mV (0.01v), otherwise the output will 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.
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).
DESIGN POINTS

It is best to keep Rin as high as possible as this sets the input impedance, and in most applications, the higher the better, preferably above 10K and Rf < 10M

But for maximum gain, you will get away with Rin=4.7K and Rf=4.7M.

The supply voltage generally ranges from about 9v to 28v

Shown right is a graph comparing input and output signals. The gain is set at -10 for better illustration

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 one or more of the following:

  1. Increase the power supply voltage - but do not go above 28 volts.
  2. Reduce the input signal using a volume control (preset or potentiometer).

Illustration of 'clipping' with supply of 12v and gain of -20
COMPONENT DETAILS
Operational Amplifier
The Op-Amp can be any general purpose type. Some FET types work well also. Useful types are:
  • LM741
  • CA3140
  • TL071
  • TL081

.
    Package Details

    Most op-amps come in an 8pin DIL package.
    IC Connections

    		You can also get packages with 2 or 4 op-amps inside, such as the TL072 or TL074.

    For more information on op-amps - go to our Datasheets page.
    Written by Phil Townshend - 2008
    www.edutek.ltd.uk - Working Electronics For Students & Teachers