Bench Amplifier

Circuit : Andy Collinson
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This bench amplifier is a useful addition to the workshop. It has a high impedance front end so as not to load circuit, and a small 325 mW audio amplifier with a voltage gain of 200. This circuit may be able to amplify strong signals from a crystal radio or used as a general purpose bench amplifier. bench amplifier circuit

The bench amplifier offers a high input impedance and uses a 2N3819 FET is source follower mode. Input impedance is set by R1 and is 2.2 Meg. C1 insures that no DC output from the circuit being analysed will affect the amplifier. The source follower load is R2, this will have a small DC voltage across it, which is removed by C2 before being amplified by IC1.

In this circuit, the LM386 acts as a non-inverting amplifier with voltage gain of x200. VR1 is a logarithmic potentiometer and acts as the volume control. The gain is boosted to 200 by the inclusion of C3 across pins 1 and 8. The IC is available in an 8 pin DIL package and several versions are available; the LM386N-1 which has 325mW output into an 8 ohm load, the LM386N-3 which has 700mW output and the LM386N-4 which offers 1000mW output. all versions work in this circuit.

The IC works from 4 to 12 Volts DC, 12 Volts being the maximum recommended value.

LM386 Datasheet
A data-sheet in PDF format can be downloaded from the LM386 Datasheet.

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