Mini Audio Signal Generator


A small audio test
generator is very useful for quickly tracing a signal through an audio
unit. Its main purpose is speed rather than refinement. A single
sine-wave signal of about 1 kHz is normally all that is needed:
distortion is not terribly important. It is, however, important that the
unit does not draw too high a current. The generator described meets
these modest requirements. It uses standard components, produces a
signal of 899 Hz at an output level of 1 V r.m.s. and draws a current of
only 20 µA. In theory, the low current drain would give a 9 V battery a
life of 25,000 hours. The circuit is a traditional Wien bridge
oscillator based on a Type TLC271 op amp. The frequency determining
bridge is formed by C1, C2 and R1–R4. The two inputs of the op amp are
held at half the supply voltage by dividers R3-R4 and R5-R6
respectively.

Circuit diagram:

Mini Audio Signal Generator Circuit

Mini Audio Signal Generator Circuit Diagram

Resistors R5 and R6 also form part of the feedback loop. The
amplification is set to about ´3 with P1. Diodes D1 and D2 are peak
limiters. Since the limiting is based on the non-linearity of the
diodes, there is a certain amount of distortion. At the nominal output
voltage of 1 V r.m.s., the distortion is about 10%. This is, however, of
no consequence in fast tests. Nevertheless, if 10% is considered too
high, it may be improved appreciably by linking pin 8 of IC1 to ground.
This increases the current drain of the circuit to 640 µA, but the
distortion is down to 0.7%, provided the circuit is adjusted properly.
If a distortion meter or similar is not available, simply adjust the
output to 1 V r.m.s. Since the distortion of the unit is not measured in
hundredths of a per cent, C1 and C2 may be ceramic types without much
detriment.


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