A Variable Audio Frequency Oscillator Using Op-amp 741

The frequency of the AF oscillator can be varied over a wide range. It can, therefore, be used for AF amplifier testing and also as a code practice oscillator.

Circuit Description of A Variable Audio Frequency Oscillator Using Op-amp 741

The circuit of a variable audio frequency oscillator using op-amp 741 is mainly a Wein Bridge oscillator whose frequency is varied using the gang condenser. While using the IC1 741 (operational amplifier), a split power supply has been avoided by using capacitors C1 and C2. For R₄*, a 150-Ω resistor, we can use a thermister or even a bulb that controls the amplitude of the oscillation.

While using the oscillator, an 8Ω speaker with a 10 µF capacitor connected as in the circuit will provide a good amount of volume. For testing, I connected the output pin number 6 of the IC1 to the input of the instrument to be tested via a 0.01 µF capacitor. The circuit works satisfactorily off a 3V to 9V supply.

PARTS LIST OF VARIABLE AUDIO FREQUENCY OSCILLATOR USING OP-AMP 741

Application of Variable audio oscillators

Repair of audio gear is one where you inject an audio signal (usually a very pure sine wave) into an audio amplifier, view the output waveform, and observe its linearity at various audio frequencies between 20 Hz and 20 KHz. For an amplifier that is “dead”, i.e. it appears to be “on” and you’ve injected a signal (sine wave) you can point-test sections of the circuitry to determine where and why the signal is not making it to the output stage, or perhaps the tone-controls (if the amp has them) are not doing what they’re supposed to be doing.

Another area is for speaker testing: You can test various types of speakers, such as woofers (low frequency), mid-range speakers (most of what we hear and that is in music and speech), and tweeters for higher audio frequencies, to see whether they’re performing within their frequency range. Yet another area is to derive a frequency response plot (sometimes referred to a BODE plot) when testing an audio amplifier to determine its overall frequency response (gain/loss-vs-frequency) as well as it’s power bandwidth (power-vs-frequency). Still, yet another area is filter design testing (active or passive filters) to determine the general response of the filter, for example, how well is a low-pass filter (used to filter out high frequencies) working or how will a high-pass filter (used to filter out low frequencies) is working based on the calculated cutoff points for the respective filters in question. There are more applications as well such as testing modulation circuits, etc.