The Mini-3 VCOs

The VCOs are based on unijunction transistors. The op-amp is the voltage summer. The PNP & NPN transistors provide a constant-current source that charges the .001 capicitor. The .001 capicitor and the unijunction transistor form a relaxation oscillator. As the voltage at the input increases the current supplied by the constant-current source increases causing the .001 capicitor to charge faster increasing the frequency of the relaxation oscillator. The voltage across the .001 capicitor is a linearly increasing ramp until the unijunction transistor is fired then drops as the .001 capicitor starts to charge again. This produces a sawtooth waveform thats applied to a FET buffer transistor and a comparitor that is used to convert the sawtooth to a variable pulse-width square wave.

Testing The VCOs

Apply +- 12VDC to the curcit, then hook the output to a monitor amp.

The frequency of the oscillator should increase when the RANGE trimmer is ajusted.