Posted on Aug 11, 2012

Unijunction transistors Ql and Q2 are both connected as relaxation-type, sawtooth oscillators. Transistor Ql is the low-frequency control oscillator and Q2 is the tone generator. Sawtooth waveforms are produced at the emitter terminals. Without R4 connecting the two emitters, each oscillator operates independently, with its frequency determined mainly by the rc time constant. With the values shown, Ql operates from 1. to 1.5 Hz and Q2 operates from 400 to 500 Hz.

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When R4 is connected between the two emitters, it couples the low-fr€quency sawtooth from Ql directly across capacitor C2. That coupling causes the frequency of the tone generator to increase, along with the rise in sawtooth voltage from Ql. The tone generator"s frequency drops to its lower design value when Cl discharges and produces the falling edge of the sawtooth.

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