The principle of voltage-frequency converter circuit with linear sawtooth output

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As shown, the circuit is actually a voltage controlled oscillator whose output is connected to the discharge terminal of 555 (pin 7), thus outputting a sawtooth wave.
The operational amplifier A and the capacitors C1 and R1 are connected to form an integrator, and the junction FET VT1 (3DJG) is used as an integrator discharge reset switch, which is controlled by the 555's 3-pin output. VT2 and R4~R2 are connected to form a constant current source. When 555 outputs a high level and a half period, VT2 is turned on and C4 is discharged. During the low period, VT1 and VT2 are turned off, and VTa charges C4 with a constant current to ensure linear growth of the sawtooth voltage on C2.