Electronic – Operating principles of a sawtooth wave generator bootstrap circuit

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In one of my assignments, I was asked to explain the working principles of the sawtooth wave generator shown below. The input voltage is a spiked waveform obtained from a differentiator circuit.

I realise that the transistor is supposed to act as a switch and C1 is getting discharged by it, but what I don't understand is the role of Vcc, capacitor C2 and the op-amp.

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Best Answer

Hmm that looks quite a clever circuit - it largely forces a constant current into C1 by keeping the DC voltage across R1 fairly constant between discharge events. I may be wrong but, if you have a simulator you could try it out.

but what I don't understand is what is the role of Vcc, capacitor C2 and the Op-Amp.

\$\color{red}{\text{(the diode is important too)}}\$

Once the discharge event has finished, C1 starts to charge and the bootstrap capacitor C2 lifts the cathode voltage of the diode and reverse biases it. Now, the diode doesn't have any role and a constant voltage is applied across R1 thus, it feeds a constant current into C1 and you get a linear charging voltage.

For this to be fairly linear, C2 needs to be many times bigger in value than C1.

Take a bow on presenting me a circuit I've never seen before.