circuit explanation

[Achelius]

Hello from Greece !!!

This is a Voltage-controlled oscillator (VCO). Αn oscillator that its frequency is controlled using a voltage signal. The VCO described here provide both triangular and square wave output. The control voltage can be varied between 5 mV to 5 V, to produce oscillation frequency from 10 Hz to 10 KHz.

Can someone please explain to me briefly the operation of the circuit ?

thank you

[Ken256]

Re: circuit explanation

This looks like a circuit from a national semiconductor datasheet
It might be fun to build and see how it works.

The LM101 opamp is configured as an inverting integrator. The LM111 is a comparator and produces the square wave from triangle wave.

Basic equation for opamp integrator is Vo = -1/R1C1*int(Vin*dt). Vin is constant for half the cycle, so integration is simple: Vo = t*(-Vin/R1C1), producing a ramping voltage. But this circuit is a little different because a tuning voltage is applied to non-inverting input of LM101.

It looks like the square wave output is being used to switch the JFets Q1, Q2. When sq wave is high P-Ch Q2 turns off, N-Ch Q1 on. When sq wave is low Q1 is off, Q2 on.

Pin 3, (Non-Inv) input of LM101 gets held at the tuning voltage divided by 2 (R2 and R5). Input to integrator at R1 gets switched between two voltages: Input tuning voltage via Q1 and ground via Q2.

This is how the integrator gets an input voltage of constant magnitude, but differing polarities. For example, if the tuning voltage is +4.2 volts: Pin 3 is +2.1 volts. R1 gets switched between +4.2 volts and ground. So the integrator sees (2.1V - 4.2) = -2.1V and (2.1V - 0) = 2.1V giving positive and negative output ramps.

D3/D4 are 5V zeners. It looks like they set the switching point at about +/- 5.6 volts. The comparator has a lot of positive feedback (hysteresis, R9). I'm feeling lazy, maybe you can analyze this comparator more in depth.