analog pi control of an old inverter

Thread Starter


Joined Oct 15, 2014
Hello everyone,

I found this circuit in an old inverter ,which i tried to re-trace the circuit.
Im confused at this circuit which i think is the a PI control loop.

I tried to simulate it in ltspice but getting some wierd results.
Firstly there is no DC gain present.Only when i give different AC values i see some difference in the output dutycycle.

I check a few books and tried to derieve the transfer function and the co-efficients.
Please correct me if i am wrong:

Kp = R2/R3 = 5.62
Ki = 1/(C1*R3) = 21276.5

Will the output be (Vout) = -(R2/R3 + (1/C1*C3)) *Vin??

Many thanks


Last edited:

Thread Starter


Joined Oct 15, 2014
That circuit is an integrator with DC gain equal to the open-loop DC gain of the op amp.
In real life, if not part of a feedback loop, the output will likely go to one of the rails due to input offset and stay there.

I don't see any C3.
Thanks for the reply.
I made a typo in the formula.I meant (Vout) = -(R2/R3 + (1/C1*R3)) *Vin


Joined Mar 14, 2008
You formulas are incorrect because they do not have a frequency related S or jW term for the capacitive reactance.
The impedance of a capacitor varies with the applied signal frequency and thus any expression for the transfer function of a circuit with capacitors must include a frequency term.
Read this for more info.