[SOLVED] Why does Gain Bandwidth Product affect this circuit?

[roys]

I am using the PIC16F1508's PWM peripheral to generate a 15.8KHz PWM frequency (0-5V amplitude) which then gets fed into an active low pass filter (see spice file).

The spice file contains 3 similar circuits. The only difference is the opamps used in the circuit have different GBWP: 10MHz, 180Khz, and 14KHz.



The 10MHz GBWP amplifier outputs a relatively smooth DC signal which is desirable. The 180KHz output looks unstable and the 14KHz doesn't seem to work at all.

Could someone please explain why the 10MHz GBWP does a better job filtering the 15.8KHz PWM signal than the other two amplifiers?

Also, why is the 14KHz opamp output higher than the supply rail?

I would like to get this circuit to work with a 14kHz GBWP amplifier... any suggestions on how to improve this design?

 

[FvM]

By changing the filter from second to third order Butterworth with "absorbed" first order, you'll notice that the GBW as such isn't the problem. It's the PWM carrier feedforward and OP dynamic overload that caused the problem. You should consider that the low speed OPs are also micropower types with relative high output impedance and limited AC drive capability.

 

[crutschow]

In an active filter there is feedback at the input frequency so the op amp has to respond to the high frequency rise-time components of the PWM signal in order to perform the proper filter function. Thus, as you have discovered, you need a high GBWP op amp to filter a high frequency, even if the filter output is a low frequency.

Using an RC filter at the input to reduce the high frequency components below the GBWP of the op amp, which is basically what FvM suggested, will also work.

Your 14kHz op amp may be oscillating since it is stable only with a closed loop gain of >5 (didn't you notice that in the spec?) and you are using it at a gain of 1. You need an op amp that is stable with a gain of 1. As to why its output is above the supply voltage, that may be a simulation artifact of the oscillations coupled with the input signal.