Part Number: OPA551
Other Parts Discussed in Thread: TL082
I am designing ac constant current source using OPA551. The current source is <100mA
this is my pspice simulator
and this is wave output on R8
when i made it with actual circuit it not woking. This is wave form R8 on Osciloscope
I don't know why wave on R8 is clipped
Thanks for Kai to duplicate the customer's schematic in Tina!
Hi Le Hung,
Your original schematic in pspice is not stable, when it is simulated.
I modified the schematic slightly. I added a zero to compensate the Q at approx. 500kHz. It looks pretty good now, see the attache image and file.
Best,
Raymond
Hi Raymond,
unfortunately, there's still another issue:
It seems to have to do with the value of R5 (in the above simulation).
Kai
Hi Le Hung and Kai,
Kai is correct that the my modified circuit did not simulate for a longer duration.
After I removed the high pass filter, C2, another Q is popped up. The Q is disappeared after TL082, U3 buffer is removed. If U3 is removed, U2 is not needed either, therefore I removed both buffers. Technically, both buffers should not be there, because they create more phase lag in the feedback loop.
It seems that the high pass filter (C2 and R5) is interfering with the stability of the feedback loop. Thus I removed HPF from the simulation. However, if you have to have it, please place a buffer after the HPF (before R6).
The circuit simulates good now. I did it up to 2 seconds, see the image below.
Best,
Raymond
Hi Le,
omitting the high pass filter C2, R5 is a good idea. Unfortunately, the circuit is still not yet stable, even not with optimum phase lead compensation as recommended by Raymond:
This has to do with the fact that the TL082 introduce an additional phase lag which cannot be fully compensated by the help of C1.
I would do it this way:
Kai
