Op amp circuit to drive 20uF load

[ack88]

Op amp circuit

Hello,
Thanks in advance for reading.
I am trying to drive a 20uF load with the opamp circuit attached .I have used AD825 op amp.Basically I am giving [Vac + (Vdc/2) ] at non inverting input and getting Vac+Vdc at output. Maximum Vac is 100mV at 20kHZ.
The output is not stable with this configuration. Do i need to put an transistor output power stage or something else for this?
Thanks

 

[Harald Kapp]

The problem is not the driving strength of the OpAmp but the phase shift induced by the capacitive load. This leads to positive feedback, making the circuit instable.

The best solution would be to use an OpAmp that is capable of driving large capacitive loads.
Second best would be to decouple the output and feedback node of the OpAmp from the capacitor: add a series resistor between OpAmp and capacitor, but leave the feedback connection at the OpAmp output.
You could try a transistor buffer stage, but you need a very low output impedance, otherwise the phase shift will persist.

By the way: Why do you need such a large capacitor at the OpAmp's output?

Regards,
Harald

 

[Harald Kapp]

Maybe you can replace the large capacitor by an active low pass filter, thus relieving the OpAmp from the large currents flowing in and out of the capcitor?

Harald