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Subject: OpAmp stability when driving transistor load

Date: 04/15/03 at 8:43 PM
Posted by: Sidney Chi
E-mail: sidney.chi@qualitau.com
Message Posted:

Hello,
I went into some oscillation problem (approx. 15MHz & 1.5V peak to peak sine wave) when driving transistor load with opamp.

The opamp (negative fb) is in unity-gain configuration with switching relays inside the feedback loop. Input to the opamp is simple dc and output (connected to a transistor via a cable) will oscillate when connected to a transistor load.

I have read several documents that states that oscillation is caused by the interaction between opamp output resistance and load. What could be the solutions to avoid oscillation? Could an isolation resistor be used? If yes, should it be connected inside or outside the feedback loop?
Thanks.

sidney


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