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Subject: Re: OPAMP buffer instability

Date: 03/29/01 at 10:47 PM
Posted by: John Roberts
E-mail: johnrbeer@aol.com
Message Posted:

In Reply to: OPAMP buffer instability posted by ordepat on 03/27/01 at 12:26 PM:

There may be a simple answer but first some basics. Amplifier in-stability is generally caused by phase shift or time delay in the total negative feedback path. The criteria for stability is that the product of amplifier open loop gain and feedback network attenuation be less than unity before the phase shift reaches 180'. Obviously if you have total gain greater than unity and 180' of phase shift you now have positive feedback and an oscillator.

The case you describe, 100% negative feedback, is the worst case for stability as there is no feedback network attenuation. Even though the opamp may be rated unity gain stable (read the spec sheet), capacitive loading at the output may interact with the opamps intrinsic output impedance to cause enough additional phase shift for the circuit to be unstable.

I gather from your topology.. + input to gnd, with 100% negative feedback, that you are using the opamp for some alternate reference gnd? You need to be careful of how you load this opamp.

A possible simple fix is to use some resistive decoupling. If your circuit can afford it, a 10-50 Ohm resistor between the opamp output and the rest of your circuit could help. The power supply impedance should be low (caps at power pins).

Hope this helps.

JR


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