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[SOLVED] output impedance of opamp

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Toru Fujinami

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Hi,
I have a question about output impedance of the opamp.

In basic circuit theory class, we say that output impedance of the opamp is very small so that output voltage does not drop within opamp itself.

However in transistor level OPAMP design class, we learn that gain of the Opamp is typically Gm*Ro where Gm is transconductance and Ro is the impedance looking into output. To get a high gain, we want to increase Ro.

Aren't these two statements contradicting each other?

Thank you
 

An op amp adjusts its resistance so that the output produces the correct voltage to whatever load is attached. That is, within limits. An LM741 can only power a load of several hundred ohms, causing voltage to drop 5 percent.

In regard to gain, the amount is valid only until the output voltage reaches the supply rails, and then your gain figure becomes invalid.
 

It depends on how you look at it. If you consider the opamp to be voltage controlled voltage source, then the output impedance is very low. If you consider it to be voltage controlled current source (operational transconductance amplifier, OTA), then the output impedance should be very high. Choose the correct model for your analysis. Both are one and the same.
 
However in transistor level OPAMP design class, we learn that gain of the Opamp is typically Gm*Ro where Gm is transconductance and Ro is the impedance looking into output. To get a high gain, we want to increase Ro.

The voltage gain of a single transistor stage without a stabilizing emitter resistor is G=-gm*R with R being the effective resistance across which the output voltage is developed (for example: Collector resistor in parallel to ta load resistor). However, sucha stage is not an "opamp".
More than that, you can model a complete operational amplifier as a transconductance stage gm working upon a (low) internal output resistance.
But this is just a model - nothing else.
 

The output resistance of a typical opamp is maybe 100 ohms. The gain at DC and low frequencies (below 10Hz) is extremely high which reduces the output impedance when negative feedback is used so the output impedance could be much less than 1 ohms at DC and low frequencies.
 

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