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Op-amp cct output impedance

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dknguyen

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How do you calculate the output impedance of an op-amp circuit? Because if you assume an ideal op-amp has an output impedance of zero, wouldn't that mean that anything you do would not change the output impedance (since most impedance components are placed such that they are in paralle with the output reducing output impedance even farther? (that is, unless you placed something between the output of the op-amp and the load it's driving.)

Let's assume the op-amp can drive enough current through those impedances parallel to it's output to generate the correct output voltage.
 

audioguru

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An opamp has an output resistance of about 100 ohms. Its internal gain is about 200,000 from DC to about 10Hz. So its output impedance from DC to 10 Hz is about 500 micro-ohms when it has no gain.
Its internal gain drops at the rate of 20dB per decade of frequency so at about 3MHz the internal gain is 1 and its output impedance is 75 ohms.

Most opamps limit their output current to about 30mA peak.
 

dknguyen

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But is that right when I say that most circuits won't increase the output impedance of an op-amp? Because from my understanding, that seems to be the case. Feedback resistors and such seem to all go in parallel with the output just lowering the output impedance further (but increasing output drive current at the same time)/
 

audioguru

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An opamp with 100% feedback so its closed-loop gain is 1 has a very low output impedance from DC to 10Hz. But if it has some closed-loop gain then the output impedance is higher.

An opamp can't drive a load resistance (or a feedback resistance) less than about 1k ohms which is much higher than its output impedance.

An opamp's output current is limited to about 30mA peak. That is the total current in the load and in the feedback resistors.
 

Optikon

New Member
dknguyen said:
But is that right when I say that most circuits won't increase the output impedance of an op-amp? Because from my understanding, that seems to be the case. Feedback resistors and such seem to all go in parallel with the output just lowering the output impedance further (but increasing output drive current at the same time)/
I think you are confusing output impedance of the opamp itself with output impedance of *your circuit* that includes an opamp among other things like feedback resistors.

Nothing you can do will affect the opamp itself. It has a biased transistor output stage which has some output impedance. You dont get a pin that has a wire to that cirucit so you cannot affect it.

Now, the output impedance of your circuit is a different matter. I fyou use feedback then the *EFFECTIVE* output impedance o fthe circuit can be lowered by the gain. but you have technically done nothing to the opamp itself.

Now if you put a series resistor in the output of an opamp, you have increased your circuit output impedance but again, you have doen nothing to change the output impedance of an op amp.

Clear as mud? :D
 

dknguyen

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Of the circuit I mean. But the fact that feedback changes the output impedance of the op-amp circuit itself answers my question.
 
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