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opamp output impedance, how to calculate?

post #1 of 4
12/1/06 at 10:03am
Thread Starter 
Hello,

I need to know how to calculate the output impedance of opamps, esp in cloed-loop mode. I can measure it at any frequency, and calculate and measure input impedance of a closed loop circuit.

output impedance/resistance of the given opamp (TL071) is not available in the data sheet - i.e. no graph.

Thanks in advance
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post #2 of 4
12/1/06 at 12:10pm
You can expect it to be low, usually <100 ohms.

Remember the output impedance varies with gain and frenquency, sometimes that information is given in the datasheet (such as AD8610), sometimes it's not. So when you don't have such information available you need its internal circuitry to do the calculation. But most opamps have output impedance <100 Ohms.
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post #3 of 4
12/1/06 at 7:30pm
Thread Starter 
I know... and less than 100 Ohms was my guesstimate too

Looking at the schematic at page 5 of http://focus.ti.com/lit/ds/symlink/tl071.pdf it seemsto have a resistance of 128 ohms in series with the output, and 64 ohms leading from an NPN emitter, and 64 ohms coming from a PNP collector. Is it really possible to guess the output impedance from this?
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post #4 of 4
12/1/06 at 8:50pm
It's roughly the intrinsic output impedance (~170 Ohms seems reasonable for this opamp) divided by the loop gain. This results in approximately 170/200,000=850 uOhm at low frequency for a unity gain follower version of this opamp.
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