[sdiy] Outputs and Impedances

Tony Kalomiris weplar at gmail.com
Sun Oct 28 10:24:06 CET 2018

Besides load resistance the output current limit of the op-amp must not be
exceeded. (which is what the OP was concerned about IIRC)

To take an example from one of my textbooks (Sedra/Smith Microelectronic
Circuits 5th Ed, page 94),

"the output current limit of the 741 is +/- 20mA - this includes the
current in the feedback as well as the current supplied to the load.
If the circuit requires a larger current, the op-amp output will saturate
at the level corresponding to the maximum allowable output current."

In the example (2.5) we have Rload = 1 K, R1 (or Rin) = 1 k, R2 (or Rf) =
9 K, and 15 volt supplies.
For a 1v peak signal input you would have 10v peak output (Av= 1 + R2/R1).
Across that 1 K load resistance gives 10 mA.
Then you calculate the current in the feedback network , or, 10 v (output
signal) / ( 9 K + 1 K) = 1 mA, for a total current of 11 mA.
"Well under the limit of 20mA"

So to calculate the minimum load resistance,

(10/Rmin + 10 /(9 K+ 1K))= 20 mA (Iout(max) for a 741)
Rload (min) for this example would be 526 ohms.



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