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Simulated on Multisim 14.0

In the above circuit, without RL the circuit behaves quite normally with a non inverting gain of 50 for any input at V1 till the output saturates to 15V.

Things however get weird when I connect a heavy load RL=75 ohms as shown.

For V1 = 10 to 25 mV the gain stays at 50 as expected. But as V1 is further increased, the gain starts to drop rapidly with a gain of about 35 at 50mV of V1 and about 19 at 100mV of V1. This drop however is not seen when I am driving loads of say 50 kohms; the gain stays constant. I am confused and cannot find any solution as to why this would happen. What is the significance of connecting a heavy load and how does it impact the gain?

The op amp model used is a 741 with an open loop gain of 10k, open loop input impedance of 10 kohms and open loop output impedance of 100 ohms. Offset voltage was set to 0. Same circuit with the op amp model changed and used a "model" (no specific name)

The properties changing dialog box of the "model" op amp

UPDATE: Okay tinkering with values in the dialog box led to some interesting results. Following jonk's comment about current compliance I went on to change the "Output Short Circuit Current" from 25 mA to 100 mA and it seemed to do the trick. Now even at 120 mV of V1 the gain seemed to stay at about 50.

So what is the deal with this "Output Short Circuit Current"? How does it limit the performance of my op amp? Help much appreciated!!

  • 2
    \$\begingroup\$ Most opamps have very limited current compliance and cannot drive arbitrary loads. Without looking at the datasheet, I generally try to ask for less than 5 mA, though many do support more. The 741 probably can't handle the heavy loading. Have you looked at a datasheet on it? \$\endgroup\$ – jonk Nov 4 '19 at 6:57
  • \$\begingroup\$ will i upload the multisim database on the particular op amp? \$\endgroup\$ – nn08 Nov 4 '19 at 7:00
  • \$\begingroup\$ No. I'm asking you to look at the datasheet. Not a multisim .subckt. The output should have some specifications. My guess is that it isn't design to drive your low loads. But since I don't have the time to check myself I'm asking you to look. Chances are, it is right there in black and white. \$\endgroup\$ – jonk Nov 4 '19 at 7:04
  • \$\begingroup\$ Well even then the weird thing is I then used a model op amp (no specific name given ,on the schematic it shows up as OPAMP_5T_VIRTUAL where I can freely change the parameters) and it gives the same kinda observations. \$\endgroup\$ – nn08 Nov 4 '19 at 7:04
  • 1
    \$\begingroup\$ Jonk has pretty much said it all... If you want to drive higher loads I’d suggest using a transistor in order to buff the output. Here’s an example: allaboutcircuits.com/technical-articles/… \$\endgroup\$ – rr1303 Nov 4 '19 at 10:00

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