I have a question about open loop gain. In the circuit below, I want to measure the open loop gain of the error op-amp. I'm curious if the Current Limiting op-amp will affect my gain.

What does it mean when they say Open loop voltage gain @ Rl = >= 10M, Change of Vo = 1v to 4v

enter image description here


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ Which controller chip is this diagram from? \$\endgroup\$ – Nick Alexeev Apr 19 '16 at 2:22
  • \$\begingroup\$ microsemi.com/document-portal/doc_view/11119-sg1524b-datasheet \$\endgroup\$ – hybridchem Apr 19 '16 at 2:24
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    \$\begingroup\$ The gain should be roughly 1000, so my offset voltage between the + and - should be tiny? \$\endgroup\$ – hybridchem Apr 19 '16 at 2:25
  • \$\begingroup\$ You say you have a question but you don't say what it is. Don't be shy. \$\endgroup\$ – Andy aka Apr 19 '16 at 7:23
  • \$\begingroup\$ I didn't mean to be vague. My question is how do I measure the gain. It's Vo / V1 - V2 which should be 60db.. So I put a signal into the Error amp with a tiny offset? \$\endgroup\$ – hybridchem Apr 19 '16 at 11:57

Open loop gain is infinite high of an op-amp by design. See https://en.wikipedia.org/wiki/Open-loop_gain Of course, in real opamps the gain can not be infinite, but it is quite high. In real opamps the output can not go to infinite high or low, but will be clipped by the min-max of voltage rails.

In the datasheet you can see an open loop gain of 78dB, which roughly corresponds to 100,000,000 times of amplification.

However, your circuit does not seem to be open-loop. It is a driver for a closed-loop system. The transistors drive the transformer, and there is a feedback from that fed to your error amplifier, this makes this circuit a closed loop control.

  • \$\begingroup\$ You're talking about the top circuit? How would one measure the open loop gain then. \$\endgroup\$ – hybridchem Apr 19 '16 at 14:15
  • \$\begingroup\$ So the spec says Open Loop voltage gain from Vo (1 to 4)v.. So I adjust my signal in Vdc till I see 1V out to 4V out and then Do 3V/Change in input? \$\endgroup\$ – hybridchem Apr 19 '16 at 14:47
  • \$\begingroup\$ I am unsure what are you willing to achieve. The top circuit says a 78dB open loop gain. A generic opamp like TL081 has similar open loop gain like 80dB. However the lower circuit makes no sense. The output of the opamp will be clipped by power supply rails, and you'll see a rail-to-rail square wave on the output. \$\endgroup\$ – Gee Bee Apr 19 '16 at 16:54
  • \$\begingroup\$ Ignore that. I'm just trying to get the output working properly. I Currently have the senses tied low, and a input signal in with a 1mV Vdc Offset and I'm not seeing anything on the output. \$\endgroup\$ – hybridchem Apr 26 '16 at 19:02
  • \$\begingroup\$ So with the circuit I just updated, I'm getting a very large gain. Does that make sense? \$\endgroup\$ – hybridchem Apr 26 '16 at 19:16

Open loop gain can be measured:

When S1 and S2 are closed, Ios still flows in the 100-Ω resistors and introduces an error in Vos, but unless Ios is large enough to produce an error greater than 1% of the measured Vos it may usually be ignored in this calculation.

The open-loop dc gain of an op amp can be very high; gains greater than 107 are not unknown, but values between 250,000 and 2,000,000 are more usual. The dc gain is measured by forcing the output of the DUT to move by a known amount (1 V in Figure 4, but 10 V if the device is running on large enough supplies to allow this) by switching R5 between the DUT output and a 1-V reference with S6. If R5 is at +1 V, then the DUT output must move to –1 V if the input of the auxiliary amplifier is to remain unchanged near zero.

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