How to measure open loop freq response
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 29th July 2002, 03:03 AM #1 yves diyAudio Member   Join Date: Feb 2001 Location: earth How to measure open loop freq response For amps with open loop response which are typically 100db or so, how do you go about measuring the open loop freq response? Obviously, this is a routine measurement done by the opamp manufacturers. Any thoughts? TIA, Yv
 29th July 2002, 09:04 AM #2 jan.didden   diyAudio Member     Join Date: May 2002 Location: The great city of Turnhout, BE Measure open loop etc Generally it is done by modifying temporarily the feedback loop, so that the DC feedback stays intact to keep the amp in DC balance, but making the AC feedback zero. For instance, you can split the resistor going from the output back to the input stage, and put a cap from the split point to ground. To make the measurements easier, you should also attenuate the input signal 100 times or more, otherwise you overdrive the amp with just a tiny little input signal. Details depend on your particular amp of course. Cheers, Jan Didden
 29th July 2002, 11:33 AM #3 Circlotron   diyAudio Member     Join Date: Jun 2002 Location: Melbourne, Australia Remaining signal on the (almost) virtual earth. Maybe you could hook up the opamp as an inverting gain of 1amplifier and drive it to full output swing with a swept frequency. The higher the internal gain at any particular frequency the less residual signal on the inverting input. So if the opamp has an open loop gain of 100,000 and swings 10 volts then the change in voltage between the + and - inputs will be 10/100,000 volts = 100uV. Just guessing here, I don't really know for sure. GP.
 29th July 2002, 11:38 AM #4 jan.didden   diyAudio Member     Join Date: May 2002 Location: The great city of Turnhout, BE Measure open loop etc Circlotron, You're absolutely right. I have seen it done that way. The output level was kept constant, and the input level plotted vs frequency. But this will involve measuring reliably very low input levels. The method described above avoids that. But there is no reason it shouldn't work. Jan Didden
 30th July 2002, 12:15 AM #5 Nelson Pass   The one and only     Join Date: Mar 2001 If you have an Audio Precision, you can typically tap across the diff pair going to the balanced input of the AP and sweep. This avoids having to modify the circuit. If you don't have an AP, build a high impedance instrumentation amp out of op amps, and measure across the input diff pair with it.
 30th July 2002, 03:20 AM #6 yves diyAudio Member   Join Date: Feb 2001 Location: earth Jan, Your method still involves a very small signal, which is ok if the signal is not too small. Let's assume R1 is resistor from opamp output to diffpair inverting input and R2 is shunt resistor which is connected from the inverting input to ground thru a large value capacitor so that the low freq 3db point is well below the first open loop pole. If I understand your concept correctly, you split R1 into 2, say, R1 from output to R11 to inverting input. You connect a large cap from R1/R11 junction to ground. If the open loop gain is say, 100db, then it still means the signal has to be attenuated to very small levels. I was trying to avoid this as the noise can be not insignificant, no? NP, I don't have an AP set, does your method have the feedback network intact? If so, how does one get the open loop response - Vout / Vdiff_at_input? thanks Yv
 30th July 2002, 11:51 PM #7 Nelson Pass   The one and only     Join Date: Mar 2001 So maybe it loads the inputs with a couple pF. If your circuit is typical, it has a couple K impedance to ground on the feedback loop, so no problem. Open loop is Output voltage / diff input voltage. If you need a bandpass filter to take out any noise, DIY

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