How to calculate unity gain frequency for an OPAMP in unity fb configuration? - Page 1
Forum section: electronics / projects

author: kankanas
author_rank: Contributor
timestamp: July 04, 2020, 12:43:29 pm
content: Hello,My query is related to the circuit configuration needed to check unity gain frequency of an opamp. I tried to find and verify(from datasheet) the unity gain frequency of an OPAMP by putting it in inverting configuration with a gain of 20 and then with noninverting configuration with 31 and with another gain so far in these two configurations. Mostly it validates with the datasheet.Now the query is, is it possible to measure the unity gain frequency with buffer configuration(i.e ideal unity feedback configuration)?As in the unity feedback case, the maximum gain starts itself from 0dB, remain flats up to certain frequency, and then attenuate. Which location of frequency should we consider to actually refer it as unity gain frequency?Thanks in Advance.KS

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 04, 2020, 01:33:14 pm
content: There is no "universal answer" for this and it depends on the opamp you use. Some opamps are not even unity gain stable, and you also have to consider other factors such as slew rate.When you increase frequency, initial amplitude differences are likely to be very small, and phase shifts are probably much easier to distinguish on an oscilloscope.Normally opamps are only used in frequency ranges far below this limit because the external circuitry around the opamp does not work anymore as intended. You'd be wiser to use it as a guide for "Absolute Maximum Values" and stay well below it. I.e: If you want to use a transistor to switch 24V, then use a transistor that can handle 50V.

author: magic
author_rank: Super Contributor
timestamp: July 04, 2020, 02:05:37 pm
content: I think you will find that closed loop gain is about -3dB at the unity gain frequency.Rationale:Draw a ground-centered sinewave - this is your differential input voltage, Vin+ - Vin-.Draw an equal ground-centered sinewave delayed by 90°~120° - this is your output, Vout.Draw the sum of those waveforms - this is your Vin+, the input signal which produces such closed loop output response.The ratio of Vout to Vin+ amplitude is your closed loop gain. I suppose it will be -3dB if phase delay is exactly 90°.Why? Because we know thatVin- = Voutbecause it's wired for unity gain. ThereforeVin+ = Vout + (Vin+ - Vin-)

author: kankanas
author_rank: Contributor
timestamp: July 04, 2020, 08:31:25 pm
content: Hi, Thanks for your reply.Pardon as I did not get fully the method you described.You mentioned a ground centered sinewave, that is being applied to Vin+(noninverting terminal). Due to the slew rate of OPAMP (I am assuming) the delay is associated with the output of 90 to 120 degrees. I did not understand the reason for summing the two waveforms. Can you please tell me the objective of the analysis? (My understanding is that you are proving 3dB frequency is UGF in unity feedback configuration)Thanks in advance.RegardsKS

author: kankanas
author_rank: Contributor
timestamp: July 04, 2020, 08:35:09 pm
content: Thanks for the information about the importance and limitation of UGF where we can use it.Regards,Kankana

author: magic
author_rank: Super Contributor
timestamp: July 05, 2020, 06:21:28 am
content: You should probably read / watch some video about opamp fundamentals.An opamp strictly reacts to the difference between Vin+ and Vin-, not just Vin+ or anything like that.The output isalwayslagging behind the differential input voltage (Vin+ - Vin-) to some degree dependent on frequency, because of internal compensation. This is true for every closed loop configuration imaginable, as long as the inputs and output are within limits of normal operation.That's why there is a predictable phase relationship between (Vin+ - Vin-) and Vout. Since Vout = Vin-, we can add that differential voltage to Vout and what we get is the signal Vin+.