I'm trying to analyze how op amps work in this circuit:
TDA7265 bridge amplifier
To be specific, I'm not sure in what mode the op amps work here and how to calculate their gains.
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\$\begingroup\$ I haven't looked, but are you saying that the datasheet doesn't provide sufficient information for you? \$\endgroup\$– jonkCommented Jan 31, 2022 at 19:50
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\$\begingroup\$ @jonk Well, explanation of how the circuit works is not included in datasheet \$\endgroup\$– SpicyHoloCommented Jan 31, 2022 at 19:58
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\$\begingroup\$ Looks to me as though they are using a simple resistor divider to feed back a certain percentage of the output to the NFB input. Do you not know how to compute the resulting gain from that? What's the open loop gain? Do they tell you that much (they had better!) Oh... and do they tell you anything about the minimum closed loop gain? \$\endgroup\$– jonkCommented Jan 31, 2022 at 20:01
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\$\begingroup\$ @jonk To be honest, I'm not sure how to calculate that, but from the datasheet minimum closed loop gain >= 25dB and open loop voltage gain is 80dB \$\endgroup\$– SpicyHoloCommented Jan 31, 2022 at 20:09
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\$\begingroup\$ Okay. That's sufficient. \$80\:\text{dB}\$ is \$A_{v_o}=10^{^\frac{80}{20}}=10000\$. The \$NFB=\frac{560\:\Omega}{560\:\Omega+36\:\text{k}\Omega}\$, so the resulting closed loop gain is \$A_{v}=\frac{A_{v_o}}{1+A_{v_o}\cdot NFB}\approx 65\$ or \$\approx 36\:\text{dB}\$. That's above the minimum, so that's to the good. You should memorize this stuff (after understanding it.) Haven't you ever looked at how NFB affects closed loop gain before? If not, I could derive it in an answer along with the above info. But I'm happy enough if these comments answer your need, too. \$\endgroup\$– jonkCommented Jan 31, 2022 at 20:29
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2 Answers
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To analyze the closed-loop gain assuming ideal op-amps you can do a few things to simplify. Ignore the supply bypassing. Ignore the Zoebel networks (R5, C7, R6, C9). Ignore the coupling capacitors and assume DC coupling with a bipolar supply (there is some biasing involved that is not shown). Ignore C8 for now.
Then you have this circuit:
simulate this circuit – Schematic created using CircuitLab
The bottom of R3 is a virtual ground so Vout of OA1 is: (1+36/1.12)\$\cdot\$ V1 = 33.1 \$\cdot\$ V1.
|Output| of OA2 is a bit less (no '+1' factor): -(36/1.12)\$\cdot\$V1 = -32.2\$\cdot\$ V1, so voltage gain is 65.3 or 36.3dB.
answered Jan 31, 2022 at 21:12
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1\$\begingroup\$ Thanks! I was just calculating It using this method. It's good to know that I actually did it correctly. \$\endgroup\$ Commented Jan 31, 2022 at 21:21
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The voltage gain of the top amplifier is (36k/560)+1= 65.3 and the same for the bottom amplifier. Since the two amplifiers are bridged then the output gain is double the single amplifier gain so the total gain is 130.6 times.
