simulate this circuit – Schematic created using CircuitLab


How to find the voltage gain of the amplifier (Vout/Vin)?

I wish to know this in terms of Resistance numbers...if possible.

Thanks and best regards, Vishal Sapre

  • \$\begingroup\$ It's not a linear amplifier with that diode in series with the output but if you ignore that gain is 11.0454545 \$\endgroup\$ – Andy aka Oct 14 '15 at 11:48
  • \$\begingroup\$ Hi Andy, Could you please explain how you arrived at that figure. \$\endgroup\$ – Vishal Oct 14 '15 at 11:51
  • \$\begingroup\$ Nowadays anyone who can build something with an Arduino thinks they're doing "Electronics". But I agree, if you can't figure out an opamp feedback network you're not into "electronics". Let alone at a "Pro" level. \$\endgroup\$ – Bimpelrekkie Oct 14 '15 at 12:10
  • \$\begingroup\$ Thanks FakeMoustache. May be I should reframe my question. I understand that this is non-inverting combination where the gain is given as: 1 + (Rf/Ri). However in this case, there is a constant voltage (although very small about 0.045 volt) being put on the inverting pin. This means to bring the input differential to zero, the output will need to swing a little more positive than otherwise required. I want to know if this line of thinking is correct? Also in this case, what should be considered as Ri. Is it 1K or a parallel combination of 1K||220K? \$\endgroup\$ – Vishal Oct 14 '15 at 12:27
  • \$\begingroup\$ You've already accepted an answer, but the answer to your followup question/comment is: superposition. \$\endgroup\$ – Fizz Oct 14 '15 at 12:42

It's not exactly an amplifier - I reckon it's an attempt at a signal rectifier (half wave). But, ignoring the diode in the output, the gain of this non-inverting amplifier is 1 + Rf/Rx where Rf is the 10k feedback resistor and Rx is the parallel combination of R3 and R2.

For signal changes (i.e. gain analysis) the small offset produced by V1 thru R3 is null to the analysis.

Gain = 1 + 10k/(1k||220k) = 1 + 10/0.995475 = 1 + 10.04545

Note that R4 only comes into play when frequencies are very high and the non-inverting input capacitance starts to erode the input signal.

  • \$\begingroup\$ Thanks Andy. In case there is no signal at Vin (its floating). Will the negative bias (45mV) on inverting pin have any effect on the output. \$\endgroup\$ – Vishal Oct 14 '15 at 12:30
  • 1
    \$\begingroup\$ If the input is floating the output is undefined and anywhere between either power rails. Put a 1M resistor down to ground if unsure. \$\endgroup\$ – Andy aka Oct 14 '15 at 12:33

Thats one

thats two

This is the answer I think. But like it is said above, it is not an amplifier.


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