Still confused about OpAmp gains

atferrari

Joined Jan 6, 2004
4,798
The image below was created by member Audioguru, maybe a decade ago. I copied it for my own reference and it was good I did.

Here you have it. Take your time to understand it.


Audioguru.gif
 

BobaMosfet

Joined Jul 1, 2009
2,123
I posted previously about getting an OpAmp to work. @Audioguru again kindly posted the circuit on the right. I built that circuit and it "kind of" works but I still have a few questions.

I changed the R47K with 5K and I changed the R15K with 1K so I am expecting a gain of -5. I put my frequency generate on a constant 1V DC. My question:

  • My second scope channel shows 9.2V why this value and not 5V
  • If I change the voltage of my power supply by 1V - the value of the second scope channel changes by about 0.4/0.6V so if I put my power supply on 20V the Scope shows an output of 10.4. whilst at 18V it shows 9.2V. I would have expected the gain to remain the same if we stay within the operating range of the OpAmp which with 1V input, a gain of 5 should be well withing the +9 / -9 range?

I'm probably missing something very obvious but can't figure out what it is.

View attachment 224770
@doubledutch1962
A better way to learn, rather than the forum is here (Dave's EEV Blog), then what we're saying will make more sense:

 

Audioguru again

Joined Oct 21, 2019
6,785
1) Messy wiring/noise: I never appreciated how much impact that had on what I was trying to do.
I have made thousands of circuits but have not used a solderless breadboard for about 50 years because the mess of wires cause wrong connections and LOTS of noise pickup. Almost every time somebody makes a thread about a circuit that does not work properly it was because of a solderless breadboard. I properly design a circuit and its parts layout then solder its parts and a few short jumper wires on a stripboard. The parallel copper strips are easily cut and have perforated holes for the parts and jumper wires to be soldered.

2) Combine (1) with the wrong type of capacitors. Changing them to Electrolytic made a huge difference
Post 17 talks about the inductance of my C2 capacitor combining that to my 5K R3 resistor, I think really messed up the Amplification ratio. So when I significantly increased the value of R4 to 500k things became a lot more obvious and I could see real (although clipped) amplification.
No. You changed the 0.1uF ceramic capacitor to a 100 times higher value (amount of capacitance) 10uF electrolytic. A 10uF ceramic capacitor would do the same except it would be microphonic.
I said the 32k ohms of reactance (AC resistance) of your 0.1uF capacitor C2 adds to the 5k input resistor making the gain 39k/(32K + 5K)= 1.054 times at 50Hz. The value of an input coupling capacitor must be simply calculated for it to pass a low frequency without loss.

Here is a graph of the low frequency loss caused by the reactance of a 0.1uF capacitor feeding a 5k resistor and with a 10uF capacitor:
 

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