I need some help verifying the transfer function of this amplifier circuit stage. The below circuit has a gain of 20 from the result of Rf and Rg.
The problem I am having is that I have 2 transfer functions in a feedback loop. G(s) and H(s), the op amp open loop transfer function and the closed loop transfer function. When they combine in feedback loop I get a final transfer function of G(s)/(1+G(s)*H(s))
My output transfer function however seems to have gain bode plot of less than unity!
- Green curve = output transfer function
- Blue curve = G(s) op amp open loop transfer function
- Orange curve = close loop H(s)
Shouldn't the amplifier stage, well, amplify? I can clearly see that the math works out to make it less than unity gain, but how is the output voltage going to be amplified? For example: at DC the math works out to be -26dB, gain of ~1/20. Similarly, the closed loop gain is around +26dB.
When applying H(s) to G(s), in feedback it became net negative. But to apply a real voltage at the input and expect an output, let's say for 1 volt input @DC, Vi=1, Vo=ViTF -> Vo=10.05011=0.05011.
The problem here is I am missing 1/x somewhere to get the proper gain of 20. I thought the transfer function is Vo=TF*Vi not Vo=1/TF *Vi?