# Frequency selective operational amplifier circuit

I have also question about this question? Q1.What factors mean that the potentiometer wiper position relating to k=o.5 is not necessarily equal to the mid point when measured outside the circuit? Q2. Set the input frequency to ,firstly 25kHz and secondly 200Hz and observe the variation in output amplitude at these frequencies as you vary the position of the potentiometer. Discusssion the action of the circuit at different frequencies in relation to the frequency-dependent gain profile seen in your plots. • OK I see the question you have a question about BUT I don't see the question YOU have about the question(s) listed AND neither do I see the plots referred to in Q2 – Andy aka Apr 30 '14 at 17:37
• At first, I think it would be wise to calculate the transfer function of the circuit (assuming an IDEAL opamp as long as nothing else is specified). For this calculation, I think you can neglect the resistor R4. This part does contribute only very little to the overall function. But it is necessary to allow a stable operating point (DC feedback). – LvW Apr 30 '14 at 18:37
• Thanks for answer my question! could you answer question 1? Q1 Q1.What factors mean that the potentiometer wiper position relating to k=o.5 is not necessarily equal to the mid point when measured outside the circuit? – user41146 Apr 30 '14 at 20:22
• I think @LvW is correct, your answer comes from calculating the transfer function (-Zf/Zi). This will explicity answer your question. If you need help calculating the transfer function, that's another question :) – John D Apr 30 '14 at 20:39

## 1 Answer

If the pot is not linear (i.e. audio taper), k=0.5 will not be midpoint. However linear would be best.

However midpoint balances impedance ratio of gain so it is unity gain to nearly DC and well below audio range due to C2R4. Otherwise Pot to fixed ratio looks like 16 to 20 db boost or cut treble amp. Which is flat in the 25kHz range.

Since C is huge impedance compared to R1 and R2 in the low f range, gain is unity regardless of pot position.