# Op Amp and capacitors

Discussion in 'Electronics Homework Help' started by pianolife, May 20, 2013.

1. ### pianolife

3
0
Nov 15, 2012
Dear members of Electronics point,

I am back to disturb you for a question I am stuck with.

I am asked to sketch a Bode Plot (magnitude only), for the circuit in the picture, showing the frequencies where the midband gain has fallen by 3dB.

I see that there are both a LPF and an HPF, so the first formula that comes into my mind is omega=1/RC. The problem is how to "relate" it to the op amp, can anyone help me?

All the best

File size:
21.8 KB
Views:
96
2. ### Laplace

1,252
184
Apr 4, 2010
So what is the DC gain of the amplifier? What component provides the feedback loop at DC? What is the high-frequency gain of the amplifier? What component provides the high frequency feedback loop? What components provide the feedback loop in the mid-band?

So it seems that there is a low-band and a 3 dB corner frequency coming out of the low-band, then a 20 dB/decade change in gain transitioning to the mid-band, a 3 dB corner frequency for entering the mid-band and another 3 dB corner frequency for leaving the mid-band, then a 20 dB/decade change in gain transitioning to the high-band, and finally a 3 dB corner frequency for entering the high-band.

For each of these four 3 dB corner frequencies, determine how each capacitor interacts with the resistor(s) or whether the capacitor can be considered a short or an open circuit at that corner frequency. Depending on which capacitor is shorted or open, the resistors might need to be considered as being in series or in parallel to determine the RC time constant of the corner frequency. Hint: the corner frequencies of the mid-band are the simplest to find.

3. ### duke37

5,364
769
Jan 9, 2011
I know nothing about Bode plots so cannot help with your question. I was however interested in the circuit which I had not seen before.

I simulated it with 5spice and it is a band pass filter in the audio range. Simple and effective. The 3dB points can be determined from the response curve.