# Op amp frequency/gain control

Discussion in 'The Projects Forum' started by jetbat, Jan 5, 2010.

1. ### jetbat Thread Starter Member

Oct 21, 2009
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Hi.
I have some questions about controlling the frequency gain of an op amp. In figure 1, capacitor C1 is setting a high pass filter at 329Hz.

Question 1. Would the low frequencies from the output of the op amp have phase shift like a passive high pass filter output? Or do they remain in phase?

It seems like they should remain in phase but I am still new to this stuff.

Now what I want to do is make a variable high pass filter. In figure 2, I added a pot to bypass C1 to ground. So when the pot is turn down all the way, the capacitor is bypassed and the low frequencies are all amplified. Turned up full, the low frequencies roll off at 329Hz.

I am not sure if this will work because I can't seem to find any info on what effect a resistor parallel with the capacitor to ground would have on the circuit.

Question 2. Will a resistor parallel with C1 to ground change the way the circuit works? Would the lows still roll off at 329Hz? Or would it change the over all gain of the op amp?

Any help would be appreciated.
Thanks
Scott

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2. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
Before pursuing this further, you need to be aware that
1. The high pass corner is at 3290 Hz, not 329 Hz.
2. The minimum (low frequency) gain in the noninverting configuration (your circuit) is unity (one).
Is this what you want?

3. ### jetbat Thread Starter Member

Oct 21, 2009
14
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Oops. C1 is suppose to be .22uF.

And yes the low frequencies are at unity gain. But with the higher frequencies driven with a gain of 10, it should be the effect I am looking for. And that is roll off the low end and be able to adjust it.

4. ### Ron H AAC Fanatic!

Apr 14, 2005
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The way you proposed works, but not too well. I would do it like this.

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5. ### jetbat Thread Starter Member

Oct 21, 2009
14
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That's nice. I had not thought about doing it that way. I will give it a try. Thanks.

A couple of questions.

1. What would the total gain at the input of U3 be? 20?

2. Are the low frequencies coming from U1 in phase with those from U2? Or are they shifted to 90° like a passive high pass filter?

Thanks again
Scott

6. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
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The gain, as you can see from the Bode plot (attached), is 11 (20.83dB). Av=1+22k/2.2k=11. It is not 20 (or 22) because the wiper is, by superposition, an attenuating summing node.
The phase shift at low frequencies is actually zero degrees, because the cap, being an open circuit, contributes no phase shift, and the gain is unity. As you can see, it reaches a peak of about +56° at 100Hz when the pot wiper is all the way to the bottom.

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7. ### Ron H AAC Fanatic!

Apr 14, 2005
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An equivalent passive high pass network (without the gain) has the same low frequency phase shift characteristics.

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8. ### jetbat Thread Starter Member

Oct 21, 2009
14
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Thanks so much the help.
Scott

9. ### Ron H AAC Fanatic!

Apr 14, 2005
7,050
657
I had another thought. Don't know if you could use it. It has variable high frequency boost instead instead of variable low frequency cut.
The minimum gain is one. The high frequency gain is Rf/Rs+1.
The advantage is it only requires one op amp.

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