following up my op-amp question - still confused

I get a gain of 14.90

Doesn't 1+Rf/R1 = 11 set the lowest possible gain at dc?

The lowest gain is unity gain because at low frequencies the capacitor is open and at high frequencies the capacitor is a short to ground so your gain is 1+Rf/R1.

But my calculated and the simulation are different. Does anyone know why????

Thanks

Oops, my screwup. For reason unknown I took the RC as being parallel even though it clearly said series.

Anyway,

the circuit gain =

(Ri + Rf)Cs + 1 ============= RiCs+1

so for

Ri=2000 Rf=20000 C=4e-6 f=20

the gain is 7.831

dave y.

Wow!!!!! for 20k, 2k, 4uF and 20Hz I get exactly 7.8305 for a gain (dividing my output by the input on the oscilloscope). This entire time I was trying to find out why the exact gain can't be calculated and now it looks like you solved the gain by calculationing and got exact (ideal) answers. Remember, I'm using an ideal op-amp and exact value components, I don't expect this to be true in the real world, but for purposes of understanding the math behind it, I can use ideal components to verify my math is correct.

I am not sure of your method though. It looks like you didn't use polar cordinates at all and came up with an exact gain. Why using polar cordinates (as everyone else did) does my gain not come out exact and whichever method you used it came out correct???

If this is easier in an email, you can reach me at snipped-for-privacy@aol.com

Thanks

There was nothing special, just complex arithmetic. The steps are

1. Formulate the gain as a laplace equation

(Ri + Rf)Cs + 1 =============== RiCs+1

1. Substitute jw for s, where w=2pi*Fsig

1. Find the magnitude of the complex gain

You can do this by hand if you must, but I wouldn't. You can use Matlab or an HP calculator if you have one on hand, or you can also do complex math in Excel. As far the comment about using polar coordinates, you have to convert to rectangular coordinates to do the additions required, so polar doesn't get anywhere with this problem.

Also, you don't have to do any complex arithmetic at all if you want to first convert the equation after step 2 into a magnitude term using the usual steps (multiply the bottom by its complex conjugate, etc).

dave y.