i need to design a 2nd order low pass filter (fc=4khz)?

Question

I need the exact commercial values for R's and C's and the op-amp model.
The circuit will further be used to multiplex speech signal in a small telephone switch.

Answer 1

Just use a simple 741 in the basic inverting configuration, where

R1 = resistor from the input signal to the opamp input
R2 = resistor connecting opamp input and output

you'll choose R2 and R1 to get the overall gain you want where

Gain = R2/R1

second order means you'll have either have 2 inductors, 2 capacitors, or 1 of each.

Now compute your time constants. We'll just use the same one twice. this will give you a 40db/decade drop off at the corner frequency

tau1 = tau2 = 4000*2*pi = 25132.74

R*C = 1/tau = 1/25132.74 = 0.00003979

I'll tell you how to get this thing built as opposed to a design that works on paper. I'm assuming you just need to get this thing working and are more concerned with the multiplexers and other stuff.Also, I'm not sure if my equations are exactly correct. between

I'm gonna use capacitors. Before we worry about capacitor values, go ahead and chose your R2 and R1, and build this circuit on a breadboard. Power the op amp with +/- 9 or 12 volts. hook up your input and measure your gain to make sure you are straight. Then detach input and replace it with a short to ground.The output resistance of an op amp is, I think really low. Hook up an ohmmeter and measure it, and write that # down. well mark that as Ro. Measure the resistance across R2, write that down, as Req2.
Take the input short off, and measure between where the input goes to R1 and ground,this will be Req1. replace the short.

The first capacitor will go across the leads of (in parallel with ) R2.
Take that number above and divide it by Req2 to get C2

C2 = 0.00003979/Req2

I think the equation for Req2, which is the resistance that C2 "sees"
should be:

Req2 = R2//(Ro+R1)

All capacitors are open circuits at o hz, and shorts at infinity. Thus C2 will short out R2 as frequency goes to infinity, and make the gain, R2/R1 = 0

Now, I suggest that you put you put C1 in parallel with the input. That is between the input to R1 and ground. This way, as frequency goes to infinity, you see that the input becomes shorted, and output becomes zero.

C1 = 0.00003979/Req1

since the input to an op amp is very large, the equation for Req1 should be:

Req1 = R1 + R2 + Ro

Finally, if you have the time and know how to use Ltspice, make sure your circuit is stable. If you want to know, post another Question and I'll answer it.

Good luck!