transfer functions

[jasimtariqjt]

Can anyone please tell me how to find a transfer function by looking at an op-amp circuit. I also want to know how to go backwards by looking at a transfer function and designing the op-amp circuit.

 

[crust]

Well, one way is to just look up the circuit, most of them are in a standard configuration. The other way is to write the circuit equations and solve for whatever you are interested in, lets just say Vo/Vi as an example in a non-inverting amplifier circuit.

From the perspective of the - terminal of the op-amp, no current will flow into this terminal. the + side of the opamp is at ground. With a model resistor between the two, no current will flow. Thus, the - terminal is also at ground potential (usually a tiny dc offset, but close enough to 0). So at the - node, you have a current Vi/R1 and a current Vo/R2. Now, you have everything you need to solve the transfer function. Vi/R1 + Vo/R2 = 0. When you solve it you get Vo/Vi = -R2/R1.

If you have a transfer function and want the network, you can usually either recognize the transfer function, or build the simplest circuit that represents it (since some components may have been collapsed).