calculate load and input impedance

[Nescafe]

Hi all,

I'm new here and this is my first post.

I have play with the state variable filter recently and need assistance or help to determining the impedance of the partial circuit.

Could someone please tell me how to calculate the load and input impedance (the formula) from two schematics that I attached and what I need to calculate are:
- output load for U1a pin 1 and U1b pin 1
- input impedance for U2a pin 3, U3a pin 3, U2b pin 5, U3b pin 5
- assuming the C impedance is constant to make it easier

Thank You

 

[Miles Prower]

There's no need to calculate anything. Since these are noninverting op-amp designs, just get the Zi from the spec sheets. The Zo is pretty much going to be ~75R, and significantly smaller than the Zi. Just figure Zi= INF and Zo= 0 and you won't introduce any significant errors.

 

[steveB]

The input impedances into the noninverting input can usually be considered infinite for most modern opamps.

The load impedances at the pin 1 terminals of the first opamps are going to be approximately R1+R2||R4 , at least at low frequency and DC.

This is because the R4 goes into the inverting terminal of the other opamp, which looks like a virtual ground connection, at least when the feedback is operating and the opamp is not hitting the rails. Hence R2 and R4 both go to ground in your analysis, and are in parallel. The R1 must be added because it is in series with the parallel combination.

 

[Nescafe]

Thank You Miles Prowner & steveB