I want to compute the voltages and currents in a circuit containing R,L,C, opamps and voltage sources. I perfectly understand how to do this for circuits containing only R,L,C and voltage sources, using mesh or nodal analysis. But as soon as a circuit contains an opamp, I'm stuck.
In the literature I found, it is explained only by example (typically the inverting and non-inverting opamp), but no general "algorithm" is given.
Can somebody please explain to me how to construct the DE system for circuits of arbitrary size which contain opamps?
Best Answer
I'm assuming if you want to construct your ode systematically, you're actually thinking about doing this on a computer -- basically writing your own simulator.
Probably the key difference between the op-amp and R, C, L, or V elements is that those devices are all one-port devices while the op-amp is a two-port. The first port is the input pins, the second port is the output pin and the ground pin.
To set up the equations, you can treat the input as a very high resistance, in the gigaohm range (better yet, pick a real op-amp and use the value from its datasheet). It's probably not a good idea to make the input resistance infinite the way we do for hand calculation, because that could lead to an unsolvable set of equations if there's no other path to ground for one of the nodes in your circuit.
The second port is just like a voltage source, and you set it up in your equations the same way, but with the extra feature that its voltage depends on two other nodes in the circuit -- the input pins. Again, you probably won't want to assume infinite gain like in hand calculation, just use a high number, maybe 10^6, or use the open loop gain value from a real part.