Created at: 2025-11-30
Note: In electronics positive feedback is usually a "bad thing" and negative feedback is a "good thing" because for most applications it's desirable to control the gain (through negative feedback) rather than to drive it to extremes (saturation - positive feedback).
In this type of connection, the op amp output will be connected back to the inverting terminal.
The output voltage is "sent back" to the negative terminal, so that means that the original equation:
Vout = A*(V+ - V-)
Now becomes
Vout = A*(V+ - fVout)
Where f = a fraction of what Vout is sending back.
Because A is a very large number:
Vout/A = V+ - fVout
Therefore
0 = V+ - fVout
In practical terms, it means that if the op amp sense any voltage difference between its inputs, it will respond by feeding back as much current/voltage through the feedback network as is necessary to keep the input difference equal to zero! This is only valid for negative feedback.
Negative feedback examples are:
In this case the output sends back a response to the positive terminal (non-inverting terminal):
Vout = A*(V+ - V-)
Becomes:
Vout = A*(fVout - V-)
Where f = a fraction of what Vout is sending back
Because the fVout component is positive, it will drive the op amp into saturation mode, i.e: fVout augments the Vout output, which augments the fVout componend, so on so forth.