How smart are op-amps?

Thread Starter

dsharp02

Joined Aug 8, 2015
17
I often read that op-amps will "do whatever it can (within the limits of their power supply) to make the voltage on the inverting and non-inverting inputs equal." Can an op-amp recognize if the response to its actions are having the opposite of a desired effect and reverse its response?

For example, suppose I have 2v being applied to the non-inverting input, and 1v being applied to the inverting input. The op-amp will start to ramp its output voltage in an attempt to raise the inverting input up to the voltage of the non-inverting input. Suppose that the feedback network negates the output of the op-amp so for example if the op-amp is outputting 2v that gets converted to -2v before being fed back to the inverting input.

Would the op-amp recognize this reversed response and start dropping its voltage in order to get the two outputs to be equal? Or would it just peg its output at the positive rail?

Thanks,
 

GopherT

Joined Nov 23, 2012
8,012
I often read that op-amps will "do whatever it can (within the limits of their power supply) to make the voltage on the inverting and non-inverting inputs equal." Can an op-amp recognize if the response to its actions are having the opposite of a desired effect and reverse its response?

For example, suppose I have 2v being applied to the non-inverting input, and 1v being applied to the inverting input. The op-amp will start to ramp its output voltage in an attempt to raise the inverting input up to the voltage of the non-inverting input. Suppose that the feedback network negates the output of the op-amp so for example if the op-amp is outputting 2v that gets converted to -2v before being fed back to the inverting input.

Would the op-amp recognize this reversed response and start dropping its voltage in order to get the two outputs to be equal? Or would it just peg its output at the positive rail?

Thanks,
Op amps are not SMART, they are simply amplifiers that amplify the difference between two inputs. if you add negative feedback, you can get the output to force the inverting input to match the non-inverting input. If you use positive feedback, then you can even add hysteresis. The op amp is dumb as a brick of silicon that it is made of.
 

dannyf

Joined Sep 13, 2015
2,197
What's inverting and noninverting depends on the topology.

Suppose that the feedback network negates the output of the op-amp
In this case, you have essentially swap'd the inverting / non-inverting inputs of the opamp. If the wiring is correspondingly swapped (aka negative feedback still applies), the output will track. Otherwise, you get positive feedback.
 

GopherT

Joined Nov 23, 2012
8,012
I often read that op-amps will "do whatever it can (within the limits of their power supply) to make the voltage on the inverting and non-inverting inputs equal." Can an op-amp recognize if the response to its actions are having the opposite of a desired effect and reverse its response?

For example, suppose I have 2v being applied to the non-inverting input, and 1v being applied to the inverting input. The op-amp will start to ramp its output voltage in an attempt to raise the inverting input up to the voltage of the non-inverting input. Suppose that the feedback network negates the output of the op-amp so for example if the op-amp is outputting 2v that gets converted to -2v before being fed back to the inverting input.

Would the op-amp recognize this reversed response and start dropping its voltage in order to get the two outputs to be equal? Or would it just peg its output at the positive rail?

Thanks,
it would peg it's output at the positive rail - unless you have an inverse system (aka: two wrongs make a right).
 

mcasale

Joined Jul 18, 2011
210
Someone already said op-amps are not smart. Very true.

Think of an op-amp amplifier as a feedback control system. The op-amp amplifies the error signal at its inputs (V+) - (V-) (which also includes inherent offset voltages) by a HUGE amount. The resistor networks provide feedback to one, or both, inputs to make it behave (amplifier, buffer, etc.). The op-amp is just a dumb amplifier with HUGE gain (which varies with frequency).
 

KL7AJ

Joined Nov 4, 2008
2,224
I often read that op-amps will "do whatever it can (within the limits of their power supply) to make the voltage on the inverting and non-inverting inputs equal." Can an op-amp recognize if the response to its actions are having the opposite of a desired effect and reverse its response?

For example, suppose I have 2v being applied to the non-inverting input, and 1v being applied to the inverting input. The op-amp will start to ramp its output voltage in an attempt to raise the inverting input up to the voltage of the non-inverting input. Suppose that the feedback network negates the output of the op-amp so for example if the op-amp is outputting 2v that gets converted to -2v before being fed back to the inverting input.

Would the op-amp recognize this reversed response and start dropping its voltage in order to get the two outputs to be equal? Or would it just peg its output at the positive rail?

Thanks,
Negative feedback inherently does that. No brains involved. :)
 
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