- Many op-amp circuits use a closed-loop configuration,
which uses external circuitry to route some portion
of the output signal around to feed it back
into the op-amp as one of the inputs,
creating a feedback loop.
That feedback loop enables the op-amp
to effectively monitor its output signal
so it can keep from overcompensating for changes
to the input signal.
That helps the op-amp stabilize its behavior
to produce and maintain the desired output voltage.
I use a similar form of feedback to stabilize myself
when I'm riding a bike.
Imagine for a moment that I'm an op-amp.
One of my input are the map directions
that I'm getting from my phone
and the other input is looking ahead at the road
to see where I'm going.
My output, based on those inputs, is to turn my handlebars
to the left or to the right.
Changing my output causes my bike to turn,
and by looking ahead to see where I'm going,
I can see if I've turned too much or too little,
so I can adjust my output again to compensate.
Right now, my phone is telling me to go straight,
so I'll set my bike in that direction and keep on going.
But I'm not just going to set and forget my bike.
Remember, I'm using closed-loop feedback here,
so I'm going to continuously watch the road ahead
to see if I drift to the left or the right,
so I can compensate by moving back and forth.
- [Female] Turn right.
- Now my phone is telling me to turn right,
so I'll turn my handlebars, look ahead to see where I going,
and when I see that I've turned far enough
in that direction, I'll adjust my output.
Now sometimes when I'm making really sharp turns,
I might overshoot where I want to go a little.
But because I have that feedback,
I can make those small adjustments I need to,
to stay right on course.
But what happens if I don't have that feedback?
What if I'm only paying attention to my cell phone?
- [Female] Turn left.
- Well, now it's telling me to turn left
and so I'll respond to that input
by turning my handlebars to the left.
But I'm not looking where I'm going,
I'm only paying attention to the phone.
This is what's known as an open-loop scenario,
because I've broken that feedback loop I get
by looking ahead to see where I'm going.
I'm going to keep my output in this direction
until the phone tells me to do something else,
so I might be going in circles for a while.
When an operational amplifier is used
with external feedback, the op-amp will attempt to change
its output voltage in whatever way is necessary
to make the difference between the two input terminals zero.
This rule, along with the rule that op-amp inputs
don't draw any current,
are the two golden rules of operational amplifiers.
I can represent that process of riding my bike
as an op-amp circuit by pretending like the instructions
I get from my phone's GPS,
telling me to turn to the left or to the right,
are an input to the non-inverting op-amp terminal.
The output from the op-amp is the position of my handlebars
to turn the bike to the left or to the right.
And the feedback loop is me observing how much I'm turning,
which gets fed back into the input terminal.
If my GPS says to turn slightly to the right,
and the op-amp's output
is already turning slightly to the right,
the feedback loop will see that the input and output signals
are the same, so the difference between the value
at the inverting and non-inverting input terminals is zero.
The output will remain stable there
because the circuit is in equilibrium.
If the GPS says to take a hard left,
now there will be a difference
between the two input terminals
because the output state is still turning slightly
to the right.
The op-amp sees that difference,
so it will adjust it's output towards the left
to reduce the difference.
If for some reason the op-amp overshoots
when it's making the change
and turns the output too far to the left,
or if it undershoots and doesn't turn far enough,
the op-amp will see those differences
and adjust the output accordingly
until the two input signals match.
This type of feedback is called negative feedback
because some portion of the output signal
is being fed back
into the inverting, or negative, input terminal.
So that feedback value is getting subtracted
from the non-inverting input signal.
There are a lot of op-amp configurations
that use negative feedback
because it enables the circuit to achieve stability quickly.
I'll show you several examples
of circuits using negative feedback later in this course.
When the feedback signal is routed back
to the non-inverting input terminal,
that creates positive feedback.
With positive feedback,
the output tends to quickly go to one of two states,
saturating at the positive supply rail value,
or the negative supply rail value.
Positive feedback can be useful in certain circumstances,
but it's a lot less common to use the negative feedback,
so I won't be focusing on positive feedback in this course.
Looking back at the bicycle analogy,
when I remove the feedback loop
to use the op-amp in the open-loop configuration,
its output signal is completely separated from the input.
This is similar to when I don't pay any attention
to where my bike is going
or how much I've turned the handlebars.
The op-amp has no idea how the output and input signals
relate to each other,
or whether the output is even the correct value.
The op-amp will simply look at whatever signal
it's given as input, and then blindly and faithfully
try to set to output accordingly.
When my GPS tells me to turn right,
I'll turn the handlebars to the right.
But I can only trust that I've turned them
to the correct position to get me where I want to go,
because I don't have any feedback.
Due to the op-amp's incredibly high open loop gain,
I'll usually end up turning the handlebars way too far
to the right, until they saturate
because they've turned as far as they can go.
It's straightforward to put this analogy relating feedback
to riding a bicycle in terms of electrical signals.
The instructions I receive from the GPS,
telling me to turn to the left or to the right,
correspond to the positive or negative input voltages.
And the output position of my handlebars,
turning it to the left or to the right,
corresponds to positive or negative output voltages.