# Quick Question: Open Loop inverting opamp

#### Braeden Hamson

Feb 18, 2016
237
I've been Googling for a little bit, I can't seem to get an answer. What is the circuit for an open loop inverting op-amp? Does it even exist, or can this only be done in a non-inverting format? If it can't be inverting Then how do I make a very high gain inverting amplifier?

Thanks

#### dorke

Jun 20, 2015
2,342
It exists,but should not be used !!!
Basically it is an amp. that doesn't have feedback (i.e. connection of output to input).
without feedback an amp. would have all possible disadvantages in the world.

It sounds strange but,
Negative feedback is actually very Positive,
a bit of an "oxymoron"

You should use a closed loop amp. and define "very high".
The way to get high gain would be to cascade lower gain amps, all with local closed loop.

Last edited:

#### Braeden Hamson

Feb 18, 2016
237
Thank you, I've seen many many versions of this circuit. I'm pretty new to these circuits, how do I determine the amplification (A)?

#### Harald Kapp

##### Moderator
Moderator
Nov 17, 2011
12,713
A tad on the expert side of explanations: http://www.analog.com/media/en/training-seminars/design-handbooks/Op-Amp-Applications/Section1.pdf
More down to earth: http://www.physics.unlv.edu/~bill/PHYS483/op-amps.pdf

Think of a good opamp as having "infinite" gain (to prevent comments on this: I'm well aware that there ain't no such thing as infinite gain, it is only a useful simplification).
A useful circuit almost always requires feedback to bring the opamp into a useful operating range.
Without feedback, a very small input voltage will drive the opamp's output in saturation.

#### dorke

Jun 20, 2015
2,342
Thank you, I've seen many many versions of this circuit. I'm pretty new to these circuits, how do I determine the amplification (A)?

If you use a commercial op.amp it is given in it's datasheet.
Given a specific circuit with all parameters of the transistors and other devices you can calculate it.

In both cases it can be measured,
taking care of "problematic issues" in the operation and measurement,
Not the place to elaborate on here.

In the case of a legacy op. amp like the LM741.
The datsheet states a minimum "Large signal voltage gain" of 25V/mV (i.e 25,000)
The typical value at 25C(room temperature) is 200,000 !

They also give it in a graph:
Note,it is given in dB ,106dB=20* log(20,000)
and it is at very very low frequencies under 10Hz(5) !

Replies
2
Views
1K
Replies
3
Views
695
Replies
3
Views
2K
Replies
4
Views
598
Replies
11
Views
928