# 741 Op amp question

Discussion in 'Homework Help' started by notoriusjt2, Sep 19, 2010.

1. ### notoriusjt2 Thread Starter Member

Feb 4, 2010
209
0

can anyone point me in the right direction here? I have been looking through my textbook for the last hour looking for some kind of equation

first of all what does the +- 3 V hysteresis have to do with anything?

2. ### Ghar Active Member

Mar 8, 2010
655
72
This op amp circuit has positive feedback, no negative feedback.

An op amp amplifies the voltage difference from plus to minus inputs.
An overall positive input makes the output go positive, an overall negative input makes the output go negative.

Without negative feedback the gain of the op-amp is enormous, meaning for very small inputs the output goes as high as it can go in the appropriate direction, here it's defined as Vsat.

Knowing this, you can figure out that inputs that will make the output switch state.
The positive input terminal of the op-amp will have a voltage:

Vout * Ri/(Ri +Rf)

When the negative input, Vin, crosses this threshold the output will flip.
(i.e. the op-amp changes states when V+ = V-, referring to the input terminals.)

Hysteresis of +/- 3V means that Vin cannot change the output unless it exceeds those values.

Last edited: Sep 19, 2010