# OP AMP input V Max

Discussion in 'Homework Help' started by dwr, Oct 17, 2005.

1. ### dwr Thread Starter Member

Apr 10, 2005
14
0
I'm doing a question in my text and I think the answer in the back of the book is wrong.

The task is to find the largest input voltage without creating distortion(clipping)

There is an op-amp circuit where the input is on the non-inverting input and +15V and -15V on the repective rails.

Ri= 680ohm
Rf=4.7Kohm

Limits- Isc=25mA GBW=1Mhz, Slew Rate= 0.5v/uSec

When I calculate I get these answers... Gain =Rf/Ri+1 =7.91

since Isc and the slew rate limits are well below the limits of 28v by the rails I use them as my limit.

therefor I calculate the limit by the rails by dividing the rails max -2 dividing by the gain

so 28vp-p/7.91=3.54Vmax.

But for some reason the book says the answer is 4vp-p.

any help or confirmation of my answer is appreciated.

2. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Hi dwr,

The closed loop gain you computed for the non-inverting opamp configuration of 7.91 is correct.

The book's answer of 4 Vp-p would be more consistent with a closed loop gain of 7. It is as though the answer in the book failed to take into account the need to add 1 to the ratio of Rf/Ri when computing the gain of the non-inverting opamp stage.

It is possible that the book's answer was derived based on an inverting opamp stage. In that case the gain would be -Rf/Ri. Since the problem was to find the magnitude of the input that would result in the clipping condition then the negative sign would be of no concern.

At least that in my take on your observation.

hgmjr

3. ### dwr Thread Starter Member

Apr 10, 2005
14
0
That's what I thought as well... double checked and the input is indeed on the non-inverting input.

Thanks,

it's always good to have input to help verify something like this.