# Schmitt Trigger with zener diode as a feedback

#### Xufyan

Joined Aug 3, 2010
114
Hello everyone,

please look at these two circuits (attachments) , both has been solved by me but the value of Vutp and Vltp in the 2nd circuit is incorrect :/

the book says its 2.54volts and -2.54volts while my output is 1.72v and -1.72v,
what is the problem here ?

#### praondevou

Joined Jul 9, 2011
2,942
The OPAMP will try to adjust zero voltage difference between it's inputs. The difference between output and inverting input is clamped to 5.4V. The max voltage drop on R1 will therefore also be 5.4V.

You can then calculate the current through R1 and R2. The current through R2 will give you the voltage drop on R2 which is also the turning point.

The opamp output voltage is therefore not +-5.4V but the sum of the voltage drops of R1 and R2.

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• Xufyan

#### t_n_k

Joined Mar 6, 2009
5,455
If the +ve feedback factor is K, then the output voltage excursions will be given by

Vout=±(Vz+Vf)/(1-K)

Since Vout-K*Vout=(Vz+Vf)

So if K=47k/147k=0.32, Vz=4.7V & Vf=0.7V then

Vout=±5.4/0.68=±7.94V

The Vutp and Vltp values will then be ±K*7.94 or ±0.32*7.94 = ±2.54V

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#### T.Jackson

Joined Nov 22, 2011
328
The OPAMP will try to adjust zero voltage difference between it's inputs.

That's an OP AMP in a nutshell.

Bit of an overkill for a Schmitt trigger though. Assuming that we even know what one is, or even need one for that matter
.

thankyou 