Schmitt

Trigger

All circuits we have discussed so far are having an op-amp in

open loop operation at its core. Since open loop gain is very high, even for a

small Vd , output will change. This means that a steady signal

hovering around (just above or below) threshold may trigger the circuit many

times due to noise. For example, in a zero-crossing detector, input sine wave

may have HF noise superimposed on it. So, during the expected zero crossing of

input, due to noise, actual amplitude may cross zero more than once. This will

trigger the circuit more than once, something that we don’t wish for as we are

interested only in the number of times

the signal cross zero. A solution to this problem is to implement a circuit

which has some kind of memory. Such a zero- crossing detector circuit will

change output only if input is sufficiently high (more than some positive

voltage, say V) while increasing and only if input is sufficiently small ( less

than a negative voltage, say -V) while decreasing. If this V is more than the

amplitude of noise (which is the usual case), only actual zero crossings by the

signal trigger the circuit.

Schmitt trigger is one such circuit which is a comparator

with hysteresis introduced due to positive (regenerative) feedback.

Inverting

Schmit trigger

Consider the basic Inverting Schmitt trigger circuit

given in the left page for explanation purpose.

Here, = ?Vout

Let the saturation levels of the op-amp be VCsat = L+ and -VCsat

= L- . When the input is largely negative, VN

Given in

Fig 4 b is a circuit which have

different magnitudes of VTH and VTL.

Here, we

use resistors R and mR for positive feedback and nR to set the desired value

for VP . Operation is straightforward as we have seen in the case of

simple circuit discussed above. Only difference is in the design procedure

which is mentioned below:

We can

write VP in terms of VCC and Vout using

superposition as follows

For getting the desired

VTH and VTL ,

replace VP with VTH ( or VTL) and Vout

with L+ ( or L- ) and solve for m and n. Choose resistor

by assuming a suitable value of R, say 3.3 k? .

Non –

Inverting Schmitt Trigger

This also similar to

the previous case but the main difference is that input is given to inverting

terminal. Consider Fig 5 a .

This is the simplest

Non Inverting Schmitt Trigger circuit. Here also, we use the same notations we

used in the inverting case. The main thing to note here is that VP

is not fixed. Using superposition, we can write VP as follows:

It should be noted that

output switches its state when VP cross zero in either direction.

Consider the input to

be largely negative. This means that

output will be at L- . When we increase Vin , VP

also strart to become less negative. At one particular input voltage, VP

cross zero. Corresponding input voltage is denoted as VTH . Once VP

goes positive, Vout becomes L+

, increasing VP further. This means that for further

rise in Vin , there is no change in Vout ( Even if Vin

is kept as 0 just after output has changed, Vout won’t trip back to

L- as VP is still positive due to feedback) .

Putting VP =

0 , Vout = L- and Vin = VTH in the

above eqn, we get

When we go in the other

direction, Vout will remain as L+ until VP

cross zero. This crossing occur at an input voltage of VTL . Once Vi

decrease below VTL , output becomes L- , which further

decrease VP which ensure output won’t change for further decrease in

Vin. ( Even if Vin is kept as 0 just after output has

changed, Vout won’t trip back to L+ as VP is

still negative due to feedback)

Using equation x , we

get

As we have seen for the

inverting case, to shift the switching levels, we use external voltage . Fig 5 b is a Non Inverting Schmitt Trigger in

which we can design threshold levels. Ra and Rb are used for setting

VN and R1 and R2 are used as feedback

resistors. Working is similar to the simple case but with different threshold

voltages. Design can be done using the following equations.

For switching, VP

should cross VN where

i.e., (Using

superposition)

Or

Substitute Vin

= VTH ( or VTL ) and Vout = L- ( or

L+ ) to obtain expressions

for threshold voltages as given below :

And

Solve for R1

, R2 ,Ra and R4

using above expressions ( Assume some parameters to get remaining )