Fig 1 gives the schematic circuit of Schmitt trigger. It forms an important bistable ator and differs from the basic Eccles-Jordon bistable multivibrator of Fig 1 in that (i) from output point C_{2} of T_{2} to the input of transistor T_{1} is missing and (ii) the feedback through resistor R_{E}. Using transistors, the circuit is basically an emitter coupled binary. It is called Schmitt trigger after the name of the inventor of the vacuum tube Version.

In this circuit, also there are two stable states. This results because of positive feedback with loop gain greater than unity. Operation Let us assume initially that the loop gain is kept less than unity (one) by using small value of resistor R_{C1}. Then there is no regeneration. Hence the circuit does not operate as a binary (flip-flop), but may be used as an amplifier with input voltage v_{i} applied at B_{1} as shown in Fig 1 and let v_{0} be the output voltage at C_{2}.

If transistor T_{2} is conducting, there result a voltage drop across R_{E}, which raises the potential of emitter of T_{1}. Hence with small input v_{i}, T_{1} is cutoff. The output at C_{2} is then given by,

……(1)

As v_{i} is progressively increased, circuit will respond only when T_{1} reaches its cutin value say V_{2}. Then T_{1} conducts. With T_{1} conducting, circuit amplifies and since is positive, output v_{o} rises in response to the rise in v_{i}. As v_{i} continues to rise, voltage at B_{2} continues to fall and the voltage at E_{2} continues to rise. Ultimately a value of v_{i} equal to V_{1} is reached when T_{2} is turned OFF. Voltage v_{o} equals Vcc and the output again becomes insensitive to changes in v_{i}. Solid curve in upper diagram of Fig 16.10 shows the variation of v_{o} with v_{i} for loop gain less than one.

Next let the loop gain be increased by increasing R_{c1}. This causes negligible change in the value of cutin voltage V_{1}. However, in the region of input voltage v_{i} where amplification occurs, voltage gain , increases and hence the slope of the curve increases. This slope continues to increase with the increase of loop gain until the loop gain becomes 1 for which value the circuit becomes regenerative and the slope becomes infinite.

The curve is then vertical as shown by the dotted line in the upper diagram of Fig 2. Finally, with loop gain greater than unity, the slope of the response curve reverses its sign i.e., v_{o} increases with decrease in v_{i} as shown in the lower diagram in Fig 2. The curve assumes the shape of letter S.

**Hysteresis: –** With loop gain more than 1, as v_{i} rises from zero volt, v_{o} remains at the lower level (V_{CC}-I_{c2}.R_{c2}) until v_{i} reaches V_{1}. When v_{i} exceeds V_{1}, a line drawn vertically cuts the response curve only at the upper level V_{CC}. Hence with v_{i} > V_{1}, the circuit makes an abrupt change to this higher level V_{CC}. Next let v_{i} be initially in excess of V_{1} and let it be gradually reduced. Then the output continues to remain at its upper level V_{CC} until v_{i}, reaches a value V_{2} shown in Fig 2. For still lower value of v_{i}, the output makes an abrupt transition to the lower level. The circuit thus exhibits hysteresis i.e. to cause a change in direction, we must first go beyond the voltage at which the reverse saturation takes place.

**Application of Schmitt Trigger**:

Schmitt trigger and conventional bistable multivibrator (Eccles-Jordon circuit) may be used for the same applications. However, Eccles Jordon circuit because of its symmetry, is preferred in applications where the circuit is required to be triggered back and forth between stable states. In schmitt trigger, the base of transistor T_{1} is not involved in the regenerative switching. Hence when the circuit switches between two stable states, the voltage of the basic terminal does not change. Hence schmitt trigger circuit is preferred wherever this free base terminal is needed. Further the resistance R_{c2} in the output circuit of T_{2} is not required for the operation of binary. Hence this resistance may be suitably selected to achieve any desired output voltage amplitude. Further capacitive loading of the output terminal C_{2} does not slow down the regenerative action.

### Important applications of Schmitt trigger circuit are as below:

- As an amplitude comparator
- As a squaring circuit
- As a flip-flop circuit