Schmitt Trigger using Op-amp

Schmitt Trigger using Op-amp

In a basic comparator, a feedback is not used and the op-amp is used in the open loop mode.

As open loop gain of op-amp is large, very small noise voltages also can cause triggering of the comparator, to change its state.

Such a false triggering may cause lot of problems in the applications of comparator as zero crossing detector.

The comparator circuit used to avoid such unwanted triggering is called regenerative comparator or Schmitt trigger, which basically uses a positive feedback.

1. Inverting Schmitt Trigger

The Fig. 3.28.1 shows the basic Schmitt trigger circuit.

The inverting mode produces opposite polarity output. This is fed back to the non-inverting input which is of same polarity as that of output. This ensures positive feedback.

When V_in is slightly positive than Vrefz the output gets driven into negative saturation at - V_sat level.

When V_in becomes more negative than -V_ref, then output gets driven into positive saturation at + V_sat level.

Thus output voltage is always at + V_sat or - V_sat but the voltage at which it changes its state now can be controlled by the resistance R₁ and R₂. Thus Vref can be obtained as per the requirement.

Now R₁ and R₂ forms a potential divider and we can write,

+ V_ref is for positive saturation when V_o = +V_sat and is called upper threshold voltage denoted as V_UT. 

- V_ref is for negative saturation when V_o = - V_sat and is called lower threshold voltage denoted as V_LT.

The values of these threshold voltage levels can be determined and adjusted by selecting proper values of R₁ and R₂.

The output voltage remains in a given state until the input voltage exceeds the threshold voltage level either positive or negative.

The Fig. 3.28.2 shows the graph of output voltage against input voltage. This is called transfer characteristics of Schmitt trigger.

The graph indicates that once once the output changes its state, it remains there indefinitely until the input voltage crosses any of the threshold voltage levels. This is called hysteresis of Schmitt trigger. The hysteresis is also called dead band or dead zone.

The difference between V_UT and V_LT is called width of the hysteresis denoted as H.

The Schmitt trigger eliminates the effect of noise voltage.

The noise voltages less than the hysteresis H, cannot cause triggering.

For positive V_in greater than V_UT the output becomes -V_sat and for negative Vin less than V_LT, the output becomes + V_sat this is called inverting Schmitt trigger.

The waveforms are shown in the Fig. 3.28.3.

2. Non-inverting Schmitt Trigger

The Fig. 3.28.4 shows the non-inverting Schmitt Schmitt trigger circuit. The input is applied to the non-inverting input terminal of the op-amp.

To understand the working of the circuit, let us assume that the output is positively saturated i.e. at +V_sat. This is fed back to the non-inverting input through R₁. This is a positive feedback.

Now though V_in is decreased, the output continues its positive saturation level unless and until the input becomes more negative than V_LT. At lower threshold, the output changes its state from positive saturation +V_sat to negative saturation -V_sat. It remains in negative saturation till V_in increases beyond its upper threshold level V_UT. The transfer characteristics is shown in the Fig. 3.28.5.

V_A = Voltage at point A = I_in R₂ = V_UT

As op-amp input current is zero, I_in entirely passes through R₁.

If sinusoidal input is applied to the non-inverting Schmitt trigger, the input and output waveforms can be shown as in the Fig. 3.28.6.

Example 3.28.1 For a Schmitt trigger shown in the Fig. 3.28.7, calculate threshold voltage levels and hysteresis. Assume V_sat =0-9 V_CC.

Solution :  

3. Schmitt Trigger Applications

One important application of trigger is sine to square wave converter. It can be used to eliminate comparator chatter in signal shaping and in ON/OFF control. It is a building block of relaxation oscillators.

a. Schmitt Triggers for Eliminating Comparator Chatter

Chattering can be defined as production of multiple output transition as the input signal swings through the threshold region of a comparator.

This happens due to the fact that a.c. noise is present in the practical circuits.

Fig. 3.28.8 shows input signal with a.c. noise and how comparator output chatters. Even if noise is very less, it takes a very small noise spike to cause chatter due to high comparator gains.

This problem can be solved by using trigger circuits. This is because they exhibit hysteresis.

In case of hysteresis, as soon as the input signal crosses the present threshold level once, the output changes its state and activates the other threshold level, so that the input signal must swing back to the new threshold in order to make the output of the circuit to change its state again. 

Refer Fig. 3.28.9, by making hysteresis width greater than the maximum peak amplitude of noise, a dead zone is created such that noise within this zone no longer causes multiple output transitions.

b. Schmitt Triggers in ON/OFF Controllers

In ON/OFF controller, such as temperature controller, when temperature is below setpoint heater is made ON and when it is above setpoint heater is made OFF.

Now consider that temperature is just below the set point and heater is ON.

As soon as the temperature reaches the set point, the comparator which is controlling the heater makes it OFF.

The smallest temperature drop following the heater OFF will make comparator to go in its activate state and switch on the heater. As a result, the heater will be cycled ON and OFF by the comparator at a rapid pace. This is not desirable.

Temperature usually need not be regulated to such a sharp degree. By allowing a dead band of a few degrees we can ensure a comfortable environment and reduce heater cycling significantly. This is possible by introducing hysteresis in the comparator using Schmitt trigger.

4. Comparison of Schmitt Trigger and Comparator

The various differences and similarities between Schmitt trigger and comparator are given in the following table.

Example 3.28.2 Design and draw the circuit of Schmitt trigger with V_UT - 25 mV, V_LT = -25 mV, V_in = 1 V_{p_p}, voltage swings ± 14 V. Calculate R₁ R₂ and R_OM

Solution :

Choose R₂ = 1 kΩ i.e. R₁ = 559 kΩ

The designed circuit is shown in the Fig. 3.28.10.

Review Questions

1. Explain the working of inverting Schmitt trigger. Derive the equation for the trigger points.

2. Explain the working of noninverting Schmitt trigger. Derive the equation for the trigger points.

3. Design Schmitt trigger having upper and lower threshold of 120 mV. Input to this circuit is 1 V peak to peak triangular wave cf 100 Hz. Draw the hysteresis loop.

[Ans.: V_at = ± 15 V, R₁ = kΩ, R₂ = 1 kΩ]

4. What is the difference between basic comparator and Schmitt trigger.

5. With a neat circuit diagram, explain the working of Schmitt trigger using op-amp.

May-15, 16, Marks 8