Schmitt Trigger: Understanding Operation, UTP/LTP, and Applications

This page explains the basics of a Schmitt trigger. It compares the Upper Trip Point (UTP) and Lower Trip Point (LTP) of a Schmitt trigger and includes equations for UTP and LTP within the Schmitt trigger hysteresis curve. It also mentions popular Schmitt trigger ICs like the 74LS132 and 4093B, along with their applications.

Figure 1: Schmitt Trigger Circuit

The Schmitt trigger is fundamentally a bi-stable multivibrator where feedback is provided via a common resistor between the emitter circuits of two cascaded transistors. Typically, in a bi-stable circuit, a resistor connects the collector terminal of one transistor to the base terminal of the other. The circuit operates in two states based on the input voltage, Vi.

• Low State: If the input voltage Vi is zero, transistor Tr1 is OFF, and transistor Tr2 is in saturation mode. Here, Ve (emitter voltage) is 2V, the same as the collector voltage of Tr2. Vo (output voltage) is 2V and remains at 2V until Vi is less than 2V.
• High State: If Vi increases above 2V, Tr1 becomes forward biased and conducts, while Tr2 is cutoff. At this state, Vo is 6V. There is no voltage across R4 since the current through it is zero. This state persists even if Vi is increased further.
• Returning to Low State: To return the circuit to the “Low State”, Vi needs to be reduced. This brings Tr1 into saturation mode and Tr2 into cutoff mode.

This operation creates a hysteresis curve, as illustrated in Figure 2.

UTP vs LTP - Difference Between UTP and LTP in a Schmitt Trigger

Figure 2: Schmitt Trigger Hysteresis Curve

• The Upper Trip Point (UTP) is the value of Vi that triggers the Schmitt trigger circuit, causing Vo to jump from the “LOW” state to the “HIGH” state.
• The Lower Trip Point (LTP) is the value of Vi that causes Vo to jump from the “HIGH” state to the “LOW” state.

In Figure 2, the UTP is approximately 2V, and the LTP is approximately 1V.

• Non-inverting Schmitt trigger:

  $\frac{\Delta Vo}{\Delta Vin} = \frac{Vo_+ - Vo_-}{UTP - LTP} = \frac{R_F}{R_E}$

• Inverting Schmitt trigger:

  $\frac{\Delta Vo}{\Delta Vin} = \frac{Vo_+ - Vo_-}{UTP - LTP} = 1 + \frac{R_F}{R_E}$

Popular Schmitt Trigger ICs - TTL 74LS132, CMOS 4093B

Figure 3: Pin outs of Schmitt Trigger ICs (TTL 74LS132, CMOS 4093B)

Both the ICs are Quad two input NAND Gates.

Schmitt Trigger Applications - Rise Time Improver, Noise Remover, Switch Debouncer, Square Wave Oscillator

Figure 4: Schmitt Trigger Applications - Rise Time Improver, Noise Remover, Switch Debouncer, Square Wave Oscillator

Here are some applications of the Schmitt Trigger:

In all these applications, both inputs of the Schmitt trigger IC are joined together, so the gate functions as an inverter.

• Rise Time Improver: As shown in Figure 4a, this application converts a slowly changing input into an output with a fast rise time. It can also convert a sine wave into a square wave.
• Noise Remover: This application, depicted in Figure 4b, eliminates unwanted high-frequency spikes present in the input signal.
• Switch Debouncer: Figure 4c shows this Schmitt trigger application preventing contact bounce at the switch due to the discharging function of the capacitor C.
• Square Wave Oscillator: This application is shown in Figure 4d.