Schmitt Trigger


To design and study a Schmitt Trigger using Op-amp 741.


In electronics, a Schmitt trigger is a circuit with positive feedback and a loop gain greater than 1. The circuit is named a "trigger" because the output retains its value until the input changes sufficiently to trigger a change. In the non-inverting configuration, when the input is higher than a certain chosen threshold, the output is high. When the input is below a different (lower) chosen threshold, the output is low, and when the input is between the two levels, the output retains its value. This dual threshold action is called hysteresis and implies that the Schmitt trigger possesses memory and can act as a bi-stable circuit (latch or flip-flop). There is a close relation between the two kinds of circuits: a Schmitt trigger can be converted into a latch and a latch can be converted into a Schmitt trigger.

Schmitt trigger devices are typically used in signal conditioning applications to remove noise from signals used in digital circuits, and closed loop negative feedback configurations to implement bi-stable regulators, triangle/square wave generators, etc.

An operational Amplifier, often called an op-amp, is a DC-coupled high-gain electronic voltage amplifier with differential inputs ad usually a single output. Typically the output of the op-amp is controlled either by negative feedback, which largely determines the magnitude of its output Voltage gain, or by positive feedback, which facilitates regenerative gain and oscillation. High input impedance at the input terminals and low output impedance are important typical characteristics.

Ideal Op-amp: The figure below shows an example of an ideal operational amplifier. The main part in an amplifier is the dependent voltage source that increases in relation to the voltage drop across Rin, thus amplifying the voltage difference between V + and V -. Many uses have been found for Op-amp and an ideal Op-amp seeks to characterize the physical phenomena that make Op-amps useful.

Fig.1 Op-amp 741

Fig.2 Ideal Op-amp

Vs+ and Vs- are not connected to the circuit within the Op-amp because they power the dependent voltage source’s circuit. These are notable, however, because they determine the maximum voltage the dependent voltage source can output. For any input voltage the ideal Op-amp has,
a. Infinite open-loop gain
b. Infinite bandwidth
c. Infinite input impedance
d. Zero offset voltage
e. Infinite slew rate
f. Zero output impedance and
g. Zero noise