Bipolar Trigger Circuits

Standard Schmitt Trigger

The most simple two transistor design. But transistors saturate and parasitc transistors have to be considdered if integrated monolithically!

Basic Idea:

The circuit has two trip points. The circuit is simple but accuracy depends on VDD and temperature (because Vbe is temperature dependent).

Rising input voltage:
Q1 is off. Q2 is on and saturated. Parasitc PNP transistor Q4 drains most of the current flowing through R1. To turn off Q1 the current trough Q2 and the current trough Q1 must flow viar R3. The voltage drop at R3 roughly calculates as:

V3=R3*VDD / (R2+R3*R2/(R2+R3)

To turn on Q1 a trip point of:

Vin_trip = Vbe+V3

must be reached. Be aware that at the trip point parasitic transistor Q3 turns on and the circuit get a low input impedance!

Falling input voltage:
Q1 turns off when the input voltage falls below:

Vin_release = Vbe + VDD*R3 / (R1+R3)

Improved Schmitt Trigger

In most ICs a reference voltage is available. This reference can be used to make the trip points of the schmitt trigger less sensitive to supply voltage changes and Vbe temperature dependence.

At the trip point the current of both collectors of Q7 flows via R1. The current the PNP sources provide is derived from the reference voltage, Vbe of Q5 and R2. So the upper trip point calulates as:

Vtrip = (Vref-Vbe)*R1 / R2 + Vbe

Choosing R1 = R2 the trip point becomes:

Vtrip=Vref

The lower trip point is reached when Q1 can not carry half of the reference current (Vref-Vbe)/R2 anymore. So the release point is:

Vrelease=(Vref-Vbe)*R1 / (2*R2) + Vbe

This lower trip point or release voltage has a Vbe dependence. Usually either the upper or the lower threshold can be made tracking the reference.