VOLTAGE COMPARATOR WITH HYSTERESIS SUITABLE FOR LOW, SUPPLY REFERENCED THRESHOLDS

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Motorola

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Abstract

The single supply voltage comparator circuit shown in Figure 1 is capable of operating with dual power supplies. However, whether single or dual supplies are used, it is well-suited for applications in which the reference voltage is near that of the most negative supply voltage. It can be easily adapted for applications which require the reference to be near that of the most positive supply voltage. The circuit is suitable for low power applications because the required reference voltage can be established with small bias currents and large resistances. Since it provides hysteresis, this comparator can operate reliably in noisy environments.

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MOTOROLA INC. Technical Developments Volume ~12 April 1991

VOLTAGE COMPARATOR WITH HYSTERESiS SUITABLE FOR LOW, SUPPLY REFERENCED THRESHOLDS

by Gordon Allen and David Harnishfeger ~

  The single supply voltage comparator circuit shown in Figure 1 is capable of operating with dual power supplies. However, whether single or dual supplies are used, it is well-suited for applications in which the reference voltage is near that of the most negative supply voltage. It can be easily adapted for applications which require the reference to be near that of the most positive supply voltage. The circuit is suitable for low power applications because the required reference voltage can be established with small bias currents and large resistances. Since it provides hysteresis, this comparator can operate reliably in noisy environments.

  The circuit is designed so that a low logic state applied to the input will generate a voltage at the emitter of Ql which is less than that of the emitter of Q2. When this occurs, Q4 turns off while Q3A and Q3B share the current supplied from 13. Since the current in Q3B is mirrored by Q.5 and Q6, and since Q4 is off, a low voltage is produced at the output.

When the input voltage rises, the voltage at the emitter of Ql will also rise. If Ql and Q2 are matched

transistors and 1111 = 1121, ~then as the emitter voltage of Ql is raised to that of Q'& the voltages at the bases of Q3A and Q3B and Q4 &ill tend to be equal. Further increases in the i...