Digital Logic Fundamentals Unit 4 – Open Collector and Other TTL Gates
An open collector gate circuit has its output connected only to the collector of a transistor, as
shown.
The output terminal is not connected to any internal source of power; therefore, as shown above,
a voltmeter indicates 0 Vdc even if the transistor is off.
In general, an open collector device requires a pull-up resistor to VCC (5 Vdc). The voltmeter
reads 5 Vdc (VCC) when the transistor is off and less than 0.5 Vdc when the transistor is on.
This circuit performs a NAND operation because the AND output is complemented (inverted) by
the open collector transistor.
The symbol for a NOT gate, or inverter, is shown. The circle at the gate output indicates a NOT
function. A logic gate that has an output complementary to the input is an inverter, or a NOT
gate.