The NOT gate or inverter
A single transistor can be used as a NOT gate. This is the simplest type of logic gate. Many such gates, linked together, are used in digital electronic devices like clocks, computers and counters. The gate output voltage is NOT the same as the input voltage. Fig. 45.19 shows one version of such a gate, made using a single transistor. When the input voltage value is low, there is no current into the base. The collector current is low, so the voltage across the load resistor is low. The output voltage between the transistor collector and emitter is high.
The reverse is true when a high voltage is connected to the input and a high current flows into the transistor base. This current is amplified by the transistor. The collector current is high, so the voltage across the load resistor is high. The output voltage V across the transistor is low.
Several versions of a truth table for the gate can be worked out as shown in Fig. 45.19:
(a) gives actual voltage values for inputs of 0 V and 6 V, but the gate may operate at a
different supply voltage. This is the least useful version of the truth table.
(b) uses the terms ‘high’ and ‘low’, without giving actual voltage values.
(c) uses ‘I’ and ‘0’ instead of ,high’ and ‘low’. These are easier to write, and show the binary connection more clearly. The diagram also shows the logic symbol for a NOT gate. No components are shown; there are different ways of making gates, from diodes, transistors or microchips. No power supply is shown on gate symbols, but it must always be there. Only the ‘box’ to show the type of gate, and the inputs and outputs, are drawn.