EX(N)OR opamp - an anlogue digital gate

Nowadays, digital techniques are finding their way into more and more analogue circuits. Fortunately, this does not always call for the use of special integrated circuits, as it is quite common to see opamps being used to provide the logic functions NOT, AND, NAND, OR and NOR. However, this does not (normally) apply to the logic functions EXOR and EXNOR. Nevertheless, the latter can be obtained by using LM 324 or LM 358 type opamps. These opamps have the advantage that their outputs can be driven to 0 volts without the need for a negative supply voltage. As can be seen from the circuit diagram, when both inputs A and B are grounded (= logic zero) point a will be low. As a result, resistor R5 will have no effect on the state of the inverting input of the opamp. Resistor R6, however, does affect the non- inverting input via diode D2. This causes the voltage at the non-inverting input of the opamp to be lower than that at the inverting input, leading to a low level at the output. If the a