Figure 3-2. Diode OR gates
7.
Thus far, we haven't used a circuit that is really complicated enough to warrant using a symbol to
replace the whole schematic. But now let's examine a complex OR circuit that might be used in
electronic equipment, the basic transistorized OR gate.
8.
Part A, Figure 3-2, shows a single positive OR circuit schematic. By itself it may look very
complicated, but imagine having as many as 20 or more of these, plus other required circuits, on a
schematic! The advantage of using a simple symbol to represent each becomes obvious. Each OR
circuit performs the same function on the inputs. Once we determine how one circuit works, we can
substitute a small symbol for the entire circuit. The advantage: much smaller and less complicated
diagrams.
a. You're probably wondering why we bother using something so complicated to do the same
thing as the simple diode OR gate circuit. Well, there are other reasons but the primary reason is to get
voltage. Using the diode CR gate, we get only what we put into it, or even slightly less.
b. For example, if the input signal to the diode OR gate is +3.5 volts, the output will be +3.5
volts or less. However, by using the transistorized OR gate of Figure 3-3, we can also get amplification.
The same 3.5-volt input can produce a required 6-volt output. The advantage: a constant amplitude
output signal even with a weakened input signal.
9.
Using 0 and -6 volts as the applied signal voltages, let's first check out the circuit operation with
no active inputs: binary 0 (C volts) is applied to both inputs A and B.
a. There is no current through R1 and the base of Q1 is at ground (0 volts).
b. Q1 is an NPN transistor. With 0 volts on its base and emitter, Q1 is cut off (any current
under these conditions is so negligible we'll call it cutoff). At cutoff, Q1 has no effect on the voltage
divider R2-R3.
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