Suppose we altered our basic open-collector inverter
circuit, adding a second input terminal just like the first:
This schematic illustrates a real circuit, but it isn't called a
"two-input inverter." Through analysis we will discover what this circuit's
logic function is and correspondingly what it should be designated as.
Just as in the case of the inverter and buffer, the "steering" diode
cluster marked "Q1" is actually formed like a transistor, even
though it isn't used in any amplifying capacity. Unfortunately, a simple NPN
transistor structure is inadequate to simulate the three PN junctions
necessary in this diode network, so a different transistor (and symbol) is
needed. This transistor has one collector, one base, and two
emitters, and in the circuit it looks like this:
In the single-input (inverter) circuit, grounding the input resulted in
an output that assumed the "high" (1) state. In the case of the
open-collector output configuration, this "high" state was simply
"floating." Allowing the input to float (or be connected to Vcc)
resulted in the output becoming grounded, which is the "low" or 0 state.
Thus, a 1 in resulted in a 0 out, and visa-versa.
Since this circuit bears so much resemblance to the simple inverter
circuit, the only difference being a second input terminal connected in the
same way to the base of transistor Q2, we can say that each of
the inputs will have the same effect on the output. Namely, if either of the
inputs are grounded, transistor Q2 will be forced into a
condition of cutoff, thus turning Q3 off and floating the output
(output goes "high"). The following series of illustrations shows this for
three input states (00, 01, and 10):
In any case where there is a grounded ("low") input, the output is
guaranteed to be floating ("high"). Conversely, the only time the output
will ever go "low" is if transistor Q3 turns on, which means
transistor Q2 must be turned on (saturated), which means neither
input can be diverting R1 current away from the base of Q2.
The only condition that will satisfy this requirement is when both inputs
are "high" (1):
Collecting and tabulating these results into a truth table, we see that
the pattern matches that of the NAND gate:
In the earlier section on NAND gates, this type of gate was created by
taking an AND gate and increasing its complexity by adding an inverter (NOT
gate) to the output. However, when we examine this circuit, we see that the
NAND function is actually the simplest, most natural mode of operation for
this TTL design. To create an AND function using TTL circuitry, we need to
increase the complexity of this circuit by adding an inverter stage
to the output, just like we had to add an additional transistor stage to the
TTL inverter circuit to turn it into a buffer:
The truth table and equivalent gate circuit (an inverted-output NAND
gate) are shown here:
Of course, both NAND and AND gate circuits may be designed with
totem-pole output stages rather than open-collector. I am opting to show the
open-collector versions for the sake of simplicity.
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