Suppose we altered our basic open-collector inverter circuit, adding a second input terminal just like

the first:

A two-input inverter circuit


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 bu®er, 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 di®erent 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 con¯guration, this "high" state was simply

"°oating." Allowing the input to °oat (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 di®erence

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 e®ect on the output. Namely, if either of the inputs are

grounded, transistor Q2 will be forced into a condition of cuto®, thus turning Q3 o® and °oating the

output (output goes "high"). The following series of illustrations shows this for three input states

(00, 01, and 10):

InputA =0

InputB =0

Output =1

InputA =0

InputB =1

Output =1

InputA =1

InputB =0

Output =1

In any case where there is a grounded ("low") input, the output is guaranteed to be °oating

("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):

InputA =1

InputB =1

Output = 0

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.

Keywords : Logic-gates, Digital, Logic, Electronic, Bolean, Algebra, Gates, 2input, TTL NAND And AND Gates
Writer : delon  |
15 Jul 2006 Sat   
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