TTL NAND and AND gates

Posted on Feb 7, 2014

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 thoug

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h 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...

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