# Homework Help: Standard TTL 2-input NAND gate - open-collector/totem pole outputs

1. Mar 8, 2013

### s3a

1. The problem statement, all variables and given/known data
Question:
Refer to Fig. 6-5. This standard TTL 2-input NAND gate uses ______ (open-collector, totem pole) outputs.

Solution:
The 2-input TTL NAND gate in Fig. 6-5 (which is attached as "TheFigure.jpg") uses a totem pole output configuration.

2. Relevant equations
N/A

3. The attempt at a solution
I don't know what the little black triangles with a line tangent to its tip signify.

Could someone please walk me through this circuit because I understand almost nothing. I don't even understand what is meant by open-collector and totem pole outputs.

What I think I get is that if the currents for inputs A and B do not go to the ground that means that the inputs are each 1/high and then, somehow, “magic happens” and the current goes through all that stuff and an output is obtained.

Any help in fully understanding what is going on with this problem would be greatly appreciated!

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2. Mar 8, 2013

### rude man

With both inputs high, Q1 is just a forward-biased p-n junction between base & collector. So current flows thru the 4K resistor to Q2 base. Q2 turns on and in turn turns on Q4 (the combination Q2-Q4 is called a "Darlington" connection). Thus the collector of Q4 goes low. Also, Q2 collector goes low, turning off Q3. Good thing 'cause otherwise Q3 and Q4 would try to short each other out.

If either input goes low, Q1 stops sourcing base current to Q2 so its collector goes high, turning on Q3 and turning off Q4. Again, good thing they're not both on at the same time!

Take away Q3 and you have an "open-collector" output. It means what it says - there is nothing connected to Q4's collector. The presence of Q3 constitutes a "totem-pole" output, so-called because Q3 and Q4 are stacked one on top of the other tlike the faces on a totem-pole.

3. Mar 9, 2013

### CWatters

They are diodes. You can ignore them for the moment.

A totem pole output has a transistors that can pull the output pin up to VCC or down to ground.

If you remove the transistor that can pull the output to VCC then you are left with an open collector output. Usualy a resistor is added to the circuit to pull the output pin up to create a logic 1.

Totem pole..
http://en.wikipedia.org/wiki/Totem_pole_output

Open collector...
http://en.wikipedia.org/wiki/Open_collector