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Combination Logic Tutorial

Transistor Transistor Logic Tutorial

Transistor Transistor Logic Diagram

If the emitters of Tr1 are open circuit as shown, then its base/collector junction is forward biased.

Tr2 will be turned on, with its collector low and its emitter high.

Tr3 will be off and Tr4 on.

The output will be low, about 0.6 volts.

If all emitters are taken high, it is the same as them being open circuit, and the output stays low.

If one input is taken low then Tr1 behave like a normal transistor with a reverse biased base/collector junction.

The base of Tr2 is no longer forward biased by this junction, and is turned off.

Tr2 collector goes high and its emitter low.

Tr3 comes on and Tr4 goes off.

The output goes high to about 3.5 volts.

A and B and C must all be high to make the output go low (a nand gate).

Tr3 and Tr4 are in a "totem pole" configuration.

They can be considered as two switches, when one is on the other is off.

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