Unknown symbol - Need help identifying a symbol on the 7400 series TTL chip

crutschow

Joined Mar 14, 2008
25,130
That't the input to the TTL circuit, which consists of a multiple emitter NPN transistor.
It acts to perform the NAND function (output is low only when both inputs are high).

When one or both inputs are low the current from the transistor's base resistor goes out the emitter(s) to ground.
Thus the middle transistor is off and the output is high, giving the desired NAND response.

When both inputs are high the current through the base is now diverted through the base collector junction to the middle NPN transistor, turning it on.
The causes the output to go low, again giving the desired NAND response.
 
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