Question

Consider two npn transistors as shown in figure. If 0 Volts corresponds to false and 5 Volts correspond to true then the output at C corresponds to :

• A. A NAND B
• B. A OR B
• C. A AND B
• D. A NOR B

Hint:

Check how the two transistors are connected between the output C and ground: here they are in parallel, and a high input at either base pulls C to 0 V.

Answer 1

The Correct Option is D

From the figure: C is connected to the 5 V supply through a resistor and to the collectors of both transistors. The collector of transistor 1 is at C; the collector of transistor 2 is joined to C by a wire that crosses the emitter wire of transistor 1 with a 'hop' (i.e. no connection). Both emitters go to ground, so the two transistors are in parallel.
If A = 0 and B = 0: both off, no current through the resistor, C = 5 V (true).
If A = 1 or B = 1 (or both): that transistor saturates and connects C to ground, C = 0 V (false).
Truth table of C: (A,B) = (0,0) gives 1; (0,1) gives 0; (1,0) gives 0; (1,1) gives 0. This is \(\overline{A+B}\).
Answer: (4) A NOR B