Semiconductor Electronics Part 2

In figure A,VA and VB are two input voltages (either 5V or 0V)
When VA=0,VB=0
Both D1 and D2 are OFF, so V0=0,
When VA=1 i.e. 5V,VB=0 i.e. 0V
D1 is ON but D2 is OFF
So, V0=1 i.e. 5V
When VA=0 i.e. 0V,VB=1 i.e. 5V
D1 is OFF and D2 is ON
So, V0=1 i.e. 5V.
Similarly, when VA=VB=1
Both D1 and D2 are ON
So, V0=1 i.e. 5V.
We get the final truth table on the basis of inputs and their corresponding output.

A	B	Y
0	0	0
0	1	1
1	0	1
1	1	1

∴ The given circuit (A) is an OR gate.
In figure B, it is a transistor in common emitter configuration which gives high output at low input and low output at high input.
Hence, this circuit behaves as NOT Gate.