JEE Main 2021 · 27 August · morning · Numerical answer · +4 / −0
A circuit is arranged as shown in figure. The output voltage V₀ is equal to ................... V.
Answer and solution
Answer 5
Solution
Physics sir · 4 steps
Check the diodes first: they short the base to ground, so the transistor stays off.
- Diodes conduct. Both D₁ and D₂ have their anodes at the middle node and their cathodes at ground. The middle node is fed from +5 V through R, so both diodes are forward biased.
- Base at zero. An ideal forward-biased diode has no voltage drop, so it holds the middle node at 0 V. The base, connected to this node through R, also sits at 0 V, the same as the emitter.
- Transistor off. With V_(BE) = 0 the base-emitter junction is not forward biased, so no base current flows and the collector current is zero. No current flows through the collector resistor either, so there is no drop across it.
V₀ = 5 V - I_C R = 5 V - 0 = 5 V
- So. The output sits at the full supply voltage: V₀ = 5 V.
The common slip: 0. Assuming the base gets 5 V and the transistor saturates gives 0 V, but the forward-biased diodes pull the base node down to ground.
The idea to keep. In a transistor switch, first find the base voltage. If something clamps the base to the emitter's potential, the transistor is cut off and the collector floats up to the supply.
Answer from the NTA final answer key. Solution by PrepWiser.