Discussion
Ebers-Moll model of BJT has two coupled back to back diodes in parallel with controlled current sources.
Which of the following is/are correct for switching applications ?
(1) If two diodes are both in forward bias, then the BJT is in a low impedance state.
(2) If two diodes are both in reverse bias, then BJT is in a high impedance state.
(3) If two diodes are both in forward bias, then the BJT is in high impedance state.
(4) If two diodes are both in reverse bias, then BJT is in low impedance state.
Answer: A
The correct statements are 1 and 2.
In the Ebers-Moll model the BJT is two back-to-back diodes, base-emitter and base-collector, with controlled current sources. When both junctions are forward biased the transistor is in saturation: it conducts heavily with only a small collector-emitter voltage, a low impedance state, like a closed switch (1). When both are reverse biased it is in cut-off: almost no current flows, a high impedance state, like an open switch (2).
Statements 3 and 4 swap these states, so every option containing either of them is wrong.
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