The book tells that we can arrive at the conclusion by reasoning. Why can the voltage at the emitter not be -0.7 V so that both the transistors would be on?
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Because of the conditions on both bases. – Feb 11 '21 at 14:42
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Why can the voltage at the emitter not be -0.7 V so that both the transistors would be on?
If that happened then there would be 1.7 volts across the base-emitter junction of Q1 and, given that the base-emitter junction is a forward biased diode, having 1.7 volts across it (for even a fraction of a second) would destroy the transistor: -
Andy aka
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So it's more like a practical limitation? But why does it not happen? What prevents the voltage from dropping to -0.7 V and damaging the p-n junction? – Deviprasad V R Feb 11 '21 at 13:48
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I modified the picture to show that to get that 1.7 volts you'll need "plenty of amps" flowing into the emitter current source and, that is just an impossibility in any sensible or meaningful circuit @DeviprasadVR – Andy aka Feb 11 '21 at 13:50
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Oh and that's because of the exponential I v/s Vbe right? Thank you I understood it now! – Deviprasad V R Feb 11 '21 at 13:54