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It is said that for a common base configuration the base is grounded, i.e. at zero potential. But at forward biased for a npn transistor, the battery used to forward bias emitter–base junction is connected to base from its positive pole. That means, the base is at some positive potential, contradicting the fact that Base should always be grounded. Where is the error in the concept??

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When we say that the base is "grounded" in a common-base amplifier, we're talking about the AC small-signal analysis of the circuit. The key concept is that there is no signal voltage on the base terminal.

This does not mean that there's no DC bias on that terminal, if it is required for the circuit to operate. But note that the base COULD be directly connected to ground if the emitter has a negative bias (assuming NPN transistor).

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"The base is at positive potential" simply means "some other connection is at negative potential relative to the base". There is no contradiction, since there exist negative potentials relative to ground.

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Even if the base is positively dc biased with a conventional voltage divider (as for common-emitter configurations) we have a capacitor between base and ground that realizes the common (ac) base condition.

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