Part III power amplifier circuits – Class ab

Class-AB with Input Buffer Transistors

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• Neglecting and
• we have;

• and;

• Class-AB with Input Buffer Transistors

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• If;

• and;

• then;

• Class-AB with Input Buffer Transistors

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• Since the voltage gain is approximately unity, the output current is;

• The current gain is;

• which is quite substantial. A large current gain is desirable since the output stage must meet the power requirements.

• Class-AB with Input Buffer Transistors

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• EXAMPLE 4

• Determine the quiescent bias currents in all transistors;
• Calculate all the currents labeled in the figure and the current gain when vI = 10 V.

• Class-AB with Input Buffer Transistors

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• EXAMPLE 4 – Solution

• (a) For vI = 0 (quiescent currents);

• Assuming all transistors are matched, the bias currents in Q3 and Q4 are also approximately 7.2 mA since the base-emitter voltages of Q1 and Q3 are equal and those of Q2 and Q4 are equal.

• Class-AB with Input Buffer Transistors

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• EXAMPLE 4 – Solution (cont’d)

• (b) For vI = 10;

• Because the voltage gain is approx. unity;

• Class-AB with Input Buffer Transistors

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• EXAMPLE 4 – Solution (cont’d)

• Class-AB with Input Buffer Transistors

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•