Part III power amplifier circuits – Class ab

Class-AB – Diode Biasing

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• EXAMPLE 3 (cont’d)

• For both Qn and Qp;

• For D1 and D2;

• Determine;

• a) IBias and VCC;
• b) The quiescent collector current
• c) iCn and iCp when the output voltage is at its peak positive value

• Class-AB – Diode Biasing

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• EXAMPLE 3 – Solution

• a)

• Select VCC = 12 V

• Class-AB – Diode Biasing

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• At the +ve peak of the output voltage;

• EXAMPLE 3 – Solution (cont’d)

• Class-AB – Diode Biasing

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• EXAMPLE 3 – Solution (cont’d)

• To maintain a minimum of 5 mA through the diodes;

• Select IBias = 20 mA

• Class-AB – Diode Biasing

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• EXAMPLE 3 – Solution (cont’d)

• b)

• Under quiescent condition (vI = 0), ID = 20 mA (neglecting iBn);

• Class-AB – Diode Biasing

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• EXAMPLE 3 – Solution (cont’d)

• Assuming Qn and Qp are matched transistors;

• Hence;

• Class-AB – Diode Biasing

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• EXAMPLE 3 – Solution (cont’d)

• c)

• At the peak +ve value of output voltage;

• Class-AB – Diode Biasing

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• EXAMPLE 3 – Solution (cont’d)

• Class-AB – Diode Biasing

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• EXAMPLE 3 – Solution (cont’d)

• Class-AB – Diode Biasing

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• EXAMPLE 3 – Solution (cont’d)

• Hence, when the output voltage is at its peak positive value;

• and;

• Class-AB – Diode Biasing

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• EXERCISE 1

• +5 V;
• -5 V;
• at its peak negative value.

• For the same amplifier as in Example 3 above, find iCn and iCp when the output voltage is;

• Class-AB with Input Buffer Transistors

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• R1, R2 and the emitter-followers Q1 and Q2 establish the required quiescent bias.
• R3 and R4 (usually of low values) are incorporated to provide thermal stability.
• The output voltage is approximately equal to the input voltage (emitter-follower)
• Voltage gain is approximately unity (one).

• Class-AB with Input Buffer Transistors

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• When the input voltage vI increases, the base voltage of Q3 increases and the output voltage vO increases. The emitter current of Q3 increase to supply the load current iO. The base current of Q3 increases. The increase in base voltage of Q3 reduces the voltage across, and the current through R1. This means iB1 and iE1 also decrease.

• Class-AB with Input Buffer Transistors

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• Also when the input voltage vI increases, the voltage across R2 increases and iE2 and iE2 increase. The input current iI accounts for the reduction in iB1 and the increase in iB2 i.e.

• (Kirchhoff’s Current Law)

•