Part III power amplifier circuits – Class ab

• Part III
• Power amplifier circuits – Class AB

• POWER AMPLIFIERS

• Class-AB

• POWER AMP

• Small biasing voltage to eliminate dead band

• Qn & Qp are assumed matched transistors

• Class-AB

• POWER AMP

• Various techniques are used to obtain the bias voltage VBB in class AB power amplifier circuit.

• Class-AB – Diode Biasing

• POWER AMP

• The base-emitter of a BJT is basically a p-n junction and hence exhibits similar characteristics as that of a diode – voltage across a diode in a forward mode is almost constant over a range of the diode current.

• Class-AB – Diode Biasing

• POWER AMP

• The above diode characteristic is utilised in biasing of the push-pull power amplifier

• In the absence of input signal (vI = 0), most of IBias flows through D1 and D2 establishing a small bias voltage VBB for the base-emitter junctions of Qn and Qp.

• Class-AB – Diode Biasing

• POWER AMP

• When vI is at its peak +ve, iBn may be large. IBias must be sufficient to supply both iBn and the current through D1 and D2 in order to maintain the bias voltage VBB.

• Class-AB – Diode Biasing

• POWER AMP

• EXAMPLE 3

• A diode biasing class AB power amplifier is to meet the following specifications;

• RL = 8 ;
• output power to load, PL = 5 W;
• peak output voltage to be not more than 80% of VCC;
• minimum value of ID to be no less than 5 mA

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