Power amplifier circuits – Class AB POWER AMPLIFIERS Part III Power amplifier circuits – Class AB

POWER AMP Class-AB Qn & Qp are assumed matched transistors Small biasing voltage to eliminate dead band

POWER AMP Class-AB Various techniques are used to obtain the bias voltage VBB in class AB power amplifier circuit.

POWER AMP Class-AB – Diode Biasing 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.

POWER AMP Class-AB – Diode Biasing 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.

POWER AMP Class-AB – Diode Biasing 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.

POWER AMP Class-AB – Diode Biasing 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

POWER AMP Class-AB – Diode Biasing 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

POWER AMP Class-AB – Diode Biasing EXAMPLE 3 – Solution a) Select VCC = 12 V

POWER AMP Class-AB – Diode Biasing EXAMPLE 3 – Solution (cont’d) At the +ve peak of the output voltage;

POWER AMP Class-AB – Diode Biasing EXAMPLE 3 – Solution (cont’d) To maintain a minimum of 5 mA through the diodes; Select IBias = 20 mA

POWER AMP Class-AB – Diode Biasing EXAMPLE 3 – Solution (cont’d) b) Under quiescent condition (vI = 0), ID = 20 mA (neglecting iBn);

POWER AMP Class-AB – Diode Biasing EXAMPLE 3 – Solution (cont’d) Assuming Qn and Qp are matched transistors; Hence;

POWER AMP Class-AB – Diode Biasing EXAMPLE 3 – Solution (cont’d) c) At the peak +ve value of output voltage;

POWER AMP Class-AB – Diode Biasing EXAMPLE 3 – Solution (cont’d)

POWER AMP Class-AB – Diode Biasing EXAMPLE 3 – Solution (cont’d)

POWER AMP Class-AB – Diode Biasing EXAMPLE 3 – Solution (cont’d) Hence, when the output voltage is at its peak positive value; and;

POWER AMP Class-AB – Diode Biasing EXERCISE 1 For the same amplifier as in Example 3 above, find iCn and iCp when the output voltage is; +5 V; -5 V; at its peak negative value.

POWER AMP Class-AB with Input Buffer Transistors 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).

POWER AMP Class-AB with Input Buffer Transistors 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.

POWER AMP Class-AB with Input Buffer Transistors 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)

POWER AMP Class-AB with Input Buffer Transistors Neglecting and we have; and;

POWER AMP Class-AB with Input Buffer Transistors If; and; then;

POWER AMP Class-AB with Input Buffer Transistors 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.

POWER AMP Class-AB with Input Buffer Transistors 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.

POWER AMP Class-AB with Input Buffer Transistors 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.

POWER AMP Class-AB with Input Buffer Transistors EXAMPLE 4 – Solution (cont’d) (b) For vI = 10; Because the voltage gain is approx. unity;

POWER AMP Class-AB with Input Buffer Transistors EXAMPLE 4 – Solution (cont’d)

POWER AMP Class-AB with Input Buffer Transistors EXAMPLE 4 – Solution (cont’d) Since Q4 tends to turn off when vI increases, iB4 is negligible. Therefore;

POWER AMP Class-AB with Input Buffer Transistors EXAMPLE 4 – Solution (cont’d) The input current; The current gain;

POWER AMP Class-AB with Input Buffer Transistors EXAMPLE 4 – Solution (cont’d) If the previous expression i.e. is used, we have; The higher gain is due the fact that the base currents of Q3 and Q4 are neglected in deriving the expression.

POWER AMP Problems 8.36 through 8.38 in the text book Class-AB with Input Buffer Transistors EXERCISES Problems 8.36 through 8.38 in the text book