Class AB Power Amplifiers
- After studying this section, you should be able to:
- Understand Class AB amplifiers.
- • Complementary operation.
- • DC bias stabilisation.
- • Temperature stabilisation.
- • Mid-point adjustment.
- • crossover adjustment.
- • AC negative feedback.
- • Bootstrapping.
- Understand the need for Quasi AB amplifiers.
Fig 5.5.1 Class AB Bias
Class AB Power Amplifiers
The class AB push-pull output circuit is slightly less efficient than class B because it uses a small quiescent current flowing, to bias the transistors just above cut off as shown in Fig. 5.5.1, but the crossover distortion created by the non-linear section of the transistor’s input characteristic curve, near to cut off in class B is overcome. In class AB each of the push-pull transistors is conducting for slightly more than the half cycle of conduction in class B, but much less than the full cycle of conduction of class A.
As each cycle of the waveform crosses zero volts, both transistors are conducting momentarily and the bend in the characteristic of each one cancels out.
Another advantage of class AB is that, using a complementary matched pair of transistors in emitter follower mode, also gives cheaper construction. No phase splitter circuit is needed, as the opposite polarity of the NPN and PNP pair means that each transistor will conduct on opposite half cycles of the waveform. The low output impedance provided by the emitter follower connection also eliminates the need for an impedance matching output transformer.
Matching of current gain and temperature characteristics of complementary (NPN/PNP) transistors however, is more difficult than with just the single transistor type as used in class B operation. Also with no emitter resistors, due to the use of emitter follower mode, temperature stability is more difficult to maintain. Class AB therefore, can have a greater tendency towards thermal runaway.
Fig 5.5.2 Applying Class AB Bias
Fig 5.5.2 illustrates the method of applying the class AB bias to a complementary pair of transistors. The two resistors R1 and R2 apply voltages to the output transistor bases so that Trl (NPN) base is about 0.6V more positive than its emitter, and Tr2 (PNP) base is about 0.6V more negative than its emitter, which is at half of VCC.
To overcome crossover distortion, the bias on the base of each transistor needs to be accurately set so that the transistors will begin to conduct as soon as their respective half cycle begins, it is therefore common for R2 to be made adjustable.
Class AB Complementary Push Pull Output Stage
The output circuit shown in Fig 5.5.3 includes many of the features and techniques described throughout Amplifier Modules 1 to 5. It shows a class AB output stage (Tr2 and Tr3) and a class A audio driver (voltage amplifier) Trl. The circuit features both AC negative feedback to reduce distortion and noise, and widen bandwidth, as well as DC negative feedback to stabilise the DC biasing. There is also some positive feedback, ‘Bootstrapping’ applied to increase input impedance and improve efficiency. Other essential features include the use of diodes to provide thermal stability, and some bias adjustments to give minimum distortion.
Fig 5.5.3 Class AB Output Stage
Tr1 driver transistor is a class A voltage amplifier fed with a variable amplitude audio signal from the input via the volume control VR1. Bias for Tr1 is provided via the potential divider R2, Vr2 & R3 from the junction of Tr2 & Tr3 emitters, which will be at half of the supply voltage.
DC Bias Stabilisation
Biasing for Tr2 and Tr3 is provided by the current flowing through the loudspeaker (which is also the output load for the amplifier), R5 and VR3. This provides an appropriate base current on Tr2 and Tr3 to make the emitters of Tr2 and Tr3 (the mid-point), half of the supply voltage. Because the base bias for Tr1 (via R2, VR2 and R3) is taken from the emitters of Tr2 and Tr3, if the voltage at the mid-point increases, the bias on Tr1 base will also increase, causing Tr1 to conduct more heavily. The collector voltage on Tr1 would therefore fall, also causing the voltages on the bases of Tr2 and Tr3 to fall. As Tr2 is NPN and Tr3 is PNP this would tend to turn Tr2 off and Tr3 on, reducing the voltage at the mid-point until it returns to its correct value of half supply.
If the voltage at the mid-point falls too far, this will result in a lowering of the bias voltage on Tr1, turning it off and increasing its collector voltage, and also the base voltages of Tr2 and Tr3. This action will increase the conduction in (NPN transistor) Tr2 and decrease conduction in (PNP transistor) Tr3, raising the mid-point to its correct voltage once more.
Dl & D2 are two silicon diodes having a similar junction potential as Tr2 and Tr3. They are connected across the output transistors base/emitter junctions to improve thermal stability. As Tr2 and Tr3 warm up, their base/emitter junction potentials will naturally fall. This would lead to over biasing and more current flow in the transistors, leading eventually to thermal runaway. D1 & D2 are usually mounted on the same heat-sinks as the output transistors. Therefore as Tr2 and Tr3 heat up, so will D1 and D2.
The junction potential of the diodes also falls, and they begin to conduct. Because the voltage between the two output transistor bases is set by VR1 to 1.2V under cold conditions, Dl and D2 are initially just cut off. However if these diode junction potentials fall due to heating, they will begin to conduct and reduce to voltage between Tr1 and Tr2 bases. This will therefore reduce the bias on the output transistors and so maintain correct class AB bias conditions.
It is important that the mid-point voltage is kept accurately at half supply in order to obtain the maximum peak to peak output signal without clipping either peak of the waveform. VR2 is made variable so that the mid point voltage can be accurately set. This adjustment should only be needed after manufacture or when any components have been replaced. With no signal applied, a voltmeter connected to the mid point and VR2 is adjusted for half supply voltage.
VR3 is the ‘Crossover Control’ and it is adjusted with a sine wave signal applied to the amplifier input and observed on an oscilloscope connected across the output load, to give minimum crossover distortion. VR3 would be adjusted, either during manufacture or after component replacement, so that the voltage difference between the bases of Tr2 and Tr3 is such that a small standing (quiescent) current is flowing in to the bases of both Tr2 and Tr3. The voltage across VR3 will therefore be about 0.6 x 2 = 1.2V.
Because the effects of VR2 and VR3 interact with each other the adjustments would normally need to be repeated a number of times, each time with decreasing amounts of adjustment until both are correct, with the mid point voltage at half supply and crossover distortion minimised.
In commercial equipment, the correct method for adjusting VR2 and VR3 would normally be given in the manufacturers manual and these instructions should be followed precisely. The mid point and crossover controls are preset controls and once adjusted during manufacture should not normally be re-adjusted except where components have been replaced.
AC Negative Feedback
AC negative feedback is provided by C2 to increase bandwidth and especially to reduce distortion. This is important, as it is not possible to entirely eliminate crossover distortion by careful biasing alone.
Tr2 & Tr3 are biased in class AB, and so must be biased just before cut off (i.e. with 0.6V between base and emitter). The bias resistor network for these transistors also forms the resistive load for Tr1. Therefore the value of R5 and VR3 is governed by the DC voltages required for correct base biasing of TR2 and Tr3.
To provide a high gain in the class A driver stage Tr1 the collector load should have as high a resistance as possible; this conflicts with the DC requirements for biasing Tr2 and Tr3. However the collector load resistor of Tr1 actually only needs to have a high resistance to AC signals; if a way can be found to give R5 and VR3 a high impedance at audio frequencies and yet retain an appropriate (much lower) resistance at DC the gain in the driver stage Tr1 can be increased.
To achieve this increase in gain, AC positive feedback (bootstrapping) is provided by C2, which feeds back the AC output signal to the top of R5. This AC signal is in phase with the signal on Tr2 and Tr3 bases, and positive feedback would normally cause oscillation, but this is prevented by the fact that Tr2 and Tr3 are operating in emitter follower mode and the voltage gain of an emitter follower is less than 1 (typically about 0.9).
Fig 5.5.4 Quasi Class AB
This means that whatever the amplitude of the signal voltage is on Tr1 collector, about 0.9 of this signal appears at the top of R5, so the AC voltage developed across VR3 and R5 appears to be only one tenth of the signal on Tr1 collector, therefore the (AC) value of resistance of VR3 and R5 appears to be ten times higher than it actually is, giving a x10 increase in the gain of Tr1 without any change in the DC resistance of VR3 and R5.
Quasi Class AB
Complementary output stages can be used effectively for power amplifiers, but as power increases above a few watts it becomes increasingly difficult to find PNP and NPN transistors with characteristics sufficiently closely matched to provide equal amplification of positive and negative half cycles. One solution is to use a quasi-complementary output stage as illustrated in Fig. 5.5.4. In this circuit a low power complementary pair (Tr1 and Tr2) are used to drive a pair of high power NPN output transistors (Tr3 and Tr4).