An ordinary amplification stage (e.g. figure 8-1) is categorized as
Class A. There is a steady DC current through the transistor and, in the
extreme, this current can be varied between zero and twice the idle value. The power efficiency of such a stage is dismal: it can only reach 50% at maximum output; with smaller signals it is much lower. In ordinary
amplification we usually don't care about efficiency, but when it comes to a power output stage, class A is ill-suited.
In a Class B amplifier two output devices are used, one for the positive-going signal and one for the negative half. There is no idle current, each device starts to conduct as soon as the signal crosses the zero
threshold.
The Voltage Penalty
As we have seen in chapter 1 (figure 1-15), depletion layers take up space. The higher the operating voltage, the wider the depletion layer. Thus the diffusions not only need to be deeper, but also more widely spaced.
Just how serious is the penalty of using large voltages in an IC? Take a look at the drawing below. It compares the required areas for minimum-geometry bipolar transistors operating at 5, 20, 40 and 100 Volts:
Max. Voltage Dimensions, um Area Ratio
5 22x29 1
20 30x37 1.7 40 52x70 5.7 100 152x220 52.4
If only a small portion of the circuitry is required to withstand a high voltage, you wouldn't want all of the devices to pay the price of large dimensions. This then calls for a more complex process, one capable of producing both shallow and narrow devices and deep and wide ones.
This is an idealized concept which does not really work in practice. It is very difficult to switch from one device to the other without either leaving a gap or having both devices conduct at the same time. The result is distortion, which becomes very noticeable at low signal levels.
The solution is a compromise: allow a small idle current so that the amplifier works in a class A mode with small signals and gradually moves to class B as the signal increases. This operation is called Class AB.
Such an amplifier is shown in figure 14-29. The two output devices are Q10 and Q14. They are large, having an effective emitter length some 200 times that of a minimum geometry transistor.
Ideally we would want one of the two output devices to be a PNP
transistor, to exploit the complementary nature of the "push-pull" output. But NPN transistors carry a much higher current than PNP ones (unless a complementary process is available); with a 5.8 Watt output capability (requiring peak currents of 1.2A) this is no minor consideration.
To deliver the high output current, the upper stage (Q8, Q10) uses a Darlington configuration. Q9 serves to by-pass leakage current at high temperature.
The lower output stage has the identical Darlington connection plus a PNP transistor. The entire four-transistor block behaves like a PNP transistor. (All PNP transistors in this circuit are fairly large, capable of carrying 3mA).
There are three base-emitter junctions between the base of Q8 and the base of Q11. Between these two nodes a voltage is provided which causes a few hundred microamperes of idle current to flow through the two output transistors. This is done with the current I2 and transistors Q6 and Q7. The VBE of Q6 is increased with the resistor divider R5/R6 to the point where the desired current is reached. Notice that Q6 tracks the VBEs of Q8 and Q10 and Q7 tracks that of Q11.
Input -12V +12V R 3 10k Q9 R 6 5k R2 1K Q5 Q4 Q2 1m I2 Q12 1 0 Q6 1 0 SUB Speaker 8 Q14 200 Q10 200 Q11 R 1 29k Q7 Q8 10 Q1 Q3 500u I1 50p C1 R 5 2.7k Q13 Speaker
The feedback resistors R1/R2 set the gain at 30dB and C1 provides frequency compensation. The slowest device in the amplifier is the
compound PNP transistor Q11 to Q14, but it is fast enough to allow a more than sufficient frequency response for an audio amplifier without creating stability problems.
One significant drawback of using only NPN power devices is voltage drop. Only ±10 Volts are
available at the output from the ±12 Volt
power supply without creating distortion. At 10Vp, however, the distortion amounts to only 0.15%. The maximum efficiency of an ideal Class B amplifier is 76%. For this circuit, with its 2-Volt drop in each output device, the maximum efficiency amounts to 62%. Thus the output transistors produce 1.7 Watts of heat each (for a 5.6 Watt output).
It is often argued that, in audio applications, peak power is rarely required and so the heat sink for the amplifier can be reduced in size. Unfortunately, in a class B (or AB) amplifier, peak dissipation occurs not at peak output, but at about 50% of maximum power.
The design of figure 14-29 requires a split power supply. There
are two ways to avoid this. We could convert the -12V connection to ground, make Vcc 24 Volts, bias the input at 1/2 Vcc and couple the speaker through a capacitor. The only problem with this approach is the size of the new capacitor: 2000uF to get a 3dB drop-off at 10Hz.
Frequency / Hertz 1k 10k 100k 1M 10M Gain / dB -20 -10 0 10 20 30 Frequency/kHertz 500Hertz/div 0.5 1 1.5 2 2.5 3 3.5 4 4.5 Spectrum(Q12-C) / V 1m 10m 100m 1 10
Fig. 14-30: Frequency response of the class AB amplifier.
Fig. 14-31: Spectrum of the output signal at full power. 0 10 20 30 40 50 60 70 0 20 40 60 80 100
Figure 14-32: Power dissipation vs. power output in a class B amplifier.
A better solution is the Bridge
Output. In essence there are two
amplifiers, 180 degrees out of phase. With no input signal, both output rest at 1/2 Vcc. As the signal appears, one output moves up, the other one down.
In this configuration we have in fact doubled the output swing. With the same total supply voltage, 25 Watts of output are generated (which requires four output transistors with a capability of 2.5A each). Efficiency is unchanged at 62%, which produces a power dissipation of 15.3 Watts.