PWM (pulse-width modulation) forms the basis for a very efficient form of power amplification. The circuit in figure 1 is a so-called class D power amplifier since the actual power amplification is provided to the PWM signal, and since it is of constant amplitude. The transistors used can function between cutoff and saturation. This allows for maximum efficiency (over 90%) and is the reason for the increasing popularity of class D amplifiers as a means of amplifying any analog signal.
Circuit Description of PWM Generation Class D Power Amplifier
The circuit of figure 1 illustrates another common method of generation of PWM and also illustrates class D amplification. The T6 transistor generates a constant current to provide a linear charging rate to capacitor C2. The uni-junction transistor, T5, discharges C2 when its voltage reaches T5‘s firing voltage. At this time C2 starts to charge again. Thus, the signal applied to T7‘s base is a linear sawtooth. That sawtooth following amplification by the T7 emitter-follower in figure 1 applied to the op-amp’s inverting input.
The modulating signal or signal to be amplified is applied to its non-inverting input, which causes the op-amp to act as a comparator. When the sawtooth waveform is less than the modulating signal, the comparator’s output is high. At the instant the sawtooth waveform becomes greater than modulating signal, output goes low. The comparator (op-amp) output is therefore a PWM signal.
It is applied to a push-pull amplifier (T1, T2, T3, T4) in figure 1, which I a highly efficient switching amplifier. The output of this power amp is then applied to a low-pass LC circuit (L1, C2) that converts back to the original signal by integrating the PWM signal at the output. The output of the op-amp in figure 1 would be used to modulate a carrier in a communications system, while a simple integrating filter would be used at the receiver as the detector to convert from pulses to the original analog modulating signal.
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PARTS LIST OF PWM GENERATOR AND CLASS D POWER AMPLIFIER
| Resistor (all ¼-watt, ± 5% Carbon) |
| R1 = 470 Ω
R2 = 1 KΩ R3, R5 = 4 KΩ R4 = 8 KΩ R6 = 3 KΩ R7 = 68 Ω R8, R9 = 220 Ω VR1 = 12 KΩ |
| Capacitors |
| C1 = 0.03 µF (Ceramic Disc)
C2 = 0.005 µF (Ceramic Disc) C3 = 1 µF, 16V (Electrolytic Capacitor) C4 = 10 Pf (Ceramic Disc) C5 = 3 pF (Ceramic Disc) |
| Semiconductors |
| IC1 = SN72709 (General Purpose Operational Amplifiers)
T1 = 2N5385 (PNP Bipolar Junction Transistor) T2,T4Â = 2N1305 (germanium PNP Transistor) T3 = 2N3551 (NPN RF Transistor) T5 = 2N2106 (NPN Amplifier Transistor) T6 = 2N711Â (germanium PNP Transistor) T7 = 2N718 (Silicon NPN Transistor) |
| Miscellaneous |
| L1 = 40 µH |