Wednesday, September 24, 2014
Simple High Power Car PDM Power Amplifier Circuit
This power amplifier, designed for use in a car, delivers 10W into 4 SZ and because it uses the principles of PDM (pulse duration modulation) its efficiency is nearly 100%. Basically it is an expanded version of the regenerative PDM amplifier described in one of my revious posts.
The block diagram is shown in figure 1. An op-amp drives a schmitt trigger, the output of which is integrated and fed back to the inverting input of the op-amp. The system regulates itself so that the voltage is the same at both inputs of the op-amp. That can only happen if the pulse width (or pulse duration) is variable, otherwise the circuit tries to change the oscil- lating frequency as a method of regulation. The hear t of the PDM system (figure 2) is made up of lC2, N1 . . . N6, T1 and T3. To build this up into a power amplifier an out-of-phase con- trol signal is formed by N7 . . . N12. This is not the ideal situation as these are not part of the feedback loop and also there is some cross-over distortion because switching takes a finite time. However the quality is improved somewhat by using a symmetrical feedback loop consisting of the components around IC1. This digital amplifier operates much the same as an analogue equivalent, which would have to be much bigger. The BD 131/ 132 transistors give an output of 10W with a total harmonic distor- tion of 0.3%.
The maximum power l without clipping (10% distortion) is about 12W. If BD 241/242 transistors are used these figures are not quite l so good because the cut-off frequency (actually the 3 dB frequency) is much lower. With a total harmonic distortion of 0.3% the output is { only 8 W and the maximum power l available without clipping is 10W. The minimum input signal to the circuit is 800 mV and current con- sumption is about 1.5 A. Because of all the ’noise’ on car voltage lines the supply must be filtered. Generally this only requires a simple LC filter with a 2200 ;.¢/25 V capacitor and a 1 ml·l inductor with a low coil resistance. ln principle more than one amplifier can be used and fed from a single low-pass filter. Because it has to be mounted in a car this amplifier should be put in a sturdy case. There is a difference in size between the BD 131/132 and BD 241/242 transistors and this must be taken into account during construction. The photo shows the method of mounting heatsinks and clearly the end result is a very compact amplifier.
The block diagram is shown in figure 1. An op-amp drives a schmitt trigger, the output of which is integrated and fed back to the inverting input of the op-amp. The system regulates itself so that the voltage is the same at both inputs of the op-amp. That can only happen if the pulse width (or pulse duration) is variable, otherwise the circuit tries to change the oscil- lating frequency as a method of regulation. The hear t of the PDM system (figure 2) is made up of lC2, N1 . . . N6, T1 and T3. To build this up into a power amplifier an out-of-phase con- trol signal is formed by N7 . . . N12. This is not the ideal situation as these are not part of the feedback loop and also there is some cross-over distortion because switching takes a finite time. However the quality is improved somewhat by using a symmetrical feedback loop consisting of the components around IC1. This digital amplifier operates much the same as an analogue equivalent, which would have to be much bigger. The BD 131/ 132 transistors give an output of 10W with a total harmonic distor- tion of 0.3%.
The maximum power l without clipping (10% distortion) is about 12W. If BD 241/242 transistors are used these figures are not quite l so good because the cut-off frequency (actually the 3 dB frequency) is much lower. With a total harmonic distortion of 0.3% the output is { only 8 W and the maximum power l available without clipping is 10W. The minimum input signal to the circuit is 800 mV and current con- sumption is about 1.5 A. Because of all the ’noise’ on car voltage lines the supply must be filtered. Generally this only requires a simple LC filter with a 2200 ;.¢/25 V capacitor and a 1 ml·l inductor with a low coil resistance. ln principle more than one amplifier can be used and fed from a single low-pass filter. Because it has to be mounted in a car this amplifier should be put in a sturdy case. There is a difference in size between the BD 131/132 and BD 241/242 transistors and this must be taken into account during construction. The photo shows the method of mounting heatsinks and clearly the end result is a very compact amplifier.
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