Showing posts with label PDM. Show all posts
Showing posts with label PDM. Show all posts
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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