The simple and obvious approach to bridge amplifier design is simply to use two identical amplifiers driven by the same signal, except that we invert the signal to one of them so that the voltage at one output is the negative of the other. Connecting the load between these two outputs then gives double the voltage compared to a single amplifier, and 4 times the power. Our two 20watt amplifiers now become one 80watt amplifier, though in practice it will be less because of internal amplifier impedances. The drawback is that the current in the load is also doubled, so each amplifier needs higher power and current rated output transistors to cope with this.
One interesting possibility is to use a switching arrangement so a 20 watt per channel stereo amplifier can be converted into a 80 watt mono amplifier. This gives the user an option to start with a single stereo amplifier and upgrade to a higher power later if required by buying a second identical amplifier and using them as two 80 watt channels.
A less obvious idea is to use the feedforward technique to avoid having to use two high quality amplifiers in the bridge. Only one side of the bridge needs to be of high quality, and an inferior, simpler and cheaper amplifier used for the other half. The distortion signal can be extracted from the poorer amplifier and applied in phase to the other side of the load via the high quality amplifier. The distortion can be mostly cancelled by this means. The use of a cheap integrated circuit power amplifier becomes possible for one half of the bridge.
Integrated circuit amplifiers have the great advantage of being a single component which can contain a great deal of complexity. They usually contain current limiting, and some form of thermal limiting. In a bridge amplifier it may be possible to obtain sufficient protection using an i.c. for one side only without additional protection in the higher quality side beyond a fuse to protect the speaker. One drawback I found when looking at i.c. specifications for easily available devices is a rather limited output current. They are in any case usually designed for maximum voltage output into an 8 ohm load, and even with 4 ohms the output may be limited. In the bridge application envisaged full output voltage swing is required with a 2 ohm load. This is because we must accept that loudspeakers with impedance falling to 4 ohms may be used, and with bridge operation the current is double the single amplifier value, equivalent to using a 2 ohm load.
One promising i.c. I found is the TDA2050 which works with plus and minus 25 volt supplies, and can deliver 50 watts 'music power' into a 4 ohm load. The peak current is limited to 5 amps, which is plenty for a single ended amplifier, but in a bridge, with 20 volt peak output voltage from each half, the peak voltage across the load is 40 volts, giving a requirement for 10 amps into a 4 ohm speaker impedance. There are no doubt i.c. amplifiers available capable of this current, but they are not likely to be cheap. One answer is to add external power transistors to the i.c. To take advantage of the internal protection circuits we must ensure that these transistors can only increase output current by a fixed factor, In the case of the TDA2050 we want to double the maximum current from 5 amps to 10 amps.
I leave the idea for interested designers to experiment with, but I did work out a simple example using feedforward with the TDA2050 and a pair of current booster transistors:
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