Class
B Amplifier
This
is a low power general purpose amplifier using standard
easily obtainable parts. The circuit is similar in design to my 'Basket Case'
amplifier but without the preamp and tone controls.
The
schematic diagram is shown below. It can be seen that the push pull
output transistors have absolutely no bias at all, and this is the
essence of class B operation. When there is no audio applied to the
input, there is no current flow in the output and the transistors are
completely turned off. Therefore the quiescent current is just that of
the
opamp
itself, which in the case of a TL071 is about 1.7mA, making the circuit
ideal for use with a battery or solar panel
The
disadvantage of class B is crossover distortion. Most
normal push pull amplifiers are biased in class AB mode, meaning the
output transistors are already conducting a little, so the 0.6 volts
required to turn them on has already been overcome. In this design, it
is the audio signal itself that has to turn on the
transistors, which takes time, and produces gaps in the output
waveform. But if an opamp with a high slew rate is used (meaning
fast), this distortion can be greatly reduced by incorporating the
output transistors within
its feedback
loop, effectively tracking and cancelling out errors in the signal.
It's not a perfect
solution, but the better the opamp the more effective the cancellation,
and although the TL071 is not the best opamp around, it does a pretty
good job. The
amplifier is configured as inverting with the volume control (also in
the feedback loop) setting the gain between 0 and
25. A veroboard layout is shown below
The
opamp is fitted in a socket that has pins 1, 5 and 8 cut off, as they
are not required by the TL071. Pin 7 is 'blobbed' with solder to the
track above it (where pin 8 would have been) to feed power to the left
side of the board without
having to use a wire link. Also not having pin 5 connected allows the
ground rail to run straight through. This all goes to make the
veroboard layout simpler
The
finished amplifier
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