This circuit is similar to the one above but uses positive feedback to
get a little more amplitude to the speaker. I copied it from a small 5
transistor radio that uses a 25 ohm speaker. In the circuit above, the
load resistor for the driver transistor is tied directly to the +
supply. This has a disadvantage in that as the output moves positive,
the drop across the 470 ohm resistor decreases which reduces the base
current to the top NPN transistor. Thus the output cannot move all the
way to the + supply because there wouldn't be any voltage across the 470
resistor and no base current to the NPN transistor.
This circuit corrects the problem somewhat and allows a larger voltage
swing and probably more output power, but I don't know how much without
doing a lot of testing. The output still won't move more than a couple
volts using small transistors since the peak current won't be more than
100mA or so into a 25 ohm load. But it's an improvement over the other
circuit above.
In this circuit, the 1K load resistor is tied to the speaker so that as
the output moves negative, the voltage on the 1K resistor is reduced,
which aids in turning off the top NPN transistor. When the output moves
positive, the charge on the 470uF capacitor aids in turning on the top
NPN transistor.
The original circuit in the radio used a 300 ohm resistor where the 2
diodes are shown but I changed the resistor to 2 diodes so the amp would
operate on lower voltages with less distortion. The transistors shown
2n3053 and 2n2905 are just parts I used for the other circuit above and
could be smaller types. Most any small transistors can be used, but they
should be capable of 100mA or more current. A 2N3904 or 2N3906 are
probably a little small, but would work at low volume.
The 2 diodes generate a fairly constant bias voltage as the battery
drains and reduces crossover distortion. But you should take care to
insure the idle current is around 10 to 20 milliamps with no signal and
the output transistors do not get hot under load.
The circuit should work with a regular 8 ohm speaker, but the output
power may be somewhat less. To optimize the operation, select a resistor
where the 100K is shown to set the output voltage at 1/2 the supply
voltage (4.5 volts). This resistor might be anything from 50K to 700K
depending on the gain of the transistor used where the 3904 is shown.
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