Description
This circuit is similar to the 3 Transistor Audio Amp
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
Circuit Diagram
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