A 2 Watt audio amplifier made from discrete components. Design uses a classic push pull output stage. Frequency response is excellent and circuit has low distortion.
This was one of the earliest circuits that I ever designed and built, in Spring 1982. At that time I had only an analogue meter and a calculator to work with. Although not perfect, this amplifier does have a wide frequency response, low harmonic distortion about 3%, and is capable of driving an 8 ohm speaker to output levels of around 5 watts with slightly higher distortion. Any power supply in the range 12 to 18 Volts DC may be used.
The amplifier operates in Class AB mode; the single 470R preset resistor, PR1 controls the quiescent current flowing through the BD139/140 complimentary output transistors. Adjustment here, is a trade-off between low distortion and low quiescent current. Typically, under quiescent conditions, current is about 15 mA rising to 150 mA with a 50 mV input signal. The frequency response is shown below and is flat from 20Hz to 100kHz:Bode Plot
|DC Voltages Measured wrt 0V|
The BC109C and 2N3906 operate in common emitter. This alone will provide a very high open loop gain. The output BD139/140 pair operate in emitter follower, allowing the amplifier to drive low impedance speakers. The signal to noise ration is shown below:Signal to Noise Ratio:
This amplifier has a S/n ratio of 115dB at 1kHz. Overall gain is provided by the ratio of the 22k and 1k resistor. A heat sink on the BD139/140 pair is recommended but not essential, though the transistors will run "hot" to the touch.Fourier Analysis
The number of samples was set to 4096 and Fourier coefficients up to the 16th harmonic were calculated. The sum of the harmonics up to 16KHz amounted to just under 2.9% total harmonic distortion, the results are plotted below.Harmonic Distortion
The second and third harmonic are the biggest contribution to overall distortion. Choosing a different amplifier design, a different biasing scheme or more evenly match components can reduce distortion accordingly. At the time this amplifier was made, I only had use of an analogue multi-meter, so all things considered, it was not too bad an effort.
Picture of my Prototype
Finally an image of the original which has stood the test of time. The BD139,140 power transistors can be seen on the left hand side, the preset near top centre, the BC109C centre right and 2N3906 is buried under a miniature screened audio cable, centre bottom.
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