This amplifier operates in Class AB mode with up to 2 watts of output power, the single 470R preset resistor controls the quiescent current PR1 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
PR1 470R 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
This amplifier circuit is DC biased so that the emitters of the BD139 and BD140 are at approximately half supply voltage, to allow for a maximum output voltage swing. R9 and R10 provide a degree of temperature stabilization which works as follows. If the output transistors are warm, the emitter currents will increase. This causes a greater voltage drop across R9 and R10 reducing the available bias current. All four transistors are direct coupled which ensures:
This amplifier circuit is DC biased so that the emitters of the BD139 and BD140 are at approximately half supply voltage, to allow for a maximum output voltage swing. R9 and R10 provide a degree of temperature stabilization which works as follows. If the output transistors are warm, the emitter currents will increase. This causes a greater voltage drop across R9 and R10 reducing the available bias current. All four transistors are direct coupled which ensures:
- A good low frequency response
- Temperature and bias change stability.
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