Free Class-AB Amplifier Bias & Vbe Multiplier Tool
Size Vbe multiplier transistor bias networks ($V_{bias} = V_{BE} (1 + R_1/R_2)$), eliminate crossover distortion, and calculate quiescent current ($I_q$) per Oliver criteria.
🔊 Output Stage Topology & Supply
📊 Bias Voltages & Power Dissipation
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Class-AB Audio Power Amplifier Biasing & Oliver's Criterion
In a complementary push-pull audio amplifier, pure Class-B operation results in audible crossover distortion because both NPN and PNP output transistors remain non-conductive when the audio waveform passes through zero volts ($pm 0.6 ext{V}$ dead zone).
The V_be Multiplier ("Rubber Diode") Circuit
To maintain a slight quiescent idle current ($I_q$) through the output devices, an adjustable voltage reference called a $V_{BE}$ multiplier is placed between the driver bases: $$V_{bias} = V_{BE,mult} \cdot \left( 1 + \frac{R_1}{R_2} \right)$$ Mounting this multiplier transistor directly onto the main output heatsink ensures thermal negative feedback: as the power transistors heat up, the sense transistor also heats up, dropping $V_{bias}$ at approximately $-2.0 ext{ mV}/^circ ext{C}$ per junction, preventing destructive thermal runaway.
Oliver's Optimal Biasing Rule
Distortion pioneer Douglas Self and Bernard Oliver demonstrated that there is an exact optimal voltage drop across the emitter ballasting resistors ($R_E$) that minimizes gain doubling and transition ripple: $$V_{RE,opt} \approx 26\text{ mV} \implies I_{q,opt} = \frac{26\text{ mV}}{R_E}$$ For a standard $R_E = 0.22\,\Omega$, the optimal quiescent current is approximately $118\text{ mA}$ (or $\sim 26\text{ mV}$ across both resistors combined for CFPs).
Frequently Asked Questions
Why should I never over-bias a Class-AB amplifier into high idle current?
Counter-intuitively, excessive bias does not reduce distortion; it causes "gm-doubling" distortion where both output transistors conduct simultaneously during small signals, creating higher total harmonic distortion (THD) than correctly biased optimal Class-AB operation.
Why are emitter resistors (RE) mandatory in BJT output stages?
BJT collector current increases exponentially with temperature. Emitter resistors provide local negative current feedback (degenerative feedback): as current rises, the voltage drop across RE increases, reducing the net V_BE drive voltage and preventing thermal runaway and current hogging in parallel output pairs.
What capacitor should be placed across the Vbe multiplier?
A 100nF to 1uF film or ceramic capacitor is wired directly across the collector-emitter of the multiplier transistor to bypass AC audio currents and prevent the multiplier from acting as an inductive impedance at high audio frequencies.