Push Pull Amplifier Bias Calculator
This calculator is for designing a complementary-symmetry push-pull diode-compensated class-B amplifier. The diodes D1 and D2 are identical and suitable types would be from the 1N4001 to 1N4007 series. The default example values shown are for a typical audio amplifier with load resistance of 8 Ω. The power supply is 9 V, and we want ±3 V swing across the output load. The circuit has to operate at the low frequency 3 dB point of 50 Hz. You find the exact beta and VBE parameters from your transistor documentation sheet. In this example, the default value for beta is 100, and VBE is 0.7 V. This calculator will compute the value of the bias resistors R1 and R2, which are identical, and required in the potential divider network. It will also calculate the proper value of the output capacitor C1, the power output of the audio expected, including the power rating of each transistor required.
Circuit Parameters
Basic Biasing Theory
Biasing a push-pull amplifier involves using a potential divider network with diodes. Diode characteristics closely match transistor transconductance characteristics, thereby making them ideal for use here. The voltage drop across D1 is equal to VBE of the NPN transistor, and the voltage drop across D2 is equal to VBE of the PNP transistor.
In these types of power output circuits, we arrange the AC load line so that each transistor operates from near cut-off to near saturation. The output signal swings around Vcc/2, which is the Q point value. The NPN transistor therefore conducts from Vcc/2 to near Vcc for the positive half of the cycle, whilst simultaneously, its current swings from zero (at the Q point) to saturation value. The NPN transistor conducts from Vcc/2 to near 0 V for the negative half of the cycle, whilst its current swings from zero to saturation.
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Push Pull Amplifier DesignPush Pull Amplifier Bias Calculator