Class B Amplifier Operation

Class B Amplifier OperationPagaduan, Jolina Z.

Class B Operation is provided when the dc bias leaves the transistor biased just off, the transistor turning on when the ac signal is applied.>> essentially no bias>> transistor conducts current for only one-half of the signal cycle

Block Representation of Push-Pull Operation

Input (DC) Powerwhere Idc - the average or dc current drawn from the power supplies

(a) using two voltage supplies (b) using one voltage supply Connection of push-pull amplifier to load

where I(p) - the peak value of the output current waveform

Output (AC) Power

If one is using an rms meter to measure the voltage across the loadIf one is using an oscilloscope, the peak, or peak-to-peakThe larger the rms or peak output voltage, the larger the power delivered to the load.

Efficiency

the larger the peak voltage, the higher the circuit efficiency, up to a maximum value when VL(p) VCC

Power Dissipated by Output Transistor

where P2Q - the power dissipated by the two output power transistors

Maximum Power Considerations

maximum output power is delivered to the load when VL(p) = VCCcorresponding peak ac currentmaximum value of average currentmaximum value of input power

maximum circuit efficiencymaximum transistor power dissipation

ExampleFor a class B amplifier providing a 20-V peak signal to a 16- load (speaker) and a power supply of VCC = 30 V, determine the input power, output power, and circuit efficiency.

For a class B amplifier using a supply of VCC = 30 V and driving a load of 16 , determine the maximum input power, output power, and transistor dissipation.