Class A Output Stage - Recap. Class A output stage is a simple linear current amplifier. It is also very inefficient, typical maximum efficiency between 10 and 20 %. Only suitable for low power applications. High power requires much better efficiency. Why is class A so inefficient ?.
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Phase, qClass B Efficiency
Average power drawn from the positive supply:
By symmetry, power drawn from +ve and –ve supplies will be the same. Total power, therefore:
To select appropriate output transistors, the maximum power dissipation must be calculated.
Just need to find the maximum value of PD to select transistors/heatsinks
E.g. VS = 15 V, RL = 100 W
Peak Output Amplitude, A [V]
PD is a quadratic function of A,
Design a class B amplifier which will deliver up to 25 W into a 4 W load.
Supply voltages must be larger than Amax so choose Vs = 15V.
But, with qJC = 1.92 °C/W
Each of the two output transistors must be able to safely dissipate up to 5.7 Watts. Using a TIP120 & TIP 125:
i.e. Either two heatsinks rated at less than 20°C/W are required or a single heatsink rated at less than 10°C/W.
Dimensions, 50mm x 50mm x 9.5mm
Accommodates two devices
Cost 60p inc VAT
Class B amplifier output