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Darlingtons

Assume hFE3 = hFE4 = 100. This means that the total hFE of each darlington is 2000. IRL,max = 1.4A => IE1,max = 1.4A. IB1,max = IE1,max/(hFE+1) = 1.4A/2001 = 0.7mA = IR1. VB1,max = 11.3 + 2∙0.7 = 12.7V => VR1 = VS - VB1 = 15 - 12.7 = 2.3V. R1 = VR1/IR1 = 2.3V/0.7mA = 3.3kΩ.

Let's now calculate the maximum power dissipation in T1 and T2. PT1 = VCE1∙IE1 = (VS - VRL)∙(VRL/RL) = (VS∙VRL - V2RL)/RL. PT1 reaches its maximum value if dPT1/dVRL = VS - 2∙VRL = 0 => VRL = VS/2 = 7.5V. PT1,max = (15∙7.5 - 7.52)/8 = 7W. Since T1 works only during the positive half periods, T1's (and T2's) maximum dissipation will be 3.5W.

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