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Bias

This looks much better. Let's now calculate the proper values for R1 and R2. Assume RL is an 8W loudspeaker, VBE = 0.7V and hFE = 20. So u2rms/RL = 8W => u2rms = 8W∙8Ω => urms = 8V => utop = 8V∙√2 = 11.3V => VRL,max = VE1,max = 11.3V => VB1,max = 11.3 + 0.7 = 12V => VR1 = VS - VB1 = 15 - 12 = 3V. IRL,max = VRL,max/RL = 11.3/8 = 1.4A => IE1,max = 1.4A. IB1,max = IE1,max/(hFE+1) = 1.4A/21 = 67mA = IR1 (IB»ID1). R1 = VR1/IR1 = 3V/67mA = 45Ω.

The quiescent diode current will be (2∙15V - 2∙0.7V)/(2∙45Ω) = 302mA. The power dissipation in R1 and R2 will be I2∙R = 3.6W each! Needless to say, this is not a very economical amplifier. And to make things even worse: every diode has a certain resistance. And even if this resistance is just 5Ω, it results in a 302mA∙5Ω = 1.5V voltage drop. This means that the voltage across the diodes increases from 0.7V to 2.2V! This means that VBE1 and VBE2 also increase. And if VBE increases, IC will also increase. Result: the quiescent collector current will be very large!

Of course we could replace the diodes with resistors, making sure VBE will always be 0.7V. However, diodes have a major advantage: thermal stability. Transistor and diode parameters are temperature-dependent. If you keep VBE (or VD) constant, IC (ID) will increase with temperature. And vise versa: if you keep IC constant, VBE decreases with 2mV/°C. So if you use resistors to keep VBE 0.7V, an increase in temperture will cause IC to rise. This may heat up the transistor, so IC increases even more, and so on... This is called thermal runaway. However, if you use diodes instead of resistors and attach the diodes to the transistors (to make sure they have the same temperature) there will be no thermal runaway: if the transistor heats up, the diode also heats up, reducing VBE preventing and increase of IC.

The only way to reduce the power dissipation in R1 and R2 and the voltage drop across D1 and D2, is to reduce the (quiescent) current flow in these components. And that is only possible with larger hFE values. So we need darlingtons!

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