Figure P12.54 shows a class AB output stage utilizing a pair of complementary MOSFETs

Chapter 12, Problem 12.54

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Figure P12.54 shows a class AB output stage utilizing a pair of complementary MOSFETs (QN , QP) and employing BJTs for biasing and in the driver stage. The latter consists of complementary Darlington emitter followers formed by Q1 through Q4 and has the low output resistance necessary for driving the output MOSFETs at high speeds. Of special interest is the bias circuit utilizing two VBE multipliers formed by Q5 and Q6 and their associated resistors. Transistor Q6 is placed in direct thermal contact with the output transistors and thus has the same temperature as that of QN and QP. (a) Show that VGG is given by VGG = 1+ R3 R4 VBE6 + 1+ R1 R2 VBE5 4VBE (b) Noting thatVBE6 is thermally coupled to the output devices while the other BJTs remain at constant temperature, show that (c) To keep the overdrive voltages of QN and QP, and hence their quiescent current, constant with temperature variation, VGG/T is made equal to (VtN + VtP)/T. Find R3/R4 that provides this temperature stabilization when |Vt| changes by 3 mV/ C and VBE/T = 2 mV/ C. (d) Using the value of R3/R4 found in (c) and assuming that the nominal value of VBE is 0.7 V and that the MOSFETs have |Vt| = 3 V and Cox (W/L) = 2 A/V2 , find |VGS |, VGG, R, and R1/R2 to establish a quiescent current of 100 mA in the output transistors and 20 mA in the driver stage.