D ***9.127 For the circuit shown in Fig. P9.127, which uses a folded cascode involving transistor Q3, all transistors have  V BE  = 0.7 V for the currents involved, VA =200 V, and β = 100. The circuit is relatively conventional except for Q5, which operates in a Class B mode (we will study this in Chapter 12) to provide an increased negative output swing for low-resistance loads.(a) Perform a bias calculation assuming  VBE  = 0.7 V, high β, VA =∞, v+ =v− =0 V, and vO is stabilized by feedback to about 0 V. Find R so that the reference current I REF is 100 μA. What are the voltages at all the labeled nodes? (b) Provide in tabular form the bias currents in all transistors together with gm and ro for the signal transistors (Q1, Q2, Q3, Q4, and Q5) and ro for QC, QD, and QG. (c) Now, using β =100, find the voltage gain vo/(v+ −v−), and in the process, verify the polarity of the input terminals.(d) Find the input and output resistances. (e) Find the input common-mode range for linear operation. (f) For no load, what is the range of available output voltages, assuming  VCEsat  = 0.3 V? (g) Now consider the situation with a load resistance connected from the output to ground. At the positive and negative limits of the output signal swing, find the smallest load resistance that can be driven if one or the other of Q1 or Q2 is allowed to cut off.

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images - D ***9.127 For the circuit shown in Fig. P9.127, which uses a folded cascode involving transistor Q3, all transistors have  V BE  = 0.7 V for the currents involved, VA =200 V, and β = 100. The circuit is relatively conventional except for Q5, which operates in a Class B mode (we will study this in Chapter 12) to provide an increased negative output swing for low-resistance loads.(a) Perform a bias calculation assuming  VBE  = 0.7 V, high β, VA =∞, v+ =v− =0 V, and vO is stabilized by feedback to about 0 V. Find R so that the reference current I REF is 100 μA. What are the voltages at all the labeled nodes? (b) Provide in tabular form the bias currents in all transistors together with gm and ro for the signal transistors (Q1, Q2, Q3, Q4, and Q5) and ro for QC, QD, and QG. (c) Now, using β =100, find the voltage gain vo/(v+ −v−), and in the process, verify the polarity of the input terminals.(d) Find the input and output resistances. (e) Find the input common-mode range for linear operation. (f) For no load, what is the range of available output voltages, assuming  VCEsat  = 0.3 V? (g) Now consider the situation with a load resistance connected from the output to ground. At the positive and negative limits of the output signal swing, find the smallest load resistance that can be driven if one or the other of Q1 or Q2 is allowed to cut off.

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