Midterm Exam for Physics 5121, October 27, 1995
20 points - closed book - calculators allowed - show your work
1) (6 Points)
a) For the circuit in figure 1 write down the equations, but do not solve them, that you would use to determine, i1 and i2.
b) In a circuit a reactive load (Z) has a sinusoidal voltage Vsin(wt) across it's terminals and a current Isin(wt + phi)flowing in it. What is the expression for the power dissipated in the load?
c) What is the R.M.S. voltage of the voltage signal V=Vo exp(jwt)?
d) What is the valency of the impurities that are added to silicon to make p-type silicon?
e) Sketch the I-V curve (current as a function of applied voltage) for a normal semi-conductor junction diode, which has a Zener voltage of 20V, for voltages between -30V and 2V. Show the values of voltages where changes occur in the slope of the curve.
2. (7 Points) Show that a leaky capacitor Cp in which the leakage resistance is represented by a shunt Rp, (Figure 2a) is equivalent to a capacitance Cs in series with a resistance Rs (Figure 2b). Find the values of Cs and Rs in terms of Cp, Rp and the angular frequency, w, of the applied alternating voltage.
3. (7 Points)
Analyze the circuit in figure 3a: (Give the voltages to 0.1V and the currents to 10%, assume hfe = 100)
a) With no sources or loads connected what are the values of IE, IB, IC?
b) With no sources or loads connected what are the values of VE, VB and VC?
c) An AC signal is coupled to the input through a large capacitor; what is the voltage gain of output1?
d) An AC signal is coupled to the input through a large capacitor; what is the voltage gain of output2?
e) What is the input impedance of the circuit as shown?
f) Replace the emitter circuit with the circuit shown in figure 3b, what are the DC (w =0) and the high frequency ( w >1/tau) gains of the circuit?
