Electronics II ELEC 312 F,G
Examination Date Time # of pages
Final December 13, 2007 3 hours 4
Instructor(s)
Dr. R. Raut, Dr. G. Cowan
Students are allowed to use only one of the prescribed calculator(s).
Special Instructions:
Attempt all questions.
Show all steps clearly in neat and legible handwriting.
Students are required to return question paper together with exam booklet(s).
Q.1 Exact analysis of an amplifier gives the following transfer function: where AM=102, wp1=106, wp2=109, wz=109.
(a) Find the gain margin and phase margin for A(s) if it is put in feedback with β=1.
(b) What will be the β for a phase margin of 60 degrees?
Q.2: For an amplifier model as shown below, you are given:
gm = 5mA/V, Rsig=150 kΩ, Rin=0.5 MΩ, RL’=10 kΩ, Cgs=2 pF, Cgd=0.5pF. The output has a wiring capacitance of 2 pF. Find an estimate for the gain-bandwidth of the amplifier.
Q.3: A BJT device (Q1) is used to operate as a class A amplifier into a load of 100 ohms. The required signal power is 0.5 watts. The amplifier is biased by a current-mirror current source circuit. The devices (Q1 to Q3) have VCE(sat) of 0.2 volts and an EB junction cut-in voltage of 0.7 volts each. Determine the required supply voltage values that can be used. Based on this supply voltage, provide a design of the current-mirror current source circuit.
Q.4: The figure below can be used as a variable phase shifter (i.e., ) by adjusting the resistance R. Derive an expression for . If the input signal has a fixed frequency of 103 rad/sec and if C=10 nF, find the values of R required to obtain phase shifts of –30o and –90o respectively.
Q.5: A transresistance amplifier having an open-circuit gain of 100 V/mA, an input resistance of 1 kΩ, and an output resistance of 1 kΩ is connected in a negative feedback loop. The feedback network has an input resistance (with port 1 short circuited) of 10 kΩ and an input resistance (port 2 short circuited) of 10 kΩ. The feedback factor is 0.1 mA/V.
The amplifier is fed from a current source of 1 mA (ac) with Rs =10 kΩ, and the system is loaded with RL =1 kΩ. Find the gain Af of the system under negative feedback.
(Question on soft-skill component)
Q.6: The figure below depicts a low frequency oscillator using an ideal operational amplifier and several R,C elements Derive the expressions for the frequency of oscillation and the condition of oscillation for this network. Provide a design for the values of the resistors to obtain a frequency of oscillation of 10 kHz. Assume equal capacitances of 10 microfarads each.
Figure 6
Some Useful Formulae
BJTs
MOSFETs (Saturation region):