Sample ELEC 311 Final Questions Diode AC small-signal analysis 1. Determine the bias voltage, VBIAS in the circuit shown in Fig. 1. When vin = 100 sin(2π1000t) mV, the total output voltage, vO(t) is as shown in Fig. 2. (10 marks) Figure 1 Figure 2 Diode DC analysis R = 1 kΩ R=1 kΩ 2. Determine the unknown quantities below. Assume that the diodes are ideal, and please show that any assumptions you make regarding the states of the diodes are valid. (10 marks) V1 = ___________. V2 = ___________. Zener Diode Analysis 3. Consider the circuit shown below in Fig 3. V+ = 12 V +/- 1V. The Zener diode has a VZ0= 5 V and rz = 10 Ω. For this question, you can assume that this model is valid for any non-zero reverse breakdown current (IZ > 0). That is, IZK = 0 mA. The resistance R has not yet been specified. There are several quantities that can be calculated for a Zener diode based regulator. As an example, one can calculate the maximum current that the regulator can supply while maintaining the desired output voltage. Please see the last row in the table. The equation in the bottom right cell of the table shows an expression for this quantity. A quick examination of this expression leads us to the conclusion that to maximize the maximum available current, we should decrease R, as indicated in the last row of the table. Please fill in the columns “Direction of R….” and “Rationale” for each of the remaining three specifications. Design Goal: V+ IR R IZ + VL - Fig. 3 IL Minimize the power dissipation with no load is connected. I.e., P=V+*IR Improve the line regulation (consider the case when no load current is connected). I.e., minimize ΔVL/ΔV+ Improve the load regulation. I.e., minimize ΔVL/ΔIL when a load current IL is connected. Increase the maximum current that the regulator can supply while maintaining a DC output voltage near 5 V. Direction of R for satisfying goal (i.e., Increase R, decrease R or R typically has little effect on the goal) Rationale V + − VZO R Reducing R will increase IMAX I MAX = Make R small RC=10 kΩ RC=10 kΩ RC=10 kΩ RB1=10 kΩ RB1=0.8 kΩ RC=10 kΩ RE=8 kΩ RB1=5 kΩ RB1=10 kΩ BJT DC analysis 4. Below are four possible configurations for establishing DC bias conditions for a BJT. Rank the four of them from best to worst in terms of the collector current’s insensitivity to β, temperature and the device’s IS. That is, the best circuit is the one whose collector current will change the least if β, temperature or the device’s saturation current (IS) changes (2-4 marks?) Best circuit: ___________. Second best:____________. Two worst circuits (in no particular order): _____ & ______. 5. Assuming β=100, for Circuit A, determine the BJT’s DC terminal voltages and currents. Please label the terminal voltages and currents on the schematic. (10 marks) RD=10 kΩ MOSFET Analysis 6. This question deals with the circuits in Figure 4, below. The purpose of this question is to analyze an amplifier that contains a biasing scheme that is frequently used in integrated circuit applications. For this questions, please use the following: • For MOSFETs M1 and M2, assume: o Vt = 0.5 V o µnCOX(W/L) = 200 µA/V2. o λ = 0 V-1 • IREF = 100 µA Figure 4 Please calculate the following: a) Please find VB and ID in the section of the circuit on the left. b) If we connect the node labeled “A” to the node labeled “VB” will the node voltage VB change? If yes, what is its new value? If not, why does it not change? (Hint: consider how much current will flow into terminal A). c) Now, with the two circuits connected, analyze the circuit to determine the DC bias conditions for M2 and the DC output voltage VO. d) Find the AC small-signal input resistance (RIN) for the node VB for the circuit in the left half of Fig. 4. e) Draw the complete AC small-signal equivalent circuit for the circuits in Fig 4 when VB is connected to A. You should represent the left half of the circuit by the resistance RIN that you have calculated. f) Find the AC small-signal voltage gain vo/vsig.
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