Department of Electrical and Computer Engineering
EE2027 Electronic Circuits
Tutorial 3
2023/2024 Sem 2
. Unless otherwise stated, you may assume temperature, T= 300 K, thermal voltage, VT ≈ 0.025 V.
. All the symbols are as defined in lecture notes.
Homework 3:
Homework 3 has 2 questions, Questions 7 and 8 of Tutorial 3. You will need to submit a softcopy of your handwritten homework to Canvas>Homework Submissions>HW3 half an hour after class (i.e., latest by 4:30 pm) on Tuesday, 26 March 2024. Failing to do that will mean zero mark for homework.
The softcopy submission of your homework must be in PDF format (in a single file) and named using the convention “<Your Name>_HW3”.
Homework questions will not be discussed in class.
Q1. The npn BJT in an amplifier circuit has the iC versus vCE characteristics shown in Fig. Q1, along with the load line of the amplifier circuit. As shown, the BJT operates at the DC bias point Q. The temperature is T = 300 K.
At the dc bias point Q, vBE = 0.69 V, vCE = 12 V, the small _change in iC owing to the small change in vBE is 48 mA/V, while the small change in iC owing to the small change in vCE is 0.013 mA/V.
(a) Does the BJT amplifier circuit operate properly at the dc bias point Q? Explain your answer, and specify the biasing conditions and calculate the voltages of the two junctions in the BJT at Q.
(b) Determine the current gain (β), Early voltage (VA), and saturation current (IS) of the BJT. Do NOT ignore the Early effect, if present, in your calculation.
[Ans: β = 80, VA = 92.3 V, IS = 1.1×10- 15 A]
(c) How will the amplifier circuit operation be affected, if the dc bias point is moved to P? You need to elaborate your answer briefly.
Q2. For the circuit shown in Fig. Q2, the diode D1 has a breakdown voltage of 6 V, and the BJT Q1 has a common-emitter current gain of 100 and saturation current of 7.6×10- 15 A. Temperature T = 300 K.
(a) Show with explanation that the voltage v1 = 6 V.
(b) The base-emitter voltage of BJT Q1 has a magnitude of 0.64 V. For R2 = 0 Ω and R3 = 2.2 kΩ, determine the collector current of BJT Q1 and R4. State the assumption(s) made and verify it (them).
[Ans: IC = 1 mA, R4 = 5.31 kΩ]
(c) For the collector current of BJT Q1 to drop to 0.3 mA, what is the required value of R2? Resistance of all other resistors remain unchanged from part (b).
[Ans: R2 = 1.26 MΩ]
Q3. A BJT circuit is shown in Fig. Q3. The pnp transistor, Q1, in the circuit has β = 100.
(a) Assuming that the base-emitter forward biased junction voltage is 0.7 V, determine the collector current, IC, for the circuit shown in Fig. Q3 using the voltage divider method at the base terminal by assuming that IR1 ≈ IR2 >> IB.
[Ans: Ic = 1.10 mA]
(b) Is the use of voltage divider assumptions valid? Explain.
(c) Based on the conclusions of part (b), determine the correct value of IC. Also calculate VE and VC, and confirm that the BJT is operating in the forward active region.
[Ans: Ic = 0.99 mA, ⅤE = 5 V , Ⅴc = 1.98 V]
[Note: In order for a number A to be considered much larger than a number B, i.e., for A >> B, A must be 10 times or more than B.]
Q4. In the circuit of Fig. Q4, the p-channel MOSFET has μp Cox = 2×10-5 AV-2, VTH = -1 V, W/L = 5, and λ = 0.01 V-1 .
(a) What is the gate-to-source voltage, VGS, of the MOSFET? Explain your answer.
[Ans: -5 V]
(b) Does the MOSFET has a channel formed between the source and drain? Explain your answer.
(c) Assuming the MOSFET is operating in the saturation region, estimate the value of the drain current, ID, with and without Channel-Length Modulation effect. Compare the two values of ID and discuss the implication.
[Ans: 0.846 mA; 0.8 mA]
(d) Verify if the MOSFET is operating in the saturation region.
Q5. The n-MOSFET in the amplifier circuit shown in Fig. Q5 is operating in the saturation region and has the following device parameters: Kn = 1 mA V-2, VTH = 1 V. The source and body of the n-MOSFET are connected together, i.e., there is no body effect. The voltage supply VDD = 5 V. The circuit designer decided that the drain current ID is to be set at 1 mA and chose R1 = 60 kΩ, and RS = 1 kΩ. Complete the design of the circuit by following steps (a) to (d).
(a) Determine the value of VG.
[Ans: VG = 3 V]
(b) Determine the value of R2.
[Ans: R2 = 90 kΩ]
(c) What is the minimum value of the drain voltage VD such that the n-MOSFET is operating in the saturation region under the given conditions?
[Ans: Minimum VD = 2 V]
(d) What is the maximum value of RD for the n-MOSFET to be operating in the saturation region in this circuit?
[Ans: Maximum RD = 3 kΩ]
Q6. For part (a) and part (b) below, select the correct statement(s).
(a)
Consider the above circuit where BJT Q1 has β= 100. Which of the following is(are) CORRECT?
(i) With only VDD2 changed to 7 V, BJT Q1 still operates in forward active mode.
(ii) With only R2 changed to 5 kΩ, BJT Q1 no longer operates in forward active mode
(iii) With only VDD1 changed to 6 V and R2 changed to 5 kΩ, BJT Q1 no longer operates in forward active mode.
(iv) With IA changed to 2.5 mA, BJT Q1 still operates in forward active mode.
(b) Consider the following statements, indicate which is(are) CORRECT.
(i) When used in an amplifier circuit, a BJT should operate in the saturation region, while a MOSFET should not operate in the saturation region.
(ii) For a BJT operating in the forward active mode, its small-signal output resistance, ro, decreases when the magnitude of the base-emitter voltage, |VBE |, is increased.
(iii) For a MOSFET operating in the saturation mode, its transconductance, gm, increases when the magnitude of its gate to source voltage, |VGS|, is increased.
(iv) In a CMOS inverter, the sizing of the N-MOSFET is 2.5 times that of the P-MOSFET. It can be concluded that the ratio of propagation delays, tPHL/tPLH is 1.
Q7. The DC biasing circuit of a BJT amplifier circuit is shown in Fig. Q7. The BJT Q1 has , = 75 and the collector voltage is required to be VC = 7 V.
(a) What mode of operation should the BJT Q1 be in for the circuit to work as an amplifier?
(1 mark)
(b) Assuming the Early effect is negligible, calculate the base, collector, and emitter currents of the BJT Q1.
(6 marks)
(c) Calculate the voltages at the base and emitter terminals of the BJT Q1 and show whether the BJT Q1 is working in the desired operation mode.
(5 marks)
(d) Assuming the Early effect is negligible, calculate the small signal parameters gm, rπ and ro of the BJT Q1.
(3 marks)
Q8. The DC biasing circuit of a MOSFET amplifier circuit is shown in Fig. Q8. The parameters of this circuit are VDD = 6 V, R1 = 100 kΩ, R3 = 10 kΩ. The threshold voltage of the MOSFET M1 is 1 V and its drain current is 0.16 mA. The temperature is 300 K.
(a) Determine if the MOSFET in the circuit in Fig. Q8 is operating in the desired mode of operation ifR2 is 100 kΩ .
(5 marks)
(b) Determine the Channel Length Modulation parameter, ，, if the drain current of the MOSFET becomes 0.1616 mA when R3 is 5 kΩ .
(6 marks)
(c) Calculate the small signal parameters, gm and ro , of the MOSFET ifR3 is 10 kΩ .
(4 marks)
Q9. An n-channel MOSFET is connected as shown in the circuit in Fig. Q9, where VSS provides a voltage across the source and body. The MOSFET drain current equations discussed in lecture are still applicable even when VSS < 0 V, and the conductance parameter remains the same.
A drain current (ID) of 3.2 mA is measured with VSS = 0 V. When VSS is changed to - 1 V, the threshold voltage of the MOSFET increases by 0.258 V compared to that with VSS = 0 V, and this results in a reduced drain current at 2.8 mA.
(a) When VSS = - 1 V, does the MOSFET experience body effect? Why?
(b) For both cases with VSS = 0 and VSS = - 1 V, it can be concluded that the gate-to- source voltage, VGS, of the MOSFET is greater than the respective threshold voltage. Explain the reason.
(c) For both cases with VSS = 0 and VSS = - 1 V, which region is the MOSFET operating in? Why?
(d) Calculate the threshold voltage (VTH1) of the MOSFET when VSS = - 1 V. Neglect channel length modulation.
[Ans: VTH1 = 1.268 V]