ECEN203 Test 1 Tuesday 21 April 2015 17:10 – 18:00 Instructions: Attempt every question. You will receive marks for your working, as well as for numerical answers. You may use a scientific calculator (but not a graphical calculator) to assist your calculations. You may also use a printed foreign language to English dictionary, if required. The test is out of 30 marks, assigned as follows: Question 1: DC Analysis [12 marks] Question 2: AC Analysis [6 marks] Question 3: Amplifier Models & Feedback [6 marks] Question 4: Operational Amplifiers [6 marks] Page 1 of 6 Question 1: DC Analysis [12 marks] 1. [4 marks] Use Kirchoff’s laws to find a) i1, 2.5 A 1.5 A b) i2, and 1A c) i3, in the circuit below. d) Also calculate the voltage drop across the 3 Ω resistor. i1 1Ω + 5.5 V 3V i2 2Ω i3 3Ω 2. [1 mark] Find the Thevenin equivalent for the circuit below. 4 kΩ 30 V + a 10 V, 2.4 kΩ 6 kΩ + 20 V b 3. [1 mark] Find the Norton equivalent for the circuit below. 8Ω a 3A 8Ω 3 A, 2.67 Ω 4Ω b Page 2 of 6 4. [4 marks] Calculate Vab for the circuit below 4.28 V a) with the load resistance, RL, connected, and 15 V b) without the load resistance, RL, connected. c) Hence, explain what source loading is. d) A simple way to reduce the effects of source loading is to shrink R1 and R2. Explain why this is not always desirable. a 30 V + R1 10 kΩ RL 2 kΩ b R2 10 kΩ c 5. [2 marks] Use superposition to find 3.1 V a) the voltage across, and 0.444 A b) the current through, the 7Ω resistor in the circuit below. 5V + 2Ω 0.5 A Page 3 of 6 7Ω Question 2: AC Analysis [1 marks] 1. [1 mark] Find an expression for the gain, i.e. Vo/Vi, of the circuit below. R Vo Vi C 2. [1 marks] Evaluate your expression for R = 10 kΩ, C = 47 nF and ω = 2000 π rad/s. Express your answer in polar form. 0.32 <-71º 3. [4 marks] Draw a Bode plot (i.e. plot gain vs frequency and phase vs frequency) for the circuit below, assuming R = 10 kΩ, C = 47 nF and ω = 2000 π rad/s. Hint: be sure to include all relevant information on the Bode plot, including axis labels, units, and coordinates. Remember to clearly label key data points and coordinates. You may approximate curves in the Bode plots as straight lines. Page 4 of 6 Question 3: Amplifier Models and Feedback [6 marks] 1. [2 marks] A voltage controlled voltage source, such as the amplifier shown in the figure below on the left, can be modelled by the circuit on the right. Explain, with the aid of circuit diagram(s), how you would measure values for a) A, and b) Zi. Zo Vi Vi Vo Vo Zi AVi 2. [2 mark] A similar amplifier to the one above with A = 10^6 is connected as shown in the circuit below, where B represents a voltage divider network such that Vf = -Vo/10. a) Calculate the gain of this circuit, i.e. Vo/Vi. Remember to show your working. 10x b) Does this circuit have positive, negative, or self-exciting feedback? Explain. Vi A ∑ Vf Vo B 3. [2 marks] Using feedback with B = -0.1 typically reduces the gain of the circuit. Give two reasons why circuits with such feedback, such as the one shown above, are far more common than an open loop amplifier circuit, such as the one show below. Hint: consider what happens to the gain of the circuit if A changes. What about the bandwidth of the circuit? Vi A Page 5 of 6 Vo Question 4: Operational Amplifiers [6 marks] 1. [2 marks] With reference to the symbols shown in the circuit diagram below, what are the two golden rules of op amp analysis? I- - V- Vo + V+ I+ 2. [2 marks] a) Find an expression for Vo/Vi for the circuit below. b) Explain the function of this circuit. -jwRC Hint: remember to include your reasoning and any relevant calculations when determining the circuit function. R C Vo + Vi 3. [2 marks] a) Find an expression for Vo in the circuit below. b) Explain what this circuit does. Rf R2 V2 R1 V1 Vo + Page 6 of 6