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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Autumn Examinations 2008/
Module Title: Electronic Engineering
Module Code: ELTR
School: School of Electrical & Electronic Engineering
Programme Title: Bachelor of Engineering in Electronic Engineering – Year 3
Programme Code: EELXE_7_Y
External Examiner(s): Mr David Denieffe & Dr. Paula O’Sullivan
Internal Examiner(s): Dr. P. O’Connor
Instructions: Answer QUESTION 1 (worth 40 marks) and TWO other questions (worth 30 marks each)
Duration: 2 HOURS
Sitting: Autumn 2009
Requirements for this examination: Graph paper, Log Tables
Note to Candidates: Please check the Programme Title and the Module Title to ensure that you have received the correct examination paper. If in doubt please contact an Invigilator.
- (i) For a non-inverting amplifier configured with (usual notation) R (^) f = 2 k Ωand R (^) g = 5 k Ω. Calculate ACL and β.
Show that
ACL =. (5 marks)
(ii) An op-amp has a UGB = 2MHz. If it is configured as a non-inverting amplifier with a gain of 20, then what is f (^) CL? If it is configured as an inverting amplifier with a gain of -20, then what is f (^) CL? (5 marks)
(iii) An op-amp has a UGB of 1.2MHz. It is to be configured as a non- inverting amplifier with f (^) CL of at least 50kHz. What is the maximum gain that the circuit can have? If the feedback resistor has a value of 15 k Ωthen what is the value of the other resistor? (5 marks)
(iv) Explain your understanding of Slew Rate (SR) as applied to op-amps.
A non-inverting amplifier with a gain of 10 uses an op-amp with a
Slew Rate (SR) of 0. 75 V / μ s. If the input is a sine wave at 25kHz,
Find the peak input voltage before distortion due to the slew rate limitation. (5 marks)
(v) Write a technical note on the AC-Performance of Op-Amps. Your explanation should include the UGB, f (^) c ( OL ), f (^) c ( CL ), and a frequency response curve to demonstrate your understanding. (5 marks)
(vi) Use circuit diagrams and frequency response curves to demonstrate your understanding of Active Filters. (5 marks)
(vii) A buck converter has an input of 60V and an output of 25V. The load
resistor is 9 Ω. The switching frequency is 20kHz, L = 1mH, C=200 μ F.
Find the duty cycle, the minimum and peak inductor current. (5 marks)
(viii) A 240VRMS , 50Hz ac signal is fed into a 20:1 step-down transformer. The output is connected to a full-wave bridge rectifier with a load of
1k Ω and a smoothing capacitor of 100 μ F.
Find VDC , VR ,VP and the % ripple. (5 marks)
- (a) For the op-amp shown in Fig.3(a), the circuit specifications are as follows:
- Input offset voltage ( V (^) IO ) at 250 C =5mV
- Offset current ( I (^) IO ) at 250 C =15nA
- Bias current ( I (^) B ) at 25 0 C =150nA
- R1 = 82k Ω and R2 = 20.5k Ω
With R3=0 Ω , then calculate (i) the error output voltage at 250 C caused by VIO (ii) the error output voltage at 250 C caused by IB (iii) the total error output voltage.
Calculate the value of R3 necessary to reduce the error caused by input bias currents.
If R3 is changed to this calculated value, then determine (i) the error output voltage at 250 C caused by VIO (ii) the error output voltage at 250 C caused by IIO (iii) the total error output voltage.
(12 marks)
(b) Explain, using a circuit diagram, how a boost converter works. A Boost Converter has an input of 5V and an output of 20W at 15V. The minimum inductor current must be no less than 50% of the average. The output voltage ripple must be less than 1% and the switching frequency is 30kHz. Find the (i) Duty cycle (ii) Minimum inductor value (iii) Minimum capacitor value. (assume ideal components, state any assumptions) (10 marks)
(c) Design a 2nd^ order VCVS, high-pass Chebyshev filter with a cutoff frequency of 8 kHz and a ripple width of 2dB. The filter should have a gain of 4. (State any assumptions.)
[A general configuration for a 2nd^ order HP/LP Chebyshev filter is shown in Fig.3(c), as well as Tables]
(8 marks)
4. (a) A 741 op-amp (UGB = 1MHz , SR = 0. 5 V / μ s ) is configured as
an inverting amplifier with a gain of -50. Find f (^) CL and then determine the maximum input signal that can be applied at this frequency before distortion starts to affect the signal.
(10 marks) (b) Write a Technical Note on Single Supply op-amps.
A sensor has an output voltage of ± 0. 2 V .This is fed into a single-supply op-amp operating off +5V and the output should be centered around 2.5V with a swing of ± 1. 5 V. Assume that because of the frequencies involved, ac coupling is not an option. Design a circuit to do this. (15 marks)
(c) Draw a circuit for a Switching Regulator of your choice and explain its operation.
(5 marks)
Table 3(c) – 2nd^ –order High-Pass Chebyshev VCVS filter Designs (2dB)
- Circuit Element Values
- Gain
- R 1 0.640 1.390 2.117 2.625 3.040 3.
- R 2 3.259 1.500 0.985 0.794 0.686 0.
- R 3 Open 3.000 1.313 0.953 0.784 0.
- R 4 0 3.000 3.939 4.765 5.486 6.
- 1 – Resistance in kilohms for a K parameter of
