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UNIVERSITY OF OSLO
FACULTY OF MATHEMATICS AND NATURAL SCIENCES
Exam in
: FYS1210 Electronics with projects
Day of exam : 28th of May 2010
Exam hours : 14:30 – 17:30 ( 3 hours )
This examination paper consists of 3 pages + 2 pages of log paper.
Appendices : 2 log-papers
Permitted materials: Small calculator, Text book
Robert T. Paynter & B.J.Toby Boydell "Electronics Technology Fundamentals".
Make sure that your copy of this examination paper is complete before answering.
We start with some circuits found in the student satellite CubeSTAR
Exercise 1
CubeSTAR is powered by solar cells. 2 such cells connected
in series provide approximately 5 volts. Without load we
measure 5 volts between A and B. When connecting a load of
12 ohms (RL) across the cell – we find that the voltage drop
to 4,5 volts.
a)
How large is the internal resistance in the cell, Ri ?
A
Ri
RL
Vsolar cell
B
Figur 1
Figure 2 shows the electronic circuit to one of the four “plasma probes” onboard CubeSTAR.
We wish to measure the density of free electrons in the ionosphere.
The probe is put to a positive potential to attract free electrons.
R1 = 5 M Ω , R2=R3=R4=R5= 30 kΩ Output from opamp 2 R6 = 1 kΩ
Battery V1 puts a +5 volt potential to the (+) input of OPAMP 1.
b)
What is the DC-potential on the ”plasma probe” ?
c)
The voltage in A (the output from OPAMP 1) is 5,5 volts.
How large is the current from the plasma probe ? (R1 = 5 M Ω)
d)
How large is the current flowing through resistor R6 ?
1
Examination FYS 1210
28. May 2010
Exercise 2
Figure 3
a.)
b.)
c.)
d.)
e.)
Figure 3 shows two operational amplifiers (opamps) connected in series.
R1 = 1kΩ , R2 = 100 kΩ, R5 = 27 kΩ, R6 = 3 kΩ og R7 = 1 kΩ
How large is the total voltage gain in this circuit (Av = Vut/Vinn) for low
frequencies ( 1 - 10 Hz)
How large is the the voltage gain – in dB - in the last opamp (U2) for low
frequencies (1-10 Hz) ?
These opamps have a 1 MHz Gain Bandwidth Product (GBW). At what
frequency will the total voltage gain in this circuit be reduced by 3 dB?
At 10 kHz the signal output from opamp U1 is measured in AA to be 0,5
volt. How large is the signal on the inverting input to this opamp?
(- in the connection point between R1 and R2)
Use the attached log paper and draw a frequency plot from 1 Hz to 1MHz.
Mark all discontinuities in the drawing.
Exercise 3
a)
b)
c)
Look at the circuit in Figure 4. How large is the input impedance Z (ohm) for
a frequency of 20 kHz ?
(Seen from the signal source V1 into C1 = 1µF and R1 = 4kΩ )
C1 and R1 make a frequency filter. Is this a High pass or Low pass filter ?
What is the ”cutoff” frequency for this filter?
This opamp has a SLEW RATE = 1 volt/µs. What is the maximum signal
voltage (Vpp) this circuit can handle at 30kHz ?
Figure 4
2
Examination FYS 1210
28. May 2010
Exercise 4
Figure 4 shows an AC-amplifier designed with a NPN BJT transistor. This transistor has a
current gain β = 200. Battery voltage V1 = 30 volt
R1 = 80 kΩ, R2 = 12 kΩ, R3 = 2 kΩ, R4 = 10 kΩ, R5 = 10 kΩ
4 a ) Draw the Thevenin equivalent for biasing the base.
How large is the Thevenin voltage VTH and the Thevenin resistance RTH ?
4 b ) How large is ICQ for this transistor ?
You may do a simplified calculation – without using Thevenin
Show your calculations.
4 c ) How large is the current drawn from the battery V1 ?
4 d ) If IC = 2 mA. What is the voltage across the transistor (Collector – emitter) VCE?
4 e ) If ICQ = 1,5 mA
– what is the transconductance - gm in this transistor?
4 f ) What voltage gain do we have ? AV = (Vout / Vin)
4 g ) We remove the capasitor C3. What is the voltage gain AV = ( Vout / Vin ) ?
4 h ) Draw the small signal equivalent for this circuit – for high frequencies.
4 i ) Explain what you understand with Miller effect. (Short)
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