OCR Electronics for A2
Advice for Teachers
Variable resistors
Overview
In this unit your students should:
•
understand graphs to show how the drain current of a MOSEFT varies with the drainsource voltage for different values of the gate-source voltage
•
find out how to use a MOSFET as a voltage-controlled resistor
•
use analogue switches and multiplexers to control the flow of signals
•
meet the transfer characteristics of a tristate
This should not require more than 5 hours of class time.
Hour
1
Suggested Activity
Launch them straight into the Resistance characteristic practical. Use a 2N7000
MOSFET. This will give them an immediate experience of the use of a MOSFET as a
variable resistor. Students who do not finish step 8 within the lesson could finish it before
the next session.
Students who finish with time to spare could start the Musical MOSFETs practical.
Ask them to study 1.1 from the text book and answer questions 1 and 2 of the Variable
resistors exercises before the next session.
Discuss their answers to questions 1 and 2 of the Variable resistors exercises.
2
Let them continue with the Musical MOSFETs practical. This shows them how a MOSFET
can be used to make a voltage-controlled volume control. Insist that they thoroughly test
each stage before assembling the next one. Not only will they need this discipline for
coursework, they will also find trouble-shooting a lot easier.
Ask them to answer questions 1 and 2 from page 20 of the text book before the next
session.
Students should go straight into the Analogue switches practical.
3
As they finish, they could answer questions 3, 4 and 5 of the Variable resistors exercises.
Ask them to complete all of the remaining questions of the Variable resistors exercises
before the next session.
Discuss their answers to the Variable resistors exercises before setting them on to
questions 3 and 4 on pages 20 and 21 of the text book.
4
As they finish, they could start the Digital to analogue conversion practical, unless they
need to spend more time on questions 1 and 2. It is unlikely that any students will finish
step 6 before the end of the session.
Ask them to answer questions 5 and 6 from page 21 of the text book before the next
session.
Students who need more time to complete the questions from the text book could use this
session to catch up.
5
Otherwise, students should continue with the Analogue to digital conversion practical.
If there is time, you could run a quick informal test of their understanding of MOSFETs as
variable resistors.
Ask them to study 1.2 from the text book before the next session.
 Michael Brimicombe
page 1
www.hodderplus.co.uk/ocrelectronics
OCR Electronics for A2
Advice for Teachers
Variable resistors
Model Answers
1
(a)
(b)
The resistance starts off very large until the gate-source
voltage reaches the threshold voltage. Then the resistance
drops rapidly to a small value as the voltage increases.
(c)
For each fixed value of the gate-source
voltage (e.g. 5 V, 4 V ... ), the drainsource current varies linearly with the
drain-source voltage, just like the
transfer characteristic of a resistor. The
steeper the line, the lower the
resistance. So the voltage at the gate
fixes the drain-source resistance.
2
(a)
(b)
Below the threshold voltage, the MOSFET
resistance is much greater than 33 kΩ, so
most of the signal at in appears at out. The
gain is a steady 1. As the gate voltage is raised
above the threshold voltage, the MOSFET
resistance drops below 33 kΩ, so less of the
signal at in reaches out and the gain of the
system drops.
 Michael Brimicombe
page 2
www.hodderplus.co.uk/ocrelectronics
OCR Electronics for A2
3
Advice for Teachers
(c)
100 mV or less.
(a)
G = 1+
Variable resistors
Rt
470
= 1+
= +15.7
Rb
32
(b)
Vgs / V
3.0
4.0
5.0
G
16
30
45
Rds / Ω
32
16
11
(c)
Vgs / V
3.0
4.0
5.0
4
(a)
G
17
32
47
When E is low (0V), the resistance between Z and H is effectively infinite. Holding E
high (+5V) reduces the resistance between Z and H to a low value, typically 100 Ω.
(b)
Negative feedback requires that the op-amp output maintains the two inputs at the
same voltage.
(c)
Total resistance = 30 kΩ, so I =
V
15 − 0
=
= 500 × 10 −6 A.
R 30 × 10 3
-6
3
So voltage drop across each 2 kΩ is V = IR = 500×10 × 2×10 = 1 V.
So Z is at +1.0 V, Y at +2.0 V and X at +3.0 V.
(d)
(e)
C = B.A , D = B.A and E = B.A
B
A
C
D
E
signal at H
0
0
0
0
0
0.0 V
0
1
0
0
1
1.0 V
1
0
0
1
0
2.0 V
1
1
1
0
0
3.0 V
The op-amp acts as voltage follower, copying the voltage at H without drawing any
current which might upset the operation of the resistor chain.
 Michael Brimicombe
page 3
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OCR Electronics for A2
5
(a)
Advice for Teachers
Variable resistors
When A is low, the resistance between S0 and Q is about
100 Ω and the resistance between S1 and Q is effectively
infinite. When A is high, S1 is connected to Q instead.
(b)
When A is high, only the top analogue switch
closes, placing a low resistance between S1
and Q. The bottom analogue switch only
closes when B is high (A is low), connecting
S0 to Q.
(c)
Each switch behaves like a resistor
when it is closed, so signals can flow
either way through them. So the circuit
can take a signal at Q and direct it to
either S1 or S0, depending on the
signal at A., as required for a
demultiplexer.
(d)
 Michael Brimicombe
page 4
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OCR Electronics for A2
6
(a)
(b)
Advice for Teachers
Variable resistors
When E is high, the signal at A is copied to Q.
E
Q
When E is low, Q is disconnected so that its
0
-
voltage is not determined by A.
1
A
An analogue switch allows two-way flow of information when it is closed, whereas a
tristate only allows information to flow from input to output when it is enabled.
(c)
(d)
E
A
X
Y
Q
1
1
1
0
1
0
0
0
0
-
0
1
0
0
-
1
0
0
1
0
X = E.A Y = E.A
(e)
 Michael Brimicombe
page 5
www.hodderplus.co.uk/ocrelectronics