Experiment No' 7

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Experiment No. 7
EE 312 Basic Electronics Instrumentation
Laboratory Wednesday, October 11, 2000
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Objectives

• Measure dynamic impedance of a forward-biased
  diode Zener diode
• Learn about small-signal techniques
• Learn about interference reduction through the
  use of proper grounding and twisted-pair
  techniques

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Background
What is dynamic impedance ?
V I
resistance
R
dV d I
rd
dynamic resistance
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V-I Characteristics
diode
transistor
rd
Tube
slope
I
Operating Point
dV dI
V
R
5
Id
Vd
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IEEE Standard Notation
iD total diode current
ID dc diode current
id ac diode current
Id ac amplitude
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vD total diode voltage
Vd
vd
VD dc diode voltage
VD
vD
vd ac diode voltage
Vd ac amplitude
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Small-Signal Condition
Id, Vd
ID, VD
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Dynamic Resistance Measurement
iD
Vd I d
rd
2Id
ID
2Vd
VD
vD
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Input Signal Too Small
iD
ID
VD
vD
noise
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noise
Measurement of rd looks simple. The problem is
that vd in the millivolt range for forward
bias. Thus, noise and stray pickup may cause
trouble if you are not careful.
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Example
How large is it ?
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Questions Where does come from ?
Answer 1. Current iac in power lines on bench
drops from ceiling 2. fluorescent lights 3. AC
machines
r
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Question How large is ?
I
r
r
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Assume our experiment is about 2 meters from the
power lines r 2 m
100 amp. peak
60 HZ
Peak value is 3.77 mV and this may be comparable
to signal amplitudes being measured!
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Must be concerned about in all parts of
circuit.
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How is this problem avoided?
remember
We have control over A. We cant do much about r
or I. So, we must minimize A.
Step 1 Make the area small
Step 2 Twist wires together
OSC.
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Twisting wires does two things,
1- Holds wires together 2- voltages induced in
adjacent sections cancel
V1
V2
1
2
V1 -V2 So induced signals cancel
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Keep track of grounded leads
Oscilloscope
R
A
B
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Single Point Grounding Use Only One Ground
Connection Such As CRO ground
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Can only one ground connection be realized? e.
g. CRO ground. Not with BNCs because the each
outer connector is another ground.
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Capacitive Coupling1. Assume 1 pF between your
circuit and 120 VAC power lines.2. 60-Hz
current I j?CV where ? 377 rad/s at f 60
Hz, C 1 pF, and V 120 VAC(rms)
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3. The voltage produced by I ZxI where Z is the
impedance I flows through. 4. Example CRO Z
1 Meg? VCRO 377x1pFx120Vx1Meg? 45 mV(rms)
130 mVpp
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Procedures

• I- Measure dynamic resistance of a Zener diode
  in the forward bias region.
• II- Simulation for Part I.
• (In Bell 242)
• III- Measure dynamic resistance in the Zener
  breakdown region.

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Components

• Zener Diode 1N4742-12VDC-0.5 W
• 2 Heathkit Resistance Substitution Boxes
• 1-kohm 10 kohm Resistors
• Decade Capacitor Box

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1- Dynamic Resistance in Forward Region
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10 VDC
A
R1

10.4V to 10.8V
ID

0.4 to 0.8V
-
dc circuit
The values of R1 and the voltage source are
selected to control the dc bias current ID.
Suppose we want ID 10 mA. Make the dc voltage
across R1 10 VDC. Assume VD 0.7 V.
V10.7 volts ID 10 mA
R11000 Ohms
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C
R1
R2
A
id


10.7V
-
ac circuit
20Vpp 1 kHz
By setting the dc power supply voltage to 10.7
VDC the FG amplitude to 20 Vpp and R2 to
10R1, the ac current peak is 10 of dc
current. I. E. ID 10 mA id 1 mA . To obtain
other values of ID id change both R1 R2 with
R2/R1 10. The dc ac voltage levels in the
circuit change very little as R1 R2 are changed
to change the currents ID id .
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C
R2
A
id


10.7V
-
ac circuit
20Vpp 1 kHz
C blocks dc current in the ac circuit C should
be large enough so that capacitance reactance is
small compared with R2 Note that R1 must be gtgt
diode dynamic resistance so that most of the ac
current goes through the diode not the dc
circuit
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Selection of R2

• The values of R2 and the function generator
  voltage amplitude Vgen should be chosen to make
  the ac current amplitude id 10 to 20 of ID.
  The corresponding diode peak ac voltage Vd will
  be 10 V to 20 of nVT where VT 25 mV at T
  290 K. (20 C). Thus Vd will be 2.5 to 5 mV for
  n 1 and the peak-to-peak diode ac voltage will
  be 5 to 10 mV.

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Fall 2000 Data Table For Forward rd
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Fall 2000 Data Table For Forward rd
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rtheoretical ?
n1 to 2
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n?
slope gives n
1/T
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Examples ID 0.2 mA n 1 rd 1X25mV/0.2mA
125 ? n 2 rd 2X25mV/0.2mA 250 ?
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2- Simulation
a- Simulate Part 1 of experiment b- Plot I(D1)
and V(2) on separate graphs c- Calculate dynamic
impedance of the diode
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mA
DYNAMIC IMPEDANCE I1 0 1 PWL(0 .5M .00249 .5M
.0025 1M .00499 1M .005 5M .00749 5M .0075
10M) R1 1 2 1.5K D1 2 0 DIODE .MODEL DIODE
D((RS2 IS2E-9 N1.8) R2 3 2 15K C1 4 3 .22U V1
4 0 SIN(0 5 1KHZ) .TRAN .05M 10M 0 .05M .PRINT
TRAN V(2) i(D1) .END
time s
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3- Dynamic Resistance of Zener in the Breakdown
Region
Choose values of dc bias current so that the dc
power dissipation in the diode is less than 1/2
of its max rated power dissipation (1/2 Watt).
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Assume Zener Diode Breakdown Voltage VZ
12V The values of R1 and the dc voltage source
are selected to control the dc bias current ID.
Suppose we want ID 10 mA. Make the dc voltage
across R1 5 VDC. Then R1 5 VDC/10mA 0.5
k?. Use the closest value which is 470 ?. The FG
peak voltage is set at 10 V. The value of R2 is
selected so that the peak ac current 10 of the
dc current 0.1 X 10 mA. Thus R2 10V/1mA 10
k?.