LAB 2 2
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University of Pittsburgh *
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622
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Electrical Engineering
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Apr 3, 2024
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INTRODUCTION PURPOSE The purpose of this laboratory exercise is to experimentally determine the parameters of a semiconductor zener diode. Task A: Determine the reverse saturation current, I
S
, and the ideality factor, n. Task B: Determine the dynamic resistance, r
d
, of the diode, Task C: Determine the reverse breakdown voltage, V
Z
.
Expected problems that need to be overcome:
The issues considered while analysing the test circuits:
1. The measurement accuracy of the instruments used.
2. The loading effects of the instruments. Loading effects due to shunt resistance
in a
voltmeter, for example, are especially problematic at very small values of diode current.
3. The resolution of the ammeter if one is used to measure current values.
4. The non-ideal characteristics of the op amp used in the test circuit.
TASK A
Schematics:
Circuit Reasoning:
An Op-amp is used as a buffer the diode voltage before connecting it to voltmeter to overcome the issues said above. LF412 op-amp is selected so that should get low input current in (+) and (-) terminals so that the current through the diode id equal to the current through resistor R. Steps and Equation Used:
Step -1 : Rj = -Vdc / IDj IDj used: 1. Id1= 1uA R1= 4033 K ohm
2. Id2 = 10uA
R2= 403.3K ohm
3. Id3 = 100uA
R3= 40.33 K ohm
4. Id4 = 1mA
R4= 4.03Kohm
5. Id5= 10mA
R5 = 0.403 K ohm
Step 2: Record Vdc from input supply and measure Vd for each (Rj and Idj) pair by DMM Step 3: Calculate Id = -Vdc (exp)/R(real); Measure R by DMM.
Step 4: Plot log (Id) vs Vd Using eq Id = Is x e^(Vd/nVt)
Where Vt= Kt=q = 0.0259
Log (Id) = Log (Is) + (1/nVt) x Vd
Readings Vdc (exp) (V)
R(real) (K ohm)
Id (exp) (A)
Vd (V)
Log (Id exp) (A)
-4.033
3880
1.362x10^-7
0.786
-2.692
-4.033
388.9
1.537 x 10^-6
0.723
-3.732
-4.033
38.49
0.000017
0.660
-4.769
-4.033
3.89
0.000185
0.598
-5.813
-4.033
0.400
0.00203
0.545
-6.865
The equation of the above graph comes out as y=17.15x-16.13
Therefor Log 10 (Is) = -16.13
Is = 10 ^ -16.13
(Log10(e) / nVt ) = 17.15 N = 2.25
Discussion Of results We see the R real resistance in the circuit is less compared to the resistance taken thus increasing the Id (current flowing ) . As reverse saturation current and
n ( ideality factor ) is dependent on Id and R and voltage drop across Zener Diode we see that the ideal condition is changed. Some conditions on which reverse saturation current is dependent on : 1 temperature , Resistance (R) Task B Schematics Two 0.1uF capacitors are used between P.S. pins of op-amp and ground to reduce the oscillations (noise).
Steps for TASK B Step 1: Add 0.1uF cap at + and – terminals of Opamp to avoid oscillations. Step 2: func Generator setting used for Vac : High z , sin wave of 1KHZ , 400mVpp and _1V offset
Step 3: Calculate Rd (Theo) = nVt/Idj where nVt is from Task A
Step 4: Measure Vin pp and Vdiode p-p using Oscilloscope for R2 to R5
Step 5: For each R calculate Rd (exp) = Vdiode p-p/(Vinp-p/Rj)
Step 6: Plot Log-log graph for log (Id) vd Rd(Theo) and Rd (exp)
NVt= 0.058
For Eg : For Id= 2.5 mA
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Rd (Theo) = nVt/Id = 0.058/(2.5 x10^-3) = 23.2 ohms
Rd (exp) = 0.013 x 23.3/0.3257 = 14.2 ohms
Rj (k ohm)
Id (mA)
Vin p-p
(V)
Vdio
de p-
p(V)
Rd (Theo) (k ohm)
Rd (exp)
(K ohm)
Log 10 Rd(Theo)
Log 10 Rd exp
Log 10 Id 387.3
0.0025
0.431
0.01
48
23.2
13.29
4.36549
4.1235
-5.602
38.49
0.025
0.4267
0.01
3
2.32
1.176
3.36549
3.0704
-4.602
4
0.25
0.4123
0.01
25
0.232
0.1212
2.36549
2.0835
-3.602
0.4
2.5
0.3657
0.01
3
0.0232
0.0142
1.36549
1.1523
-2.602
Graph 2: Log Log graph
%diff of 2 lines Log 10 Rd (Theo)
Log 10 Rd (exp)
% diff
Slope
-1
-1.0092745
1%
y intercept
-1.23651
-1.4703898
19%
Oscilloscope measurements for TASK B Plot 1 : 400 ohm. Plot 2 : 4K ohm Plot 3: 38.49k. Plot 4: 387.3K
Discussion Of results:
The dynamic resistance is dependent on current TASK C
Schematic of circuit R= V/I
= 4/1mA = 4K ohm
Power Supply (V)
DMM
4
-4.48
6.6
-4.63
9
-4.72
11.5
-4.78
14
-4.78
17
-4.87
Reverse Breakdown Voltage of Zener Diode : -4.87V
Zener Voltage : It is the breakdown voltage of the Zener diode when it is connected in reverse bias condition. It allows the diode to conduct in reverse direction. This is why Zener diode is used as voltage regulator.
Summary and Conclusion
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