Experiment #9:
MOSFET Characteristics and DC Biasing
Friday Group
Dr. Somnath
11-20-09
Ari Mahpour
Teddy Ariyatham
Jayson De La Cruz
Table of Contents
Objective ......................................................................................................................................... 3
Tools ............................................................................................................................................... 3
Theory ............................................................................................................................................. 4
Preliminary Calculations ................................................................................................................. 5
Part 1 ....................................................................................................................................... 5
Part 2 ....................................................................................................................................... 5
Part 3 ....................................................................................................................................... 6
Part 4 ....................................................................................................................................... 6
Discussion and Results ................................................................................................................... 7
Part 3 ........................................................................................................................................... 7
Part 4 ........................................................................................................................................... 7
Part 5 ........................................................................................................................................... 8
Part 6 ......................................................................................................................................... 10
Part 7 ......................................................................................................................................... 12
Conclusion .................................................................................................................................... 14
Objective
The purpose of this laboratory experiment was to design a common emitter amplifier
using a bipolar junction transistor. Prior to doing so, it was imperative to follow the collector
and base currents and beta values through the curve tracer. In previous experiments we had
exposure to the curve tracer so this did not become an obstacle in our experiment. Compared
to the last experiment, where there was a considerable amount of trouble with calculations,
the curve tracer portion seemed to be very straight forward. Using a series of methods such as
the ten points (provided by the professor) and notes from the corresponding lecture class, the
design was very straight forward, enabling the preliminary calculations portion to move
through quickly.
Tools
-
Oscilloscope
-
Functional generator
-
Power supply
-
Transistor: Q2N2222A
-
Capacitors: 10µF
-
Resistors: (Specific for each circuit design)
Theory
Using the curve tracer, the student is expected to find the corresponding beta and
current values that match up to the bipolar junction transistor that is used in the lab. In this
case, the students are instructed to use a 2N2222A transistor and must trace their own part.
Using another student’s values could potentially cause trouble since each and every transistor
was not created equally. All corresponding beta and current values can be different for each
transistor since the fabrication process is not always exact. When tracing ones own transistor,
experimental errors stay at a minimum (if none at all). Design calculations must be done by
hand and then verified using PSPICE. Both methods are necessary to ensure that the circuit will
function correctly.
Preliminary Calculations
Part 1
(Refer to calculations sheet)
Part 2
Beta = 100 (AV = 13.29)
Figure 6.2a: PSPICE Model
3.0V
2.0V
1.0V
-0.0V
-1.0V
-2.0V
-3.0V
0s
V(VS:+)
0.5ms
V(RL:2)
1.0ms
1.5ms
2.0ms
2.5ms
3.0ms
Time
Figure 6.2b: PSPICE Simulation
3.5ms
4.0ms
4.5ms
5.0ms
Part 3
Beta = 300
Figure 6.2c: PSPICE Model
By changing the Beta value we can see that the IC and IE have increased. With respect to R1 and
R2, the higher the beta value, the closer (in value) the current becomes. More specifically:
lim 𝐼1 = 𝐼2
𝛽→∞
Note: Output waveform is the same since it is measuring the input and output voltages.
AV = 13.5
Part 4
As seen on the curve tracer printout (Figure 6.1) IC = 5.4mA and IB = 47µA
𝐼𝐶
𝛽=
𝐼𝐵
Therefore, our Beta comes out to be 115.
Discussion and Results
Part 3
RC (Ω)
VCE (V)
IC (mA)
Theoretical Experimental Percent Error
1500
1500
8.16
7.55
7.48%
4.91
5.41
10.18%
These values were simply found by performing a DC analysis both by hand (using the
digital multimeter) and PSPICE. The percent errors where off quite a bit but one must take into
consideration internal capacitance and resistance values that PSPICE does not factor in. When
working at such a low current value (in this case milliamps), any small difference will look huge
on a small scale.
Part 4
VIN (V)
VOUT (V)
AV
Theoretical Experimental Percent Error
0.24
0.24
2.52
2.52
13.29
10.5
20.99%
When observing the percentage error, it can be alarming that this value is so large.
However, it is important to note that PSPICE evaluate the voltage gain differently that was
taught in the corresponding lecture class. PSPICE will evaluate the voltage gain to be the output
voltage divided by the signal voltage. The calculations that were taught in the corresponding
lecture course were to perform the voltage gain by taking the output voltage and divide it by
the input voltage. Therefore, one can easily see that there is a clear discrepancy between the
values but they should not be regarding as an error, rather a percentage difference.
Part 5
VIN (V)
VOUT (V)
Theoretical Experimental Percent Error
0.65
1.2
14.7
15
2.04%
PSPICE RESULTS FOR PART 5 (OUTPUT SWING)
Figure 6.5a: PSPICE Model
8.0V
4.0V
0V
-4.0V
-8.0V
1.0ms
V(VS:+)
1.5ms
V(CC:2)
2.0ms
2.5ms
3.0ms
3.5ms
4.0ms
4.5ms
Time
Figure 6.5b: PSPICE Simulation
The goal of this step was to find the point where our output voltage caps. This required
us to slowly increase our input signal amplitude until we reached the limit of the output
voltage, and measure that peak-to-peak voltage. Figure 6.5a shows our input voltage of 650mV,
and Figure 6.5b shows the output voltage capped at 14.69 volts, almost the same value for Rout
achieved in our lab experiment.
5.0ms
Part 6
V1 (V)
Resistance (Ω)
V2 (V)
Zi (Ω)
Theoretical Experimental
0.2
2.48
4300
6700
0.1
1.2
4300
6281
PSPICE RESULTS PART 6
Figure 6.6a: PSPICE Model
4.0V
2.0V
0V
-2.0V
1.0ms
V(VS:+)
1.5ms
V(CC:2)
2.0ms
2.5ms
3.0ms
3.5ms
4.0ms
4.5ms
V(Q1:b)
Time
Figure 6.6b: PSPICE Simulation
Input Swing
Output Swing
(Capacitor)
Using PSPICE, we found the input impedance to be approximately 4.3kΩ using the lab
maual’s method. When we set the extra resistor to a value equal to the input impedance, our
voltage across the capacitor would be ½ that of the input signal. Figure 6.6a shows our value of
4.3kΩ, and Figure 6.6b shows that we have ½ the input voltage across the capacitor.
5.0ms
Part 7
Theoretical Experimental Percent Error
V1 (V)
2.75
5.12
Resistance (Ω)
1490
1500
0.67%
V2 (V)
1.375
2.56
Zo (Ω)
1490
1500
0.67%
PSPICE RESULTS FOR PART 7
Vout (open circuit value) = 2.75017 (RL = 100MΩ)
Figure 6.7a: PSPICE Model
Figure 6.7a shows our open circuit output of 2.75V across the increased (100MΩ) load
value. Our goal was to find a load value that would give us an output voltage approximate ½
that of the open circuit voltage. We found that a 1.49k resistor gives us the ½ open circuit
voltage we wanted. Therefore the output impedance of the circuit is approximately 1.5k, which
is exactly the value we achieved in the physical experiment.
Conclusion
In this laboratory experiment we modeled a common emitter amplifier with
little errors and ease of design. The most important factor to the beginning of this
laboratory experiment was to ensure that the proper curve tracer values were
found. In addition, the ten steps were very helpful in designing the circuit in a fast
manner. In real world applications people find common emitters to be an integral
part of the basic amplifier. They are the most commonly used and easiest to
calculate. In future experiments, the laboratory manual will take the student
through more complicated amplifiers that require a bit more skill to design.