Download Experiment #5 Report

Experiment #5:
Bipolar Junction Transistors
Friday Group
Dr. Somnath
10-12-09
Ari Mahpour
Teddy Ariyatham
Jayson Dela Cruz
Table of Contents
Objective: ........................................................................................................................................ 3
Tools: ............................................................................................................................................... 3
Theory: ............................................................................................................................................ 4
Discussion and Results: ................................................................................................................... 5
Conclusion: ...................................................................................................................................... 6
Objective:
The purpose of this laboratory is to learn the basics of the bipolar junction transistors.
BJTs may be used in various amplifier circuits such as the common emitter, base, and collector.
Each has their own various properties such as negative gain, positive gain, and neutral gain. The
BJT comes in to main forms: NPN and PNP. Each packaging type must correspond to its own
circuit diagram but they perform the same way functionally. This lab focuses on determining
the Q-point property Beta and the collector current Ic through various circuit models. By
modeling for specific values, a circuit can follow whatever specifications are asked.
Tools:
-
Oscilloscope
-
Functional generator
-
Power supply
-
2N2222A BJT
-
Resistors
-
Curve Tracer
Theory:
Each BJT has its own Q-point Beta, which can be found using a tool called the curve
tracer. Figure 5.1 shows an example curve tracer run on a BJT which is inserted into the tool.
One can select different settings to show the Q-point movement for a BJT.
Equation 5.1:
𝑅𝑏 =
Equation 5.2:
𝐼𝑐 =
Equation 5.3:
𝛽 =
1
1
1
+
𝑅1 𝑅2
𝑉𝑐
𝑅𝑐
𝐼𝑐
𝐼𝑏
: 𝑉𝑐𝑒 = 𝑉𝑐𝑐 − 𝐼𝑐𝑅𝑐 − 𝐼𝑒𝑅𝑒
Equation 5.4
The pre lab suggests preliminary calculations by finding the values ahead of time which suit the
specifications asked for. The goal is to receive a Vce of 8V and an Ic of 1mA. There are three
models which are required.
Figure 5.2a
Figure 5.2a, R1 is found to be around 2000k or basically 2M ohms. There is no bottom loop in
this equation so only R1 acts as an input resistance to the base of the BJT. R2 is found to be
5000 ohms. There is also no common emitter resistance used in Figure 5.2a. The Vcc for this
circuit was calculated to be 13V.
Figure 5.2b
Figure 5.2b, R3, the input resistance, is 2000k ohms or 2M ohms. R4, the resistance on the
collector side is 5000 ohms. This time there is a resistor, R5, on the common emitter side which
is 500 ohms. The Vcc found for this model is 13.5V.
Figure 5.2c
Figure 5.2c, R1 is 97000 ohms and R2 is 10000 ohms and both as the input resistance to the
base. They reduce down to a single Rb value when they are DC biased. Rc is 5000 ohms while Re
is 500 ohms. The Vcc used for this circuit is 13.5V.
Using the values mentioned under each circuit, the specification of Vce = 8 and Ic = 1mA should
be achieved. The goals for the circuits are to come close to each other for output values.
Discussion and Results:
The first procedure asked is to find the DC Beta values at various Ic measurements. Use
Figure 5.2C with Rb = 100KΩ and Rc = 1KΩ. There is no Re (Re = 0) for this case.
Ic (mA)
0.1
0.2
0.4
0.8
1.0
2.0
4.0
Measured Ic (mA)
.107
.201
.399
.804
1.01
2.0
3.99
Measured Ib (mA)
8.3x10-4
1.5x10-3
2.96x10-3
5.9x10-3
7.4x10-3
14.4x10-3
30.1x10-3
DC Beta
128.92
134
134.79
136.27
136.27
138.88
132.56
Figure 5.3
The main point of this procedure is to use Equation 5.3 to find the Beta values. The average
beta value from Figure 5.3 is 134. This is in contrast with the PSPICE Beta value which the
manual tells the user to set as 161. That means Beta is smaller for experimental test cases.
The second procedure is to find the Q-points using the three Figure 5.2 models. This means the
values determined in the prelab are used for the actual experimental circuits.
For Figure 5.2a, Ic was found to be .882mA and Vce was 8.59V. In contrast to the PSPICE of the
same circuit model, Ic was 1ma and Vce was 8V.
For Figure 5.2b, Ic came out to be .880mA and Vce was 8.65V. The PSPICE version of the model
found Ic to be 1mA and Vce was 8V.
For Figure 5.2c, Ic was 1.024mA and Vce was 7.85V. The PSPICE run gave an Ic value of 1mA and
Vce was 8V.
The main discussion for these two data sets is that the values used were suppose to come close
to Ic = 1mA and Vce = 8V. For the PSPICE sets, the resistors mentioned in the theory were exact
and the PSPICE found Ic and Vce exactly as requested. The experimental values which involve
actual circuit building came close as well. Figures 5.2a and 5.2b found Ic values below 1mA and
Vce values above 8V. Figure 5.2c found Ic above 1mA and Vce below 8V.
The relationship shown here is that if Ic is greater, Vce is smaller and vice versa.
Conclusion:
Bipolar junction transistors are one of the greatest inventions of the 20 th century. Its
creation has pushed technology to a whole new level of electronics, one of the biggest being
the computers we use today which have millions of tiny transistors placed into a very tiny area.
Being able to understand how transistors work should be the goal of every electrical engineer.
While engineers work on components that are much bigger, they still all contain the base
devices – transistors. For the purposes of this lab, learning how to manipulate them to give
certain values will come into importance on the next experiments which involve creating
“common” amplifier circuits. There the knowledge of the Q-point will be put to use when
determining how to find the gains of the circuits.