Download Transistor Introduction, Simulation and optional

Salt Lake Community College
Electrical Engineering Department
EE1010
Introduction to Transistors using Simplified Circuits
Original: 5/7/13 Harvey Wilson, James Quebbeman, Lee Brinton
Revision 7/13/13 Harvey Wilson
Latest revision 8/20/13 by James Quebbeman
Introduction
Of all the technological advances of the 20th Century, few had greater impact than
the development of the transistor. First documented in 1947, credit for the invention of
the transistor is generally given to three Bell Laboratories engineers – John Bardeen,
William Shockley, and Walter Brattain. This breakthrough introduced the semiconductor
revolution. Just about every electrical device or automatically controlled system today
includes transistors. This lab will demonstrate the use of transistors in typical
applications – controls or switches, digital logic, and current or voltage amplification.
The effect of frequency and loading is included and measured.
Objectives
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Use a transistor to control a lamp (as a switch)
Use a transistor to perform digital logic functions (as a switch adapted for logic)
Use a transistor to amplify smaller current into larger current (gain)
Use a transistor to amplify current at various frequencies (frequency and loading
effects)
Equipment







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2 – 2N3417 or 2 – 2N3904 NPN Transistors (or equivalent)
1-100Ω, 1-390Ω, 2-1kΩ, 1-10kΩ resistors
1– 10kΩ variable resistor (may be used as a load for final circuit)
3 LED’s of any available colors (light visible)
1 IR LED (light not visible)
DMM
Scope
Function Generator
5 Volt source
Discussion
Transistors are three terminal semiconductor devices. There are two general types
of transistors:
1. Bi-junction Transistors (BJT’s)
2. Metal Oxide Semiconductor Field Effect Transistors (MOSFET’s).
Page 1 of 8
Although they differ fundamentally in the way they operate, both types perform
similar functions. Unique characteristics of each technology determine which type of
transistor to use. This lab studies BJT’s.
The three terminals of a BJT are referred to as the Base, Emitter, and Collector.
Figure 1 shows the location of the connections on the transistor we will use in our lab.
Many transistors have the terminals in locations other than the ones shown here.
Figure 1: 2N3417 NPN Transistor
Terminal Locations and Schematic Symbol
One way to consider the operation of a transistor is to think of it as an electrically
controlled switch or valve that can be gradually enabled to allow continuously more
current through the switch until it is completely conducting. For our purposes, we will
consider current flowing into the collector and out the emitter. The current (or voltage)
applied to the base determines the amount of current flowing. Thus, the transistor can
be in one of three states:
1. The “cutoff” or “off” state (a region of operation where no current flows through
the transistor).
2. The “linear”, “active”, or “partly-on” state (the region where the current flowing
through the transistor is a scaled value of the current fed into the Base).
3. The saturation or “on” state (the region where the transistor switch is fully
enabled allowing maximum current to flow).
The base voltage (or current) of the transistor determines how far the “valve” is
enabled. In other words, the Base voltage (or current) controls how much current can
flow through the transistor. It takes approximately 0.6 Volts applied between the Base
and the Emitter (VBE) to get the transistor to start conducting current. After that
threshold voltage is attained, very small increases in VBE result in large increases in
current flow.
Pre-lab
The transistor, when used in a voltage divider, has three sets of resistance values
controlled by the voltage and current to the base terminal of the transistor. These three
relative values are called “large”, medium”, and “small” and correspond to the transistor
states of “cutoff”, “active”, and “on” respectively.
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Given VCC = +5 Volts in Figure 2: Transistor as Controlled Voltage Divider,
estimate VOUT voltage for each of these three R transistor conditions.
1. large >> R1
2. medium or about = R1
3. small << R1.
Figure 2: Transistor as Controlled Voltage
Divider
Experiment
1. Transistor as a Control or Switch.
a. Connect the circuit shown in Figure 3 using a jumper wire to act as a
switch and a lamp or LED (provided by lab instructor) as the indicator.
Use a fixed approximately 5 Volt DC source such as the BK power supply.
Observe that closing the switch activates the transistor, enabling current
flow through the indicator, illuminating it, switching it to the “on” condition.
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R2
39 0ohm
V1
5V
LED1
LED_r e d
J1
R1
Ke y = Spac e
Q1
2N3904
1k ohm
Figure 3: Transistor as Control
2. Digital logic gates: NOT, AND and OR gates
As mentioned earlier, transistors are the basic building block of digital
computing devices. In this role, transistors operate as voltage controlled
switches.
Digital circuits operate at two voltage levels. The low voltage level (usually
something close to 0 Volts) represents a binary “off” or a “0”. The high voltage
(in our case something close to 5 Volts) represents a binary “on” or a “1”. The
basic logical operations of NOT, AND, and OR are easily accomplished using
transistors. The NOT gate (a.k.a. INVERTER) simply changes the state from 0
to 1 or 1 to 0. The AND and OR operations are defined by their Truth Tables
below:
Table 1: AND Truth Table
A
B
A&&B
0
0
0
0
1
0
1
0
0
1
1
1
Table 2: OR Truth Table
A
B
A||B
0
0
0
0
1
1
1
0
1
1
1
1
In this lab, we will represent the state of the output gate with an LED. A
binary “0” will be represented with the LED “off”. A binary “1” will be represented
with the LED glowing.
a. Construct the NOT gate shown in Figure 4. Verify the operation by
demonstrating when the input is “0” (i.e. the switch is open) the output is a
“1” and the LED glows. Closing the switch (applying a “1” to the input
results in a “0” output and the LED is “off”.
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R2
39 0ohm
V1
5V
J1
R1
Ke y = Spac e
Q1
LED1
2N3904
LED_gr een
1k ohm
Figure 4: Transistor as NOT Gate
b. Construct the AND gate shown in Figure 5. Verify the operation of the
gate by testing each of the four possible combinations of inputs (open and
closed switches SWA and SWB) and demonstrating the correct output.
R2
39 0ohm
LED1
LED_r e d
V1
5V
SW A
Ke y = A
SW B
Ke y = B
R3
Q2
2N3904
1k ohm
R1
Q1
2N3904
1k ohm
Figure 5: Transistor as AND Gate
c. Construct the OR gate shown in Figure 6. Verify the operation of the gate
by testing each of the four possible combinations of inputs (open and
closed switches SWA and SWB) and demonstrating the correct output.
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R2
39 0ohm
LED1
LED_r e d
V1
5V
R3
SW A
Ke y = A
1k ohm
Q1
R1
SW B
Q3
2N3904
2N3904
Ke y = B
1k ohm
Figure 6: Transistor as OR Gate
3. Current Gain
a. Connect the circuit shown in Figure 7. Measure the current passing
through the Base of the transistor through R1 and the current passing
through the Collector of the transistor through R2 by measuring the
voltage changes, then using Ohms Law to calculate the currents. Notice
how much larger the Collector current is than the Base current. The Gain
of this transistor circuit is the ratio of Collector current divided by Base
current. Gain values of between 50 and 200 are normal.
XM M 2
R2
10 0ohm
V1
5V
LED1
LED_r e d
XM M 1
J1
R1
Ke y = Spac e
Q1
2N3904
10 kohm
Figure 7: Transistor as Current Amplifier (Gain)
Page 6 of 8
4. Current Gain Affected by Frequency and Loading
a. Connect the circuit shown in Figure 8. Observe the voltage waveform
displayed on the scope. Adjust the Function Generator and Scope
controls to about as shown in Figure 9 to obtain a smooth sine wave
display at about 10kHz. Observe how the size and shape of the output
waveform degrades as frequency is increased near 5MHz. If the LED
indicator may be interfering, placing a jumper across the LED may help.
The use of a probe, first in the 1x setting, then in the 10x setting may help
identify whether the transistor or the probe or both caused the upper
frequency limitation. The probe in the10x setting provides less loading.
XSC1
G
R2
A
B
T
10 0ohm
V1
5V
LED1
LED_r e d
XFG 1
R1
Q1
2N3904
10 kohm
Figure 8: Transistor Current Gain Affected By Frequency
Page 7 of 8
Figure 9: Setup for Figure 8
Conclusions
Record any final observations and check off as usual.
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