EE 312
Electromagnetic and Electronic Devices Lab
Spring 2009
Name_____________________________
Lab 2 - Transistor Current Source
Wednesday, January 28, 2009
Lab Report Due Friday, February 6, 2009
Purpose:
To measure and analyze the characteristics of current sources.
To measure and analyze the characteristics of a voltage divider used for biasing transistor
circuits.
Equipment:
Dual DC power supply
2N3904 NPN transistor
300 Ω, 1 kΩ, and 6.8 kΩ resistors
10 kΩ potentiometer
Two Digital Multimeters (DMM)
Breadboard
Wire Jumper Kit
Connecting leads
Part I – Construct Current Source Circuit
1)
□ On the breadboard, construct the voltage divider transistor circuit shown in figure 1. Add an
ammeter to the circuit to measure IC, the current in the collector resistor. Use a collector resistor of
RC = 0 Ω, an emitter resistor of RE = 300 Ω, and voltage divider resistors of R1 = 6.8 kΩ and R2 = 1
kΩ. If not done previously, sketch the circuit layout on a breadboard layout sheet.
Figure 1 – Voltage Divider Transistor Circuit.
2)
□ Turn on the power supply and set the voltage to 20 volts and observe the ammeter. There should be
some current in the mA range. Measure the transistor voltages and record below:
VB = __________ ,
3)
VC = __________ ,
VE = __________
□ Compare the measured VB and VE above with your Prelab calculations. If they are way off, then
you will need to debug the circuit. Here are some suggestions for debugging the circuit: The high
sides of RC and R1 should both be about 20 V. So you can use a DMM set up as a DC voltmeter to
check these test points. The low sides of R1 and RE should be connected to ground, so the voltages at
these points should both be 0 V. You can use a voltmeter to check these test points. If any of these
four voltage test points are off, then there is probably a bad connection somewhere. To find a bad
EE 312
Electromagnetic and Electronic Devices Lab
Spring 2009
connection, use the voltmeter to trace the faulty test point back to the power supply. If this still does
not help, ask instructor for assistance.
4)
□ Now that the circuit is operation properly, record the current source output current, IC.
If circuit
connections have changed since step 3), then re-measure the transistor voltages and record in step 3
above.
IC = __________ mA
5)
□ Have the instructor check the circuit:
Instructor Signoff ______________
Part II – Testing of the Current Source
A current source should be able to deliver a fixed current over a range of load resistance. In this section,
the BJT current source is tested to determine the range of operation and % regulation of the current source.
6)
□ Perform measurements of VB, VE, VC, and IC for values of RC ranging from 0 to 6.8 kΩ.
Fill in
Table 1 with these measurements.
7)
□ Calculate VRc, VBE, VBC, and VCE for each value of RC.
8)
□ Insert the 1kΩ resistor for RC and apply a hot soldering iron to the transistor and observe the
Fill in Table 1 with these calculations.
collector current.
Does the current increase or decrease when the transistor is heated? ___________________.
When you remove the soldering iron, does the current increase or decrease? ___________________.
9)
□ Pull out the transistor and measure the voltage at the base test point.
VB with transistor removed = __________ ,
Calculate the percent change in VB between no transistor and transistor in with RC = 1kΩ.
% change = _____________
EE 312
Electromagnetic and Electronic Devices Lab
Spring 2009
Table 1 – Measured and Calculated Results of Current Source
Measured
VCC
RC
VB
VE
VC
20
20
20
18
20
16
20
14
20
12
20
10
20
8
20
6
20
4
20
1
V
Ω
V
V
Junction
Bias
Calculated
V
IC
mA
PD
PD
mW
mW
VRC
VBE
VBC
VCE
V
V
V
V
BE
BC
ON or
OFF
Questions for lab report:
1. In step 8, you heated the transistor with a soldering iron and observed the collector current.
Recalling from your knowledge of the properties of semi-conductors, what is the effect on
conductivity when a semiconductor is heated?
What effect will this have on the transistor currents in a transistor circuit?
2. Look up a term called “avalanche breakdown” in the text or on the internet and explain what
happens inside a transistor is driven when a transistor is over-heated by excess currents.
EE 312
Electromagnetic and Electronic Devices Lab
Spring 2009
3. From the data in Table 1, for which RC values was the transistor in saturation?
For which RC values was the transistor in active operation?
Would you say that the transistor is a more stable current source in active mode or in saturation?
4. From the data calculate the current that is 10% lower than the maximum collector current. Record
calculation below.
90% of ICmax = ____________________ mA
From Table 1, select the VRC values that have an IC that is just above and just below the 90%
ICmax value. Record data below:
VRC below 90% ICmax = ___________ V
VRC below 90% ICmax = __________ V
Use linear interpolation to calculate the guaranteed voltage that will deliver a current within 10%
of the max current. Fill in the following sentence:
Based upon measurements and analysis of these measurements, the transistor current source will
deliver a current within 10 % of _________________ (ICmax) over a voltage range of 0 to
_______volts.
5. Plot a current source regulation curve consisting of measured IC values on the y-axis and
measured VRC values on the x-axis. Use Excel or sketch by hand. On the graph, draw a vertical
line that separates the active region from the region of saturation.
6. One key to building a transistor current source that supplies a relatively fixed current over a wide
range of output voltage is a stable base voltage. Given a base voltage, VB, and an emitter resistor
RE, derive an expression for the collector current in terms of β. From this equation, explain why a
big change in β will result in a very small change in IC, provided the base voltage remains
constant.
7. One way to have a stable base voltage is to create a voltage divider stable such that (β+1)RE is
much larger than R1, so that the parallel combination of (β+1)RE and R1 is approximately R1.
Assuming β = 150, calculate the ratio (β+1)RE / R1 for the circuit used in this lab.
(β+1)RE / R1 = _________________
Does this ratio suggest a stable base voltage?
EE 312
Electromagnetic and Electronic Devices Lab
Spring 2009
Answers
Table 1 – Measured and Calculated Results of Current Source
Fixed
Junction
Bias
Calcul
ated
Measured
VCC
RC
VB
VE
VC
IC
20
0
2.539
1.881
20
6.219
20
100
2.539
1.881
19.38
6.209
20
1k
2.537
1.864
13.95
6.160
20
2.2 k
2.527
1.829
6.874
6.065
20
3k
2.525
1.814
2.402
5.979
20
3.3 k
2.440
1.722
1.829
5.583
20
3.65 k
2.320
1.600
1.669
5.032
20
6.8 k
1.834
1.102
1.127
2.815
V
Ω
V
V
V
mA
VRC
VBE
VBC
VCE
V
V
V
V
BE
BC
ON or
OFF