Page 1 of 5
Name: ______________________________
ECET 242 – Electronic Circuits
Lab 5
Transistor Measurements: Cutoff, Active, Saturation
Objective:
Students successfully completing this lab exercise will accomplish the following
objectives:
1. Learn to construct a biasing circuit for a Bipolar Junction Transistor (BJT)
2. Measure Bi-polar Junction Transistors (BJTs) in various modes of operation
and observe the characteristics of the biasing circuit.
Lab Report:
An informal lab report will be required for this lab. Include all collected data,
typed responses to discussion questions, and graphs generated using a
computer-based application. The report will be due 2 weeks after the lab
exercise is carried out.
Equipment:
Oscilloscope
Dual DC power supply
2N3904 NPN transistor (1)
470 Ω and 47 kΩ resistors
Digital Multimeters (DMM) (3)
Breadboard
Connecting leads
Jumper wires
Procedure 1: Setup
1.
Connect each source from the dual power supply to the breadboard with DMMs connected
to measure the source currents.
Dual DC Power Supply
VBB
+ -
VCC
- +
DMM
DMM
Amps
Common
Amps
Va
Vb
Common
BreadBoard
Figure 1: Dual Power Supply Connections to a Breadboard
Page 2 of 5
2.
On the breadboard, construct the common-emitter transistor circuit of Figure 2.
Figure 2: Common-Emitter Transistor Circuit
Procedure 2: Cutoff Operation
3.
Set the base supply VBB to 0 volts and the collector power supply VCC to 10 volts. Measure
the collector, emitter, and base voltages.
Note: When measuring a transistor voltage such as VB, VC or VE, connect a fixed lead from
the common terminal of the DMM to the power supply ground and touch the positive probe
of the DMM directly to the appropriate transistor lead.
Record your results below. The emitter and base voltages should be very small (less than
1 mV) or 0 volts and the collector voltage should be close to 10 volts. If either
measurement is off, then there is something wrong with the circuit. In either case, check
circuit with the instructor before proceeding to the next step.
VB = ____________
4.
VC = ____________
VE = ____________
Measure the base and collector current.
IB = ____________
IC = ____________
Both the base and collector currents should be near 0 µA in the cutoff mode. If either
current exceeds 1 mA, there is something wrong with the circuit. In this case, check with
the instructor before proceeding to the next step.
Procedure 3: Active Operation
5.
6.
Make sure VCC is set to 10 V. Increase the base power supply voltage, VBB until the
collector voltage, VC is 8 V. Measure the collector and base currents, the base voltage, VB,
and the emitter voltage, VE.
VB = ____________
VE = ____________
IB = ____________
IC = ____________
Calculate the current gain, β, and the base-emitter voltage, VBE.
β = IC / IB = ____________
VBE = VB – VE = ____________
Page 3 of 5
For active operation, the current gain should be at least 100 and the base-emitter voltage
should be about 0.7 V. If the base-emitter voltage is about 0.7 V, but the current gain is
significantly smaller than 100, then the transistor may be positioned in reverse. In this
case, flip the transistor connections and go back to step 5. If the base-emitter voltage is
not close to 0.7 V, then there is something wrong with the circuit. Check the circuit
connections or check with the instructor before proceeding.
7.
If the circuit seems to be working correctly, record the results from steps 5 and 6 in Table 1.
8.
Change the base power supply voltage, VBB, while monitoring the collector voltage, VC.
Notice that as VBB increases, VC decreases. Take additional measurements of VB, IB and IC
for collector voltages of 6, 2, 1, 0.5, 0.3 and 0.25 volts. Record your results in Table 1.
9.
Use the following equations to calculate the current gain, β, the base-emitter voltage, VBE,
and the collector emitter voltage, VCE. Record your results in Table 1. The value of VE
recorded in step 5 may be used in the calculations.
β = IC / IB
VBE = VB – VE
VCE = VC - VE
VBC = VB - VC
For the bias of each PN junction, indicate F for forward or R for reverse. The BE junction in
a transistor is forward biased if the junction voltage, VBE ≥ 0.7 V, and is reverse biased if
VBE < 0.7 V. The BC junction in a transistor is forward biased if the junction voltage, VBC ≥
0.4 V and is reverse biased if VBC ≤ 0.4 V.
Table 1: Measured and Calculated Results in Active Operation
Measured
VCC
(V)
VB
(V)
VC (V)
10
8
10
6
10
2
10
1
10
0.5
10
0.3
10
0.25
Calculated
IC
(mA)
IB
(µA)
β
VCE
(V)
VBE
(V)
Bias
VBC
(V)
BE Jctn
(F/R)
BC Jctn
(F/R)
Mode
Page 4 of 5
Procedure 3: Saturation Operation
10.
Increasing the base current up to and beyond the point where VCE = 0.3 V will drive the
transistor into saturation. We would like to calculate base currents that will cause the
transistor to operate in saturation. Calculate values to fill in Table 2 for base currents which
are 1.5x, 2x, 3x and 5x greater than the base current when VCE = 0.3 V.
Table 2: Base Currents in Saturation Mode
Calculated Base Current
IB (mA)
Desired Base Current
IB
1.5 x IB
(when VCE = 0.3 V)
2 x IB
(when VCE = 0.3 V)
3 x IB
(when VCE = 0.3 V)
5 x IB
(when VCE = 0.3 V)
11.
Fill in the base current column of Table 3 with the four base currents you calculated in
Table 2.
12.
Make sure VCC is set to 10 V. Increase the base power supply, VBB until the base current,
IB, reaches the first value in Table 3. Measure the collector current as well as the collector
and base voltages. Record these results in Table 3.
13.
Repeat the previous step for the remaining base currents. Record these results in Table 3.
14.
Complete Table 3 for saturation operation of the transistor using methods similar to those
used to complete Table 1.
For the bias of each PN junction, indicate F for forward or R for reverse. The PN junctions
of a transistor are forward biased if the junction voltage > 0 V. They are reverse biased if
the junction voltage ≤ 0 V.
Table 3: Measured and Calculated Results in Saturation Operation
Measured
VC
(V)
VCC
(V)
10
10
10
10
IC
(mA)
Calculated
IB
(µA)
VB
(V)
β
VCE
(V)
VBE
(V)
Bias
VBC
(V)
BE Jctn
(F/R)
BC Jctn
(F/R)
Page 5 of 5
Questions:
1. In Step 8, when the transistor was in active operation, you may have noticed that as the
base power supply voltage, VBB, increases, the collector voltage, VC decreases. Use circuit
analysis to explain why this is so. Hint: Show cause and effect of increasing VBB on IB
which has an effect on IC which has effect on the voltage across RC, which has an effect on
VC.
2. The BE junction behaves like a diode. When the circuit was in active operation in steps 5
through 9, you may have noticed that as the base current, IB, increased, the base-emitter
voltage, VBE increased slightly. Use your knowledge of diode I-V characteristics to explain
why this occurred.
3. When the circuit was in cutoff operation in steps 3 and 4, the emitter and base voltages
should have been close to 0 V, while the collector voltage should have been close to 10 V.
Compute VB, VC and VE using circuit analysis of cutoff operation.
4. When the circuit was in saturation, you may have noticed that as the base current
increased, the collector current did not change very much. Explain why.
5. Combine the results from Tables 1 and 3 and plot β vs. VCE. Note the knee bend in the
curve. Identify regions of your graph that represent the various modes of operation
observed in this lab.