Lab #4
Transistors - BJTs
Due: Start of NEXT Class (Monday)
Name / SN:____________________________________
TA:
__________
Name / SN:____________________________________
TA:
__________
A) Pick up the following components from the TA:
1.
2.
3.
4.
2N3904 BJT (1)
2N3906 BJT (1)
100 resistor (2)
10K resistor (2)
B) Connect each BJT in the configuration shown in Fig 1 (NPN transistor shown)
with R1 = R2 = 10K. Set the signal generator to a 5Vpeak, 100Hz square wave.
Use one scope probe to measure the input signal, the other to measure the signal
across one 10K resistor, and the math function to subtract the two. Repeat for
both resistors.
TA: ____________________________________
1.
Which transistor is an NPN and which is a PNP? What is the associated collector/base
and emitter/base forward bias voltage drop for each.
Ans:
____________________________________
NPN
Ans:
____________________________________
cb / eb voltage drop
Ans:
____________________________________
PNP
Ans:
____________________________________
cb / eb voltage drop
ECE 251 - Circuit Analysis I
Department of Electrical & Computer Engineering, UBC
2.
How do you know which transistor is which? Hint: “Google”, is not the right answer!
Ans:
____________________________________
Explanation
C) Use the NPN BJT to build the circuit in Fig 2 with Rb = 10K, Rc = 100 and
Re = 100. Connect Vc to the DC power supply set to it’s maximum voltage
and current. Connect Vb to the signal generator with a 5Vpeak, 100Hz square
wave. Use one scope probe to measure the input signal (Vb), and the other to
measure the voltage across Re (Vre).
TA: ____________________________________
1.
Calculate the theoretical gain value (Vre/Vb). Assume  = 100 and use the forward voltage drop you measured above.
Ans:
2.
Theoretical gain
Calculate the measured gain value.
Ans:
3.
____________________________________
____________________________________
Measured gain
Compute the % error. Is this an acceptable value? Support your answer.
Ans:
____________________________________
% Error
Ans:
____________________________________
Explanation
4.
Adjust VC up and down by 5V. How does this affect Vre?
Ans:
5.
____________________________________
Use a piece of wire to short Rc. How does this affect Vre?
Ans:
____________________________________
ECE 251 - Circuit Analysis I
Department of Electrical & Computer Engineering, UBC
6.
Use the equivalent circuit model of the BJT to explain your answers in parts 4 and 5.
Ans:
____________________________________
Explanation
D) Remove the wire shorting Rc. Use both scope probes and the math function to
measure the voltage across Rc (Vrc).
1.
Calculate the theoretical gain value (Vrc/Vb). Assume  = 100 and use the forward voltage drop you measured above.
Ans:
2.
____________________________________
Measured gain
Adjust VC up and down by 5V. How does this affect Vrc?
Ans:
4.
Theoretical gain
Calculate the measured gain value.
Ans:
3.
____________________________________
____________________________________
Explanation
Is any part of the circuit affected when VC is adjusted up and down by 5V? If so, show it
using your scope. Hint: If you answer is “no” then Kirchhoff must have been wrong!
Ans:
____________________________________
ECE 251 - Circuit Analysis I
What voltage is affected?
Department of Electrical & Computer Engineering, UBC
Fig. 1
Fig. 2
+
vrc
_
+
vre
_
ECE 251 - Circuit Analysis I
Department of Electrical & Computer Engineering, UBC