BJT Characteristics

ECE 2210
Basic BJT circuits
Lab 7
The purpose of this laboratory is to introduce the Bipolar Junction Transistor (BJT) as a new circuit
element. The BJT is a current amplifier which takes current from the BASE pin and outputs a higher
current through the COLLECTER and EMITTER pins. In this lab we will use the 2N3904 which is an NPN
transistor. We will start by parameterize the BJT in the Tektronic 571 Curve tracer. From the curve
tracer we will acquire the Large Signal Current gain (hFE or ) and the Early Voltage (VA). The values will
be incorporated into our SPICE and hand analysis. In the figure below is an example of the graph you
should see from the curve tracer. The x-axis is VCE and the y-axis is the collector current. The linear
horizontal region of the graph is the active region of the transistor. At the bottom of the graph is the
Large Signal Gain (hFE or ). The Large Signal Gain will vary slightly over the different regions, which is
shown in the bottom left corner for each base current.
IB=50A
IC
Active Region
IB=30A
VCE
Notice the horizontal region has a slope which becomes more pronounced as Ib increases. Tracing the
slope to where the line would intersect the x-axis yields the Early Voltage (VA). The Early voltage deals
with a particular property in AC we will discuss later.
Pre-Lab: DC Biasing Circuit
In the laboratory you will construct two different circuits. The first circuit is the DC biasing circuit which
sets the currents IC, IB, and IE for the transistor. For the circuit we have very particular values we want to
set.
Ic
VCE

4.58mA
4.5v
200

Solve for the resistors R1 and RC based on the table above.

Recreate the circuit in SPICE and run an operating point analysis (.op) to
determine the currents and voltage of the pins.
1
ECE 2210
Basic BJT circuits
Lab 7
Pre-Lab: Current Mirror
The second circuit we will examine is the Current Mirror, which is a way of controlling current. The
current mirror is controlled by a Reference Resistor called Rref. The resistor allows us to draw a constant
current through the second transistor which is connected to another circuit. For this laboratory we will
use a load resistor to examine the effects of varying the load resistance on the current mirror.

Solve for the Reference Resistor (Rref) needed to set the current through the Load Resistor (RL) to
10mA if  is 200 and V+ is 10v.

Build the Current Mirror in SPICE using the Rref from your calculations. Run an operating point
analysis on the circuit using RL =1.

The load resistor has an effect on the output. Change the load resistor to {R} and use the
following:
.step param R 1 5k 10
This will run operating point procedures starting at 1 to 5 k in steps of 10. Place Load
current on the y-axis. Notice the relationship between load resistance and load current.

Solve for the linear slope of the line before the current drops off.
End Pre-Lab
2
ECE 2210
Basic BJT circuits
Lab 7
Procedure: Parameterization
The Tektronix 571 curve tracer will vary the base current and voltage VCE and generate a number of
different curves for the 2N3904. As the base current increases the Current IC varies.





Setup the curve tracer as shown above and press START.
Once the traces reach the end of the graph press STOP.
Press the CURSOR button and examine the values of hFE shown in the bottom left corner of the
screen.
For IB = 20A and VCE = 4.5v record IC and hFE. Press COPY to obtain a printout for your
notebook.
Place the cursors at the top most IB curve. You can move the 2nd cursor by pressing the CURSOR
button again. Separate the cursors across the linear horizontal region. Record VCE and IC. We will
need these values to solve for the Early voltage (VA).
3
ECE 2210
Basic BJT circuits
Lab 7
Procedure: DC Biasing Circuit
For this part of the laboratory you are going to construct the circuit from the pre-lab using the closest
resistors you can to match your calculations. Check the Q-point currents for IB, IC, and IE along with the
pin voltages VB, VC, and VE.





Construct the pre-lab circuit using the resistor values calculated from the pre-lab. Use the
closest possible values.
Set the voltage on the DC supply to 10v and limit the current to 50mA.
Turn on the DC supply voltage.
Measure the voltages:
VC
VB
VE
Measure the currents through the transistor.
IC
IB
IE
Procedure: Current Mirror
This procedure involves current mirror from the pre-lab. Varying the load resistance will cause the
current through the second transistor to change from and ideal value. Think of the load resistor as the
Thévenin’s resistance from a more complex circuit.




Construct the Current Mirror shown in the pre-lab
Start with a 10W resistor for the Load.
Set the voltage on the DC supply to 10v and limit the current to 50mA.
Measure the current Iref to ensure your circuit is working properly.

Iref = ________________________
Next measure the load resistor current. Keep increasing the load resistance by changing
resistors. Remember to turn off the DC supply when swapping resistors.
Load Resistor
10 
100 
330 
680 
1 k
2.7 k

Current
Turn off your supply and return the resistors to their proper bins.
End Procedure
4
ECE 2210
Basic BJT circuits
Lab 7
Analysis: Parameterization


What is the Large Signal Current Gain for the transistor at IB = 20mA?
The Early Voltage is found for by solving the equation of a line and looking for the x-intercept.
Solve for the slope (m) and y-intercept (b).
VA = ______________
Analysis: DC Biasing Circuit

For your Simulated and Measured Currents solve for the %Error from your hand calculations.
%Error Simulated IC
%Error Simulated IB

For your Simulated and Measured Voltages solve for the %Error from your hand calculations.
%Error Simulated VC
%Error Simulated VB
Analysis: Current Mirror

Using your simulation as an ideal compare your measure currents and resistance.
Load Resistor
10 
100 
330 
680 
1 k
2.7 k
Simulated Current
Measure Current
% Error
5

Download Report