Challenge: BJT Amplifiers

ECE 206 BJT Amplifiers
Challenge: BJT Amplifiers
In ECE 205, you were introduced to BJT
theory. Now, you will use the BJT circuit
(Figure 1) as a current amplifier to power the
DC motor.
Our motivation is to study the characteristics
of BJT and use BJT as a switch to turn on/off
the DC motor with the help of the STEMTera.
The lab has 2 parts:
• Collecting data (VBE,VCE, IB,IC) to produce
the circuit’s IV characteristic for your
2N3904 transistor (see Figure 2).
• Using the BJT amplifier to boost the STEMTera output current to drive your DC motor with the
circuit that is shown in Figure 3. You can set the output voltage of pin 9 as HIGH.


Prelab: (in Compass, to be submitted during lab)
• Suppose you had the above curve (Figure 2) along with the known circuit test setup (Figure 1). How
would you estimate from the graph BJT parameters: 𝑉𝐶𝐸𝑆𝑎𝑡 , and β?
• For example: for 𝑉𝐵𝐸𝑜𝑛 (which is marked and provided as an example), you can say that 𝑽𝑩𝑬𝑶𝒏 is
the point on the x-axis (𝑽𝑩𝑬 ) where 𝑰𝑪 starts to increase.
Last updated 10/11/2020
Challenges:
You will need LTSpice software to design and simulate the BJT circuit in Figure 1. A basic LTSpice tutorial
is provided in the website. For the circuit design, use a voltage source instead of a function generator
with the following parameters: function = SINE, DC offset = 0.5 V, Amplitude = 1 V, and Freq = 1 Hz. You
can find a BJT component by searching ‘npn’ from ‘Select Component Symbol’. Once you create a BJT,
right click →’Pick new transistor’ → ‘2N3904’. Also set Vcc = 9 V and label a node B as ‘Vbe’.
After you finish the circuit design, go to ‘Simulate’ → ‘Edit Simulation Cmd’. Next, select ‘Transient’ tab
and make Stop time = 10, and Time to start saving data = 0. Right click on the horizontal axis and set
Quantity Plotted as ‘V(Vbe)’. Finally, right click on the center screen → ‘Add traces’ → I(R2). You should
now see an IV curve like the one in Figure 2. Record VBE,ON, the threshold at which Ic starts increasing.
Find an amplification factor (β) of the BJT by changing an AC voltage source to DC voltage source at
VBE,ON. Now, select ‘Edit Simulation Cmd’ → ‘DC op pnt’, which displays current values for Ic and Ib. The
amplification factor is then Ic/IB.
The second part of the lab is to turn on the DC motor by supplying a small current at the base of the BJT.
You will build a motor circuit as shown in Figure 3 on STEMTera. Set a pin to output a logic 1, like what
you did for the LEDs. Make sure that you can control the motor from the STEMTera board.
In summary,
• Plot IV characteristic as shown in Figure 2. Determine VBE,ON.
• Find β (Ic/IB) when VBE = VBE,ON
• Estimate the BJT transistor’s parasitic power when running the motor. You can assume the motor to
be 30 Ohms, and the transistor to be in saturation.
Report Deliverables:
• Provide descriptions with accompanying diagrams of your circuits and experimental setup; include
an annotated schematic of your experimental setups by using Fritzing.
• Include IV characteristic plot and describe how you can use this behavior to make BJT circuit as a
switch.
• Report the motor power requirement and amplifier power dissipation.
o What impact would the amplifier’s power loss have on your motor’s performance?
References:
• Chapter 16, Digital Voltmeter (DVM), Keysight InfiniiVision 3000 X-Series Oscilloscope User’s
Guide, 75019-97073, April 2013. http://literature.cdn.keysight.com/litweb/pdf/75019-
97073.pdf
• ECE 205 Lecture notes, lessons 25-27.
• ECE 110 common emitter BJT circuit,
https://courses.engr.illinois.edu/ece110/su2017/content/courseNotes/files/?commonEmitterBJ
TCircuit