Download Lab 4: Bipolar Junction Transistors (BJT)

Department of Physics, Stanford University
Physics 105, Analog Electronics
Lab 4.12
Page 1
(Final Version)
Lab 4:
Scope Probes
Intro to Bipolar Junction Transistors (BJT)
Read: Meyer Chapter 5, sections 5.1 – 5.2.4 (pp 149-158)
PRELAB
Part 1.1
A function generator can be modeled as a Thevenin equivalent circuit: an ideal
AC voltage source, Vth, in series with an output resistance, Rth. The manual for
the HP/Agilent function generator lists its output impedance as 50. Sketch a
circuit showing how to measure this output impedance using an external load.
Show what you would measure and where and specify the size of load you would
use. Think Thevenin and voltage divider.
Part 1.2.
Your oscilloscope has a stated input resistance of 1M (megohm). As in Part 1a,
sketch a circuit to measure this Rin using an added resistance. Show what you
would measure; what resistance will you add, and where it will be in the circuit.
In addition, the scope input also has a 14pf capacitance (to ground) in parallel
with Rin, leading to a frequency-dependent Zin . And, just to make it more fun,
coax cables have capacitance. Your “grabber” coax has C ~30 pF/ft, also relative
to ground. Sketch the measurement circuit above as a divider including the input
R and C of the scope, the C of your coax cable, and a resistance you add to make
a divider. Predict the 3dB point of this input circuit.
To cancel the frequency effect of the scope and cable, you want to add some C to
make a divider with the C of the scope and cable, just like you made a divider
with your added R. Hint: remember the relation between C and Zc
Part
2.
For Lab Manual Exercise 4-6, use the “Current Source”
operating curve for the 2N3904 transistor (attached and on
Coursework) to estimate the (constant) value of IC using the
operating curve data plus the two basic transistor assumptions:
VBE = 0.6V and IC = IB. (Hint: you need not calculate IB explicitly; do
Kirchhoff’s loops from base-emitter and collector-emitter. You can use the
operating curves to estimate  and eliminate IB from your equations).
+15V
Ammeter
Load (Decade
Resistor Box)
+5V
470
2N3904
Use this value of IC to estimate, in , the maximum “load”,
shown in Fig L4.6 in the Lab Manual, that will cause the current
source to saturate. The purpose of a current source is to put out a constant
820
current regardless of load; the voltage across the load can vary. The max
Figure 1
Department of Physics, Stanford University
Physics 105, Analog Electronics
Lab 4.12
Page 2
load is when IC is no longer constant as the load changes (remember, the transistor saturates
when VCE 0).
Now replace the 820 resistor in the emitter leg with a 4.7k resistor and repeat
the IC estimate and the maximum load estimate.
Part 3.
Predict VCE and the voltage drop across the bulb when fully lit. See the circuit on
the sheet labeled “Switch” attached to this lab. This can be answered by inspection—no
calculation needed.
Department of Physics, Stanford University
Physics 105, Analog Electronics
Lab 4.12
Page 3
LAB—DAY 1
PART 1.
Impedance Measurements/Scope Probes:
(25 pts)
1.1.
Measure the output impedance of your function generator using the method outlined in your
prelab. you will be adding a resistance to make a voltage divider. Do this at 50 Hz and 1Khz. Is
there a difference?
Submit: a. sketch/explanation of how you made measurement
b. quantitative result of measurement.
1.2.
Measure your scope’s input impedance using your prelab method—
a.
Start with a low frequency, 50Hz (close enough to DC), measure the attenuation to get
Rin of the scope.
b.
Now increase the frequency. You will find the scope reading drops off with frequency.
Find this 3dB frequency (one data point, not a full Bode plot).
c.
Using your Prelab sketch of the input of the scope (1M || 14pF || 30pF/ft of the grabber
coax), explain the frequency dropoff.
d.
How much C should you add in your measurement circuit, and where, to offset the effect
of the scope and coax (, ie, make a divider)?
e.
Build this and document your improvement in the 3dB point.
f.
Explain this improvement in terms of your voltage dividers and impedances.
LAB—DAY 2
PART 2.
Meet the Transistor: DC Current Source
(25 pts)
Care and Feeding of Transistors


2.1
2.2
Use the powered breadboard, which has +15/-15/+5 voltages.
Insert the transistor leads straight into adjacent holes on the breadboard, and use wires to bring the
circuit to the transistor rather than vice-versa. ie, don’t mangle, bend, fold, spindle, mutilate the
leads.
+15V
Using the diode function on the DMM, check the two diode junctions on
a 2N3904 npn transistor: BE, and BC. Measure the forward voltage
drops. You should find BE > BC. The collector area is larger than the
emitter, which means a lower resistance and hence a lower voltage drop.
Use the curve tracer to generate a set of characteristic curves for your
2N3904 transistor; your TA will demonstrate. Use the following settings:
VCE = 10V, IC = 10mA. Compare these curves with the curve attached to
the back of this lab writeup. If necessary, relabel the axes on the printed
curves to match your transistor. (take a photo with your phone to have a
record of this in case you need this).
Ammeter
Load (Decade
Resistor Box)
+5V
470
2N3904
820
Figure 1
Department of Physics, Stanford University
Physics 105, Analog Electronics
Lab 4.12
Page 4
2.3
a. Build the circuit of Figure 1. Confirm your prelab calculation for the output current with your
load (resistor decade box) at 0 ohms.
b. Increase the load in 100 steps until the current source starts to break down, i.e. IC is no longer
constant. Do this step quickly and compare to your Prelab prediction.
c. Once you’ve found the maximum load, go back to 100 and take the following data at several
loads between 100 and your maximum load: IC, VCE, VB . Use one DMM with straight probes,
not a BNC-grabber cable, for the latter two measurements—move your DMM probe from one
transistor leg to the other.
d. Plot these data, as you go, on the transistor curve at the end of the lab. From this, confirm that
Kirchhoff’s law is satisfied in the collector-emitter loop while IC is constant, and explain what is
happening to the transistor when the current source breaks down.
e. Change the 820 resistor in Fig 1 to a 4.7k resistor, and repeat Steps d, e. You will have shown
how to extend the “compliance” (read: voltage range over which the circuit operates) of the
current source to larger loads.
Submit:
1. Sketch of the circuit, indicating which instruments were connected where.
2. Raw data and calculated data to add to curve tracer plot. Calculate  ( = hfe) for each data
point you show on your plot. Explain how you calculated IB.
3. Prelab calculations for predicted IC, and the maximum load. Compare to measured
values.
4. Transistor operating curves (curve tracer) with your actual data added to the plot.
Indicate where current source starts to break down (collector current no longer constant).
5. Explain what’s happening on the plot as the load is changed.
PART 3.
(10 pts)
Transistor Switch:
+5V
+15V
Here you will see a transistor operated in saturation, and used as a
switch. The usefulness of the transistor switch is that it can use low
power signals to turn on and off higher voltage/power loads.
Use a #1813 bulb, and connect it to 15V.
1813 bulb
3
While measuring VC and VB , vary the base resistance, R, until the
light goes out--this is where the transistor goes out of saturation. Plot
your data on the 2N4401 operating curve (appended to the lab).
1k
Submit:
a. Sketch of circuit, showing what you measured, how you inferred
any quantities.
b. Transistor operating curves (standard curves attached to lab)
with your actual data added to the plot. Indicate where light
Figure 2
bulb starts to dim, and where it’s entirely off.
c. Explain what’s happening on the plot. Compare range of IB with that of Part 2.
2N4401
Department of Physics, Stanford University
Physics 105, Analog Electronics
TRANSISTOR SWITCH
Lab 4.12
Page 5
Department of Physics, Stanford University
Physics 105, Analog Electronics
Lab 4.12
Page 6