EE 2212
EXPERIMENT 8
29 November and 6 December
BJT CURRENT SOURCES AND EMITTER-COUPLED PAIR
I will not be collecting a
report for this two-week experiment. I
will review your notebooks in lab on 13 December, Thursday, with a particular
focus on reviewing your results from Experiment 8. Also, there are topics in Experiment 8 which
may show up on the Final exam.
PURPOSE
The purpose of this experiment
is to characterize the
properties of a:
Ψ Basic/Simple Current Source
Ψ Widlar Current Source
Ψ The emitter-coupled pair (DC transfer
characteristics and AC gain measurements).
COMPONENTS
Ψ LM3046 transistor array. The data sheet is posted on the class WEB
page
Ψ Resistors and potentiometers as required
for the current sources.
Ψ Three 20 kW resistors for the collector resistors of
which two should be reasonably well
matched
Ψ 4.7 kW resistor for the input voltage divider
Ψ 47 W resistor for the input voltage divider
PRELAB FOR THE CURRENT SOURCES
Compute the values of the
resistors you will need to evaluate the simple and Widlar
current sources at the indicated current levels.
GENERAL INFORMATION
Ψ In IC biasing networks, it is essential
that transistors be well matched and parameter variations track with
temperature. Figure 1 is a pin out of the
LM3046 Transistor Array. Observe that you MUST connect Pin 13, the IC substrate, to the most
negative point in the circuit or bad things happen to the IC.
Ψ The only reason there is a fixed 10 kW resistor in the circuit is to protect the
BJT against inadvertent application of a high voltage across the Base-Emitter
junction as you adjust the potentiometer.
You do not want to apply 15 volts to the base of Q1 because the chip
becomes toast (literally and figuratively)!!!
Effectively, the series combination of the 10 kW resistor and the potentiometer is the RREF.
Figure 1 LM3046 NPN BJT ARRAY
SIMPLE CURRENT SOURCE
Figure 2 is a schematic diagram
of a simple current source.
Connect the collector of Q2, (VC2)
to a 5-volt DC supply. Place a DMM in series with the Q2 collector lead to
measure current. Set IC2=IX to 1 mA. Compare this value to the reference
current. Measure all
key currents and voltages. Construct the I-V output characteristic by
changing VC2 from 0 to 5 volts. Obtain
the output resistance from the slope. Compare to a SPICE simulation.
WIDLAR CURRENT SOURCE
Figure 3 is a schematic diagram
of a Widlar current source.
For a reference current of 1 mA, compute the value of R2 required to obtain Ix
= 100 mA ±10%.
Note that VCC = 15 volts. Now connect the collector of Q2 (VC2) to a 5-volt DC
supply. Place a DMM in series with the Q2 collector lead to measure current.
You may have to change the value of R2 from the computed value to come within 100 mA ±10% .
Measure all key currents and voltages. Sketch the I-V output
characteristic from VC@ from 0 to 5 volts.. Compare these results with the
simple current source results. You will
have to measure carefully because the slope will be close to flat as you would
expect. Compare to a SPICE simulation.
.
PRELAB FOR THE EMITTER-COUPLED PAIR
Use Figure 4 and class notes for guidance to
prepare a detailed circuit diagram. Include pinouts
for the LM 3046 npn
array. From your circuit diagram and circuit specifications, calculate the
expected important Q-point
values, and Adm,
Acm, and the CMRR in dB.
DC MEASUREMENTS
Refer to the diagram and data
sheet of the LM 3046/CA3046 BJT array.
Set up the circuit in Figure 4 using Q1 and Q2 for
the emitter-coupled pair. Q3 and Q4 form a simple current source. Ground both the inputs of Q1 and Q2. Measure
the all Q-point voltages and currents using the DMM. Use the oscilloscope to also check for
excessive noise which may translate as a noisy dc voltage measurement. Pay particular attention to VOD.
Since the transistors and resistors are reasonably well matched, you would
expect VOD = 0 or reasonably close. If VOD is larger than
a few tens of mV, check your circuit and/or match the collector resistors
better. Lead dress and length is also
important. Be neat! Compare your Q-point values with the expected
and PSPICE simulations. In addition to using the DMM, look for excessive noise using the
scope even though you are measuring the dc voltage matching.
Figure 4
TRANSFER CHARACTERISTICS
The transfer characteristics of
a circuit can be displayed using the X-Y oscilloscope inputs. The amplitude of
the input must be large enough to drive the input through the entire desired
range of operation. You are particularly interested in the VOD
versus VID characteristic. Use a low frequency sinusoid or
triangular wave as the input. From a practical viewpoint, if the input signals
are noisy because of low amplitudes, you may choose to use an input voltage
divider to provide "cleaner" waveforms. Consider implementing the
100:1 voltage divider input drive circuits, Figure 5, although it doesnt have to be
100:1. The signal generators have a 100
mV minimum. By using a 100:1 external
divider, you can achieve a relatively noise free signal at the input to the BJT
bases. Keep track of the divider ratio
you finally use to scale your measurement correctly. Also observe that because
the oscilloscope does not have a floating input (i.e., one side of each
oscilloscope input is connected to ground), you will have to measure either VO1
or VO2 and scale the final results accordingly by a factor of
2 and also do not forget the sign (180°phase) differences for each of the outputs.
Show that the slope of the
transfer characteristic will be equal to |Adm/2|.
Compare your results to a SPICE simulation.
Figure 5
DIFFERENTIAL-MODE OPERATION
Set up your input signals, use 1 kHz, so that the output is reasonably linear.
You will need some level of voltage division as shown in the figure. The figure illustrates a 100:1 divider but
the actual divider value is not critical.
Use the oscilloscope and DMM to measure the differential-mode voltage
gain. Compare your results to your calculations and a SPICE simulation. Include the effect of a non-infinite Early
voltage to improve your analysis and simulation accuracy.
COMMON-MODE OPERATION
Set up your input signals,
again use 1 kHz., so that the output is reasonably linear.
You will not need an input voltage divider because the Acm is low, Figure 6,
because the common-mode voltage gain is sufficiently low that you will
have to increase the input level significantly above what you used in the
differential-mode measurement. That is do not use the 100:1 input divider. Prepare a circuit diagram illustrating how
you are measuring the common-mode voltage gain.. Measure the common-mode voltage gain and
compute the measured CMRR and convert to dB. Compare your results to your
calculations and SPICE simulations.
Figure 6
If you decide to pursue
a BSEE degree, you should at least understand the basics. Above and beyond CS1, the following provides
an important understanding of computer technology hardware.
And for
those of you with an internship this summer:
And
another sick math joke. By the way a
subtle math error in the cartoon; the angles of 30, 60, and 90 degrees the
student marked are not correct.
