Prelab 6: Biasing Circuitry
Name:
Lab Section:
VCC
R1
+
R2
VOUT
−
Figure 1: Resistive divider “voltage source”
1. Consider the resistor network shown in Figure 1. Let VCC = 10 V, R1 = 9.35 kΩ, and R2 = 650 Ω. We
can apply Thévenin’s equivalent circuit to this resistive divider to model it as an independent voltage
source with an output resistance. Find the open circuit voltage and the output resistance for this
resistive divider.
2. Now, consider a BJT voltage source such as the one shown in Figure 2. Pick the appropriate value for
RC so as to achieve an output voltage of 650 mV. Let VCC = 10 V, IS = 26.03 fA, and VT = 26 mV.
Ignore the Early effect for this calculation.
1
2
VCC
RC
IC
+
VOUT
−
Figure 2: BJT voltage source
3. Calculate the output impedance of this BJT voltage source and determine the power dissipated by the
circuit. Hint: Recall the definition of power, P = IV .
4. If we resized the resistors in the resistive divider (shown in Figure 1) to achieve the same output
impedance as the BJT voltage source, what should the values of the two resistors be so that the output
voltage stays the same? How much power will the circuit now consume?
VCC
R
IOUT
+
VOUT
RL
−
Figure 3: Resistor “current source”
5. Consider the circuit shown in Figure 3. Let VCC = 10 V and R = 10 kΩ. Roughly sketch IOUT vs.
VOUT (space is provided on next page). Note: The specific value for RL is not required for this analysis.
3
1
0.8
0.6
IOU T (mA)
0.4
0.2
0
−0.2
−0.4
−0.6
−0.8
−1
0
10
VOU T (V)
20
6. For the current source in Figure 3, find the short-circuit output current as well as the output impedance.
Note: RL represents a hypothetical load to the circuit; it can be replaced with an open-circuit for the
appropriate analysis.
7. For a given load, is it possible to increase the output impedance of the current source without decreasing
the output current and without changing VCC ? Explain.
c
University
of California, Berkeley 2008
Reproduced with Permission
Courtesy of the University of California, Berkeley and of Agilent Technologies, Inc.
This experiment has been submitted by the Contributor for posting on Agilents Educators Corner. Agilent has not tested it. All who offer or perform this experiment do so solely at their own risk. The Contributor
and Agilent are providing this experiment solely as an informational facility and without review.
4
NEITHER AGILENT NOR CONTRIBUTOR MAKES ANY WARRANTY OF ANY KIND WITH
REGARD TO THIS EXPERIMENT, AND NEITHER SHALL BE LIABLE FOR ANY DIRECT, INDIRECT, GENERAL, INCIDENTAL, SPECIAL OR CONSEQUENTIAL DAMAGES IN CONNECTION
WITH THE USE OF THIS EXPERIMENT.
Experiment 6: Biasing Circuitry
1
Objective
Setting up a biasing network for amplifiers often involves using transistors to build voltage and current
sources to deliver the right amount of biasing to the main amplifier components. In this lab, we explore how
to build these sources and fine tune their output values to achieve the optimal biasing environment.
2
Materials
The items listed in table 1 will be needed. Note: Be sure to answer the questions on the report as you proceed
through this lab. The report questions are labeled according to the sections in the experiment. CAUTION:
FOR THIS EXPERIMENT, THE TRANSISTORS CAN BECOME EXTREMELY HOT!!!
Component
100 Ω resistor
1 kΩ resistor
100 kΩ resistor
10 kΩ potentiometer
2N4401 NPN BJT
2N4403 PNP BJT
Quantity
1
1
1
1
1
2
Table 1: Components used in this lab
3
Procedure
3.1
Emitter Degeneration in Current Sources
In this section, we will examine the effect of emitter degeneration on a current source.
1. Build the circuit shown in Figure 1, setting VCC = 5 V and VBIAS = 4.35 V. As you adjust VBIAS , be
sure to check that VBE stays less than or equal to 0.65 V at all times; otherwise, your transistor may
BURN UP! (Warning: It is very easy to make a mistake and burn out a transistor. Therefore, it is
highly recommended that you check your values for VBIAS and VCC with the multimeter or oscilloscope
before connecting them to the transistor.)
2. Now measure the short circuit output current.
3. In terms of the small-signal characteristic(s) (e.g. gm , ro , β), what is the output resistance of this
current source?
4. Using ICS, sweep VOUT from 0 V to 5 V and plot IOUT vs. VOUT . What is the output impedance
at VOUT = 2.5 V? What happens to the output impedance as VOUT approaches 5 V? Explain your
observation.
1
3
2
PROCEDURE
VCC
VBIAS +
−
IOUT
+
VOUT
−
Figure 1: Transistor current source
VCC
100 Ω
VBIAS +
−
IOUT
+
VOUT
−
Figure 2: Current source with emitter degeneration
5. Now set up the circuit as shown in Figure 2 by adding a resistor to the emitter. Carefully adjust the
bias voltage, VBIAS , to maintain the same short circuit output current.
6. Using ICS, measure the output impedance for this modified circuit. Explain how the additional resistor
affects the output impedance. Note: Recall the output impedance is measured from the inverse slope
of the IOUT vs. VOUT curve, not the absolute values of IOUT or VOUT .
3.2
Current Mirror
In this part of the lab, we create a current mirror. The resultant copy current can be used to bias other
circuit elements and semiconductor devices.
1. Before building the circuit for this part of the lab, be sure to setup your potentiometer so that the
two leftmost terminals have the MAXIMUM possible resistance across them (which should be about
10 kΩ). Then, use these terminals for your initial RC . For assistance, please refer to Figure 3, which
illustrates a typical potentiometer. Please be absolutely sure that you use these terminals as the initial
resistance for your circuit; otherwise your transistor may BURN UP!
3
3
PROCEDURE
Figure 3: A typical potentiometer
VCC
QREF
+
VOUT
IREF
−
RC
Figure 4: Voltage source using a transistor
2. Now build the circuit shown in Figure 4, setting VCC = 5 V.
3. Adjust the potentiometer until IREF is approximately 2.2 mA. What is the value of RC at this point?
4. Perform load-line analysis by sketching the IREF vs. VOUT curves for both the transistor and RC . On
your sketch, indicate the fixed point solution for IREF . How should we adjust the potentiometer to
increase IREF (i.e. do we increase or decrease the value of RC )?
VCC
VCC
QREF
IREF
QCOP Y
ICOP Y
RC
1 kΩ
Figure 5: PNP current mirror
5. Now add an PNP BJT and a 1 kΩ resistor to your circuit so that it looks like Figure 5.
6. Measure the value of ICOP Y (the copy version of IC ).
3
4
PROCEDURE
7. Do ICOP Y and IREF match? If not, what are some possible reasons for this discrepancy? Hint: Begin
by considering the base currents of the BJTs.
3.3
Biasing a Common Emitter Amplifier Using a Current Mirror
A current mirror is simply a series of current sources biased by a voltage source (see Figure 6). This circuit
is useful for replicating a biasing current that is used in many places throughout your overall circuit design.
As shown in Figure 7, we can use a current mirror to bias a common emitter amplifier.
VCC
VCC
VCC
VCC
. . .
RC
Figure 6: General PNP current mirror
VCC
VCC
Q2
Q3
IC2
IC3
+
RC
+
Q1
vin
−
VIN +
−
Vout
−
RE
Figure 7: Biasing a CE amplifier with a current mirror
1. Before building the circuit for this part of the lab, be sure to setup your potentiometer so that the
two leftmost terminals have the MAXIMUM possible resistance across them (which should be about
10 kΩ). Then, use these terminals for your initial RC . Now, build the circuit shown in Figure 7, setting
VCC = 12 V, RC = 10 kΩ potentiometer, and RE = 100 Ω.
2. Use the DC offset from the function generator to set VIN to 760 mV and also apply a small signal to
the input. Approximately, what is the gain of the amplifier in Figure 7? Note: Recall that the actual
output from the function generator is doubled the amount on the display.
3. Use the function generator to apply a 20 mVpp , 1 kHz sine wave at the input and plot the output on
the oscilloscope. Because of the huge gain, the output waveform may be clipped. Attach a 100 kΩ load
3
PROCEDURE
5
resistor at the output (which should make the output no longer clipped) and measure the peak-to-peak
voltage across the resistor. Using the value of this voltage and the gain previously measured, calculate
the output impedance of the amplifier.
4. Qualitatively, how do the measured output impedance and gain compare against a common emitter
biased by a resistor?
5. Now, let us examine the effects of variations in biasing point:
(a) Adjust the potentiometer so that RC is slightly smaller. Observe how this affects the output
waveform. Please be careful to NOT burn up your BJTs while adjusting the potentiometer.
(b) Qualitatively, how does changing RC affect IC2 and IC3 (i.e. does decreasing the value of RC
increase or decrease IC2 and IC3 )? Based on your response, explain how changing RC changes
the output waveform?
c
University
of California, Berkeley 2008
Reproduced with Permission
Courtesy of the University of California, Berkeley and of Agilent Technologies, Inc.
This experiment has been submitted by the Contributor for posting on Agilents Educators Corner. Agilent has not tested it. All who offer or perform this experiment do so solely at their own risk. The Contributor
and Agilent are providing this experiment solely as an informational facility and without review.
NEITHER AGILENT NOR CONTRIBUTOR MAKES ANY WARRANTY OF ANY KIND WITH
REGARD TO THIS EXPERIMENT, AND NEITHER SHALL BE LIABLE FOR ANY DIRECT, INDIRECT, GENERAL, INCIDENTAL, SPECIAL OR CONSEQUENTIAL DAMAGES IN CONNECTION
WITH THE USE OF THIS EXPERIMENT.
Report 6: Biasing Circuitry
Name:
Lab Section:
1
Lab Questions
3.1.2 Measure the short circuit output current. :
IOUT =
3.1.3 In terms of the small-signal characteristic(s) (e.g. gm , ro , β), what is the output resistance of this
current source?
Theoretical Rout =
3.1.4 What is the output impedance at VOUT = 2.5 V? What happens to the output impedance as VOUT
approaches 5 V? Explain your observation.
At VOUT = 2.5 V, measured Rout =
3.1.6 Measure the output impedance for this modified circuit. Explain how the additional resistor affects
the output impedance.
ROUT =
3.2.3 Adjust the potentiometer until IREF is approximately 2.2 mA. What is the value of RC at this point?
1
1
LAB QUESTIONS
2
RC =
3.2.4 Perform load-line analysis by sketching the IREF vs. VOUT curves for both the transistor and RC . On
your sketch, indicate the fixed point solution for IREF . How should we adjust the potentiometer to
increase IREF (i.e. do we increase or decrease the value of RC )?
3.2.6 Measure the value of ICOP Y
ICOP Y =
3.2.7 Do ICOP Y and IC match? If not, what are some possible reasons for this discrepancy? Hint: Begin by
considering the base currents of the BJTs.
3.3.2–3 Properties of the CE amp with current mirror:
Av =
Rout =
3.3.4 Qualitatively, how do the measured output impedance and gain compare against a common emitter
biased by a resistor?
3.3.5 Qualitatively, how does changing RC affect IC2 and IC3 (i.e. does decreasing the value of RC increase
or decrease IC2 and IC3 )? Based on your response, explain how changing RC changes the output
waveform?
1
LAB QUESTIONS
3
c
University
of California, Berkeley 2008
Reproduced with Permission
Courtesy of the University of California, Berkeley and of Agilent Technologies, Inc.
This experiment has been submitted by the Contributor for posting on Agilents Educators Corner. Agilent has not tested it. All who offer or perform this experiment do so solely at their own risk. The Contributor
and Agilent are providing this experiment solely as an informational facility and without review.
NEITHER AGILENT NOR CONTRIBUTOR MAKES ANY WARRANTY OF ANY KIND WITH
REGARD TO THIS EXPERIMENT, AND NEITHER SHALL BE LIABLE FOR ANY DIRECT, INDIRECT, GENERAL, INCIDENTAL, SPECIAL OR CONSEQUENTIAL DAMAGES IN CONNECTION
WITH THE USE OF THIS EXPERIMENT.