Department of Electrical & Computer Engineering
94 Brett Rd • Piscataway • New Jersey 08854-8058
Professor Paul Panayotatos
332:364 Analog Electronics Laboratory
Laboratory Experiment II
Constant Current Sources
II.1
Introduction
Objectives
•
•
To study different designs of constant current sources
To demonstrate the utility of constant current sources as
active loads
Overview
This lab is designed to familiarize the student with the operation of two different designs
of constant current sources. In particular, the operation of a simple BJT current source
will be explored as well as the one of the so-called Wilson current mirror. The Wilson
mirror is an improved current source circuit with a more stable output resistance.
The use of a constant current source as an active load for a high gain common emitter
amplifier will also be examined. The gain of an amplifier stage is determined first with a
passive load and subsequently with a constant current source as its load.
The laboratory experiment is divided into four activities:
(A) The first activity involves the operation of a simple 2-BJT current-mirror constant
current source.
(B) The second activity involves the operation of an improved 3-BJT constant current
source (Wilson current mirror) that exhibits a more stable output resistance.
(C) The third activity involves the operation of a simple BJT single-stage CE
amplifier with a passive load.
(D) The fourth activity involves the operation of the same simple BJT single-stage
CE amplifier with a constant current sources as an active load.
The four actual laboratory experiments are designed to verify the concepts by direct
measurement of currents and voltages.
Some of the necessary theory is presented below and the prelab exercises are
designed to promote familiarity with the concepts.
Designed by M. Caggiano
Latest revision: 9/3/08 by P. Panayotatos and Steve Orbine
Analog Electronics Lab-II p.2/14
II.2
Theory
II.2.1
The Basic BJT current source1
Figure 6.9 Analysis of the current
mirror taking into account the finite !
of the BJTs.
Figure 6.10 A simple BJT current
source.
The basic BJT current mirror is shown in Fig. 6.9. Let us consider the case when ! is
sufficiently high so that we can neglect the base currents. The reference current IREF is
passed through the diode-connected transistor Q1 and thus establishes a corresponding
voltage V BE which in turn is applied between base and emitter of Q2. Now, if Q 2 is
matched to Q1 or more specifically, if the EBJ area of Q2 is the same as that of Q 1, and
thus Q2 has the same scale current Is as Q1, then the collector current of Q2 will be equal to
that of Q1, that is, I o= IREF. For this to happen, however, Q2 must be operating in the
active mode, which in turn is achieved so long as the collector voltage Vo is 0.3 V higher
than that of the emitter. To obtain a current transfer ratio other than unity, say m, we
simply arrange that the area of the EBJ of Q2 is m times that of Q1. In this case, Io= mIREF.
Next we consider the effect of finite transistor b on the current transfer ratio. The analysis
for the case in which the current transfer ratio is nominally unity -that is, for the case in
which Q2 is matched to Q1 - is illustrated in Fig. 6.9. The key point here is that since Q1
and Q2 are matched and have the same VBE their collector currents will be equal. The rest
of the analysis is straightforward. A node equation at the collector of Q1 yields
I REF = I C +
"
2I C
2%
= IC $ 1 + '
#
!
!&
Finally, since Io= IC the current transfer ratio can be found as
1
Adapted from Section 6.3.3, “Microelectronic Circuits” by Adel Sedra and Kenneth Smith, 5th Edition,
Oxford University Press, New York, 2004. Consult subsequent material as needed.
Analog Electronics Lab-II p.3/14
IO
I REF
=
IC
"
2%
IC $ 1 + '
#
!&
=
1
1+
2
!
Note that as ! approaches ", Io/IREF approaches the nominal value of unity. For typical
values of !, however, the error in the current transfer ratio can be significant. For
instance, ! = 100 results in a 2% error in the current transfer ratio.
The BJT mirror his a finite output resistance Ro,
RO =
!VO
V
= rO 2 = A
!I O
IO
where VA, and ro2 are the Early voltage and the output resistance, respectively. of Q2.
Thus, even if we neglect the error due to finite !, the output current Io will be at its
nominal value only when Q2 has the same VCE as Q1, namely at VO = VBE. As VO is
increased, Io will correspondingly increase. Taking both the finite ! and the finite Ro into
account we can express the output current of a BJT mirror (with m=1) as
IO =
I REF
2
1+
!
# VO " VBE &
%$ 1 + V
('
A
Finally, if the current IREF is taken in a simple manner as in Fig. 6.10 above, then
I REF =
II.2.2
VCC ! VBE
R
The Wilson Current Mirror2
At the cost of adding one more transistor an improved current mirror results, which
exhibits both reduced dependence of the transfer ratio on ! as well as increased output
resistance. The drawback, in addition to the cost of an extra device, is that an additional
VBE drop is required for its operation so that one must allow for about 1 V across the
Wilson-mirror output. The analysis is shown below right on the figure (Fig. 6.60) and
results in an output resistance of Ro=!ro/2 (for ! =100, 50 times as much as with the
2
Adapted from Section 6.12.3, “Microelectronic Circuits” by Adel Sedra and Kenneth Smith, 5th Edition,
Oxford University Press, New York, 2004. Consult subsequent material as needed.
Analog Electronics Lab-II p.4/14
simple mirror) and a transfer ratio
IO
=
1
2
I REF 1 +
! (! + 2)
ratio is 0.9998 or the error is 0.02% instead of 2%.
"
1
2
1+ 2
!
so that with ! =100 the
Figure 6.60 The Wilson bipolar current mirror: (a) circuit showing analysis to
determine the current transfer ratio; and (b) determining the output resistance. Note
that the current ix that enters Q3 must equal the sum of the currents that leave it, 2i.
II.2.3
The Current Mirror as an Active Load3
Figure 5.60 (a)
A common-emitter
amplifier with a
passive load of
RL||RC
3
Adapted from Sections 5.7.3 and 6.5.3, “Microelectronic Circuits” by Adel Sedra and Kenneth Smith, 5th
Edition, Oxford University Press, New York, 2004. Consult subsequent material as needed.
Analog Electronics Lab-II p.5/14
Figure 5.60 (b) Equivalent circuit obtained by replacing the transistor with its hybrid-# model.
Figure 5.60 (a) shows a simple BJT CE amplifier with a collector resistance RC and an
external load resistance RL. As is evident from Fig. 5.60(b), the combined load is RL||RC.
From the same figure it is obvious that
Rin=RB||r#≈ r# for the usual case of RB>>r#.
The open-circuit (i.e. with RL approaching ") voltage gain and the output resistance for
the usual case of RC<<r$ are:
Avo = -gm(Rc||r$) ≈-gmRc and Rout=Rc||r$ ≈Rc
Now if RC is replaced by a current source (an active load) the circuit is modified as in Fig.
6.19 (a) below4:
Figure 6.19 (a) Active-loaded common-emitter amplifier. (b) Small-signal analysis of the
amplifier in (a), performed both directly on the circuit and using the hybrid-# model
explicitly.
From the small signal analysis, performed either directly on the circuit or by using the
hybrid-# model, it follows in a straightforward way that Rin=RB||r#≈ r# as before, but that
Avo = -gmr$(>>-gmRc) and that Rout=r$ >>Rc
4
The bias resistances are not shown
Analog Electronics Lab-II p.6/14
II.3
Prelab Assignment: Calculations & PSPICE simulation
Use the computer software tool OrCAD PSPICE to simulate all four lab activities. Make
sure to bring the PSPICE results to the laboratory. In addition to being an aid in
immediately verifying measured results, they will be the basis of your Prelab grade for
this lab.
Specifically, the following items must be addressed using OrCAD PSPICE as part of the
prelab assignment:
•
•
•
Circuit drawings with the nodes labeled and with DC node voltages and all branch
currents;
Transient response (time-domain), waveform, plots for the principle nodes of the
circuits in activities D and E.
Magnitude and phase Bode plots of the voltage gains (i.e., generally Vout/Vin in dB) of
the circuits in activities D and E.
Fill in all entries in the tables provided below that are labeled “calculated”.
Analog Electronics Lab-II p.7/14
II.4
Experiments
Suggested Equipment
Protoboard
Tektronix FG501A or Tektronix AFG3021 Function Generator
Agilent 34401A Digital Multimeter
Tektronix PS 503A Power Supply
Resistors: 1 x 10 kΩ, 4 x 470 Ω, 2 x 1kΩ, 1 x 4.7 kΩ, 2 x 2.2 kΩ
Transistors: 2 x 2N3904, 1 x 2N3906
Capacitors: 1 x 100 µF
Laboratory Activities
Activity II.4.A: Simple Current Source
There will be three parts to this activity, each with a different resistance value for RL.
 II.4.A.a. Build the circuit given in
Fig. II.1 with RL = 0 %.
Fig. II.1: Simple current
source, for use in Activity A.
The transistors are 2N3904
(i) Using a DC ammeter, measure and record the DC branch currents for each
resistor. Note: Due to self heating, the circuit will take around 30 seconds to
settle. In case the measured values deviate more than 20% from the values
obtained via the OrCAD PSPICE computer simulation in the prelab make sure
to fix the circuit before proceeding.
Analog Electronics Lab-II p.8/14
Activity A. Part a: RL = 0 % DC Currents
Icalc
Imeas
% error
mA
mA
Ii (10K)
Io (RL)
(ii) Calculate the output resistance, Ro, of the current source using the equation
V
Ro = ro = A where VA = 100 V.
I CQ

II.4.A.b. Build the circuit given in Fig. II.1 with RL = 1 k%.
(i)
Using a DC ammeter, measure and record the DC branch currents for each
resistor. As before, allow the circuit to settle before taking final readings. In
case the measured values deviate more than 20% from the values obtained via
the OrCAD PSPICE computer simulation in the prelab make sure to fix the
circuit before proceeding.
Activity A. Part b: RL = 0 k% DC Currents
Icalc
Imeas
% error
mA
mA
Ii (10K)
Io (RL)
(ii) Calculate the output resistance, Ro, of the current source using the equation
V
Ro = ro = A where VA = 100 V.
I CQ
(iii) Using the two operating points, one with R L = 0 % and one with R L = 1 k% ,
determine the actual value of the output resistance and actual value of VA.

II.4.A.c. Build the circuit given in Fig. II.1 with RL = 4.7 k%.
(i)
Using a DC ammeter, measure and record the DC branch currents for each
resistor. As before, allow the circuit to settle. In case the measured values
deviate more than 20% from the values obtained via the OrCAD PSPICE
computer simulation in the prelab make sure to fix the circuit before
proceeding.
Activity A. Part c: RL = 4.7 k% DC Currents
Icalc
Imeas
% error
mA
mA
Ii (10K)
Io (RL)
Analog Electronics Lab-II p.9/14
(ii) Calculate the output resistance, Ro, of the current source using the equation
V
Ro = ro = A where VA = 100 V.
I CQ
(iii) Using the two operating points, one with RL = 0 % and one with R L = 4.7 k%,
determine the actual value of the output resistance and actual value of VA.
Activity II.4.B: The Improved Current Source Circuit
In this activity, a Wilson current mirror will be studied. There will be three parts to this
activity with different resistance values for RL in each part, similar to Activity A.
However, the values are not the same as in Activity A.

II.4.B.a. Build the circuit given in
Fig. II.1 with RL = 0 %.
Fig. II.2: Wilson current
mirror, for use in Activity B.
The transistors are 2N3904
(i) Using a DC ammeter, measure and record the DC branch currents for each
resistor. Allow the circuit to settle. In case the measured values deviate more
than 20% from the values obtained via the OrCAD PSPICE computer
simulation in the prelab make sure to fix the circuit before proceeding.
Activity B. Part a: RL = 1 k% DC Currents
Icalc
Imeas
% error
mA
mA
Ii (1K)
Io (RL)
(ii) Calculate the output resistance, Ro, of the current source using the equation
V
" !%
Ro = ro $ ' where VA = 100 V, ! = 150, and ro = A .
I
# 2&
CQ
Analog Electronics Lab-II p.10/14
•
II.4.B.b. Build the circuit given in Fig. II.2 with RL = 470 %.
(i) Using a dc ammeter, measure and record the DC branch currents for each resistor.
Allow the circuit to settle. In case the measured values deviate more than 20%
from the values obtained via the OrCAD PSPICE computer simulation in the
prelab make sure to fix the circuit before proceeding.
Activity B. Part b: RL = 0 k% DC Currents
Icalc
Imeas
% error
mA
mA
Ii (1K)
Io (RL)
(ii) Calculate the output resistance, Ro, of the current source using the equation
V
" !%
Ro = ro $ ' where VA = 100 V, ! = 150, and ro = A .
I CQ
# 2&
(iii) Using the two operating points, one with RL = 0 % and one with RL = 470 %,
determine the actual value of the output resistance and actual value of VA.
•
II.4.B.c. Build the circuit given in Fig. II.2 with RL = 1 k%.
(i) Measure and record the DC branch currents for each resistor. In case the
measured values deviate more than 20% from the values obtained via the
OrCAD PSPICE computer simulation in the prelab make sure to fix the circuit
before proceeding.
Activity B. Part c: RL = 470 % DC Currents
Icalc
Imeas
% error
mA
mA
Ii (1K)
Io (RL)
(ii) Calculate the output resistance, Ro, of the current source using the equation
V
" !%
Ro = ro $ ' where VA = 100 V, ! = 150, and ro = A .
I
# 2&
CQ
(iii) Using the two operating points, one with RL = 0 % and one with R L = 1 k% ,
determine the actual value of the output resistance and actual value of VA.
Analog Electronics Lab-II p.11/14
Activity II.4.C:
Simple Amplifier with a Passive Load
This activity is designed to examine the operation of a simple common emitter amplifier
that does not have a constant current source as an active load in order to compare it with
one that does.
(i) Build the circuit in Fig. II.3
Fig. II.3: Amplifier circuit, with a passive load, for use in Activity C. The BJT is 2N3904
(ii) With the input shorted to ground, measure and record the DC node voltages at the
base, emitter, and collector. In case the measured values deviate more than
20% from the values obtained via the OrCAD PSPICE computer simulation in
the prelab make sure to fix the circuit before proceeding.
Activity C.ii. DC voltages
Vcalc (V)
Vmeas (V)
VE
VB
VC
% error
Analog Electronics Lab-II p.12/14
(iii) Input a sinusoidal signal of amplitude 1 V rms and frequency 1 kHz to the
circuit and measure the AC voltages at the output of the function generator, as
well as at the base, emitter, and collector of the transistor.
(iv)
Determine the voltage gains of the circuit from the measurements outlined
above ( Vo Vi and Vo Vs ) in both units of V V and dB, and cross-check these
results with the computer simulation results obtained as the prelab.
Vcalc
V
Activity C. (iii) AC voltages
Vmeas
Avmeas Avmeas Avcalc
Avcalc
V
V/V
dB
dB
V/V
% error
Vs
Vb
Vc
Ve
Activity II.4.C: Simple Amplifier with a Constant Current Source as an Active Load
In this activity, observe the operation of an amplifier with a constant current source, and
note the gain improvement from the operation of the amplifier without one.
(i) Replace the collector resistor RC1 in Fig. II.3 with the active load circuit,
shown in Fig. II.4. below
Fig. II.4: Current source to be
appended to the amplifier in Activity
C as an active load, for use in
Activity D.
(ii)
Measure the DC node voltages at the bases and emitters of each transistor
to ensure correct bias. If they are not, fix the circuit before proceeding. In
case the measured values deviate more than 20% from the values obtained
via the OrCAD PSPICE computer simulation in the prelab make sure to
fix the circuit before proceeding.
Analog Electronics Lab-II p.13/14
Activity D.ii. DC voltages
Vcalc (V)
Vmeas (V)
% error
VE1
VB1
VE2
VB2
(iii)
Input a sinusoidal signal of amplitude 1 V rms and frequency 1 kHz to the
circuit and measure the AC voltages at the output by the signal generator,
and the base and the collector of the output transistor.
(iv)
Determine the voltage gains of the circuit from the measurements outlined
above ( Vo Vi and Vo Vs ) in both units of V V and dB, and cross-check
these results with the computer simulation results obtained as the prelab.
Vcalc
V
Vs
Vb
Vc
Activity D. (iii) AC voltages
Vmeas
Avmeas Avmeas Avcalc
V
V/V
dB
dB
Avcalc % error
V/V
Analog Electronics Lab-II p.14/14
II.5
Report
The laboratory report should follow the outline given in the handout titled “Laboratory
Report Guidelines.”
The following items should be addressed in the appropriate sections of the report:
•
•
•
•
II.5.1-8. DC nodal voltage analysis for each Activity in this laboratory experiment;
II.5.9-16. DC branch current analysis for each Activity in this laboratory experiment;
II.5.17-18. AC analysis (including voltage gains) for Activities B and C of this
laboratory experiment; comment on the effect of the 10K% load on the gain.
II.5.19. Comparisons of the measured results with the calculated results, as well as
with the computer simulation results obtained as the prelab.