IEEE TRANSACTIONS ON EDUCATION, VOL. 48, NO. 3, AUGUST 2005
559
On the Estimation of Frequency Response in
Amplifiers Using Miller’s Theorem
B. Mazhari
Abstract—Conventional application of Miller’s theorem in amplifiers often leaves a student with an erroneous impression that the
approximation provides an incorrect estimate for the nondominant
pole. The present paper clarifies that not only can a reasonable
estimate for both dominant and nondominant poles be obtained
through Miller’s theorem but additional insight is offered into pole
splitting, not afforded by conventional analysis.
Index Terms—Amplifiers, Miller effect, poles and zeros, transfer
function.
I. INTRODUCTION
A
course on analog circuits is an essential part of almost all
electrical engineering curricula. Besides imparting knowledge on an important subject, a course on analog circuits provides an excellent opportunity to teach important aspects of engineering design. In a senior-level or an introductory graduate
course on analog circuits, design and analysis of amplifiers constitutes a large fraction of a syllabus. In the treatment of amplifiers, analysis of frequency response of amplifiers, such as a
common emitter bipolar junction transistor (BJT) amplifier or a
common source field-effect transistor (FET) amplifier, is complicated by the presence of a feedback capacitor between input
and output ports. Similarly, estimation of poles and zeros becomes relatively more difficult in multistage amplifiers when a
compensation capacitor is deliberately added between input and
output ports so as to improve the stability of the overall amplifier [1]–[8]. In such situations, analysis of frequency response is
made easier by the application of Miller’s theorem, which essentially results in decoupling of input and output ports. As a result,
one can separately determine the input and output constants and
thereby obtain an estimate of the frequency response. Although
a fairly good estimate for the dominant pole arising out of the
input time constant can be obtained through Miller’s theorem,
the estimate for the nondominant pole turns out to be grossly incorrect [4], [9], [10]. This apparent “error” is often overlooked
in the analysis of single-stage amplifiers in which the nondominant pole plays a relatively less important role. However, in
compensation of multistage amplifiers, the exact location of this
pole must be known. In most textbooks, an estimate for this pole
is obtained through an analysis of the overall transfer function
[2]–[8]. In this paper, the author shows that a good estimate for
the nondominant pole can also be obtained through Miller’s theorem, and the errors involved in a conventional application of
Miller’s theorem are clarified.
Manuscript received March 27, 2004; revised June 3, 2005.
The author is with the Department of Electrical Engineering, Indian Institute
of Technology, Kanpur 208016, India (e-mail: baquer@iitk.ac.in).
Digital Object Identifier 10.1109/TE.2005.853070
Fig. 1. (a) Two-stage amplifier with a compensation capacitor C ; (b) smallsignal equivalent circuit of the amplifier.
Fig. 2. Small-signal equivalent circuit of the amplifier following application
of Miller’s theorem. C
=C
(1 A ) and C
=C
(1 1=A ).
2 0
2 0
II. NONDOMINANT POLE UNDER MILLER’S THEOREM
Consider a two-stage amplifier shown in Fig. 1(a) with a comconnected across the second stage to impensation capacitor
prove the overall stability of the amplifier. The equivalent circuit for the amplifier with Miller’s compensation is shown in
Fig. 1(b) in which each stage is a FET amplifier with infinite input
and , and and
impedance. The output resistances are
are the output capacitances for amplifiers 1 and 2, respectively.
The transconductance of the first and second amplifier stages
and
, respectively. An analysis of the transfer funcare
tion of the circuit shown in Fig. 1(b) reveals that there are two
poles and one zero given by the following expressions [8]:
(1)
(2)
(3)
Application of Miller’s theorem to the circuit shown in Fig. 1(b)
yields the simplified circuit shown in Fig. 2. The analysis of
input and output circuits yields the following expressions for
their transfer functions:
0018-9359/$20.00 © 2005 IEEE
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IEEE TRANSACTIONS ON EDUCATION, VOL. 48, NO. 3, AUGUST 2005
TABLE I
COMPARISON OF VALUES OF POLES AND ZEROS OBTAINED THROUGH EXACT ANALYSIS OF THE TRANSFER FUNCTION AND THROUGH THE CONVENTIONAL
METHOD USING MILLER’S APPROXIMATION. RESULTS SHOWN ARE FOR THE FOLLOWING PARAMETERS FOR THE CIRCUIT SHOWN IN FIG. 1:
R = R = 10 ; C = 1 pF; C = 0.5 pF; C = 0.1 pF; AND g = g = 100 A=V
(5)
(6)
TABLE II
VALUES OF POLES AND ZEROS IN THE TRANSFER FUNCTION OF THE FIRST AND
SECOND AMPLIFIER STAGES OBTAINED THROUGH (8) AND (14) AND (15).
RESULTS SHOWN ARE FOR CIRCUIT PARAMETERS IDENTICAL
TO THAT DESCRIBED IN TABLE I
(7)
Substitution of the low-frequency gain expression of
in (6)–(7) shows that
and
are independent of
“ ” and thus can be interpreted as poles associated with input
and output circuits, respectively. Table I shows a comparison of
these poles with those obtained through exact analysis of the
agrees well with the dominant
transfer function. Although
lies at a frequency
pole predicted by exact analysis, pole
that is lower than by almost an order of magnitude. The close
agreement between
and on one hand and the large differcan easily lead to an impression that
ence between and
analysis using Miller’s theorem works well at the input stage but
gives incorrect results for the output stage. However, this conobtained from Miller’s analysis
clusion is not true, and pole
is indeed present in the transfer function of the output stage. A
complete analysis gives the following expression for the transfer
function of the output stage:
more accurate expression for transfer function of the first gain
stage to be written, as follows:
(10)
(11)
(12)
Using dominant pole approximation [8], the denominator
of (10) can be expressed as
(13)
(8)
Equation (13) along with (10)–(12) can be used to obtain an
and
, as follows:
estimate for poles
A comparison of (8) with (5) shows that the only error that
results from use of Miller’s theorem at the output stage is that
is neglected in the approximation. The reason for
a zero
the apparent “discrepancy” between results obtained from the
application of Miller’s theorem and exact analysis is that frequency-dependent characteristics of the gain associated with
the output stage is neglected during evaluation of transfer func, the fretion of the first stage. If instead of
quency-dependent gain expression given by (5) with
is substituted in (6), one obtains a more ac, as follows:
curate expression for
(14)
(9)
can no longer be interpreted as a pole since
However,
it is a function of frequency. Substitution of (9) in (4) allows a
(15)
The complete transfer function of the first stage can now be
expressed as
(16)
Equation (16) shows that a more accurate application of Miller’s
theorem predicts the presence of one additional pole (
) and
) in the transfer function of the first gain stage bea zero (
sides the dominant pole
predicted by conventional analysis.
Table II summarizes the values of poles and zeros in the transfer
function of first and second stages. A comparison with values
lies at a frequency
shown in Table I shows that the new pole
MAZHARI: FREQUENCY RESPONSE IN AMPLIFIERS USING MILLER’S THEOREM
561
.
nected in parallel to a resistor of value
Since this resistor value is often much smaller than resistor ,
the input stage can be effectively modeled as this resistor con, thereby giving rise to
nected in parallel to capacitor
the expression (15) for nondominant pole. The only source of
error in analysis using Miller’s approximation described previously arises from the neglect of a zero in the transfer function
of the second amplifier stage (or the output circuit in single amplifier stages). If (8) is used instead of (5) in obtaining (15), one
matches with pole , obtained through analfinds that pole
ysis of the complete transfer function.
III. CONCLUSION
Fig. 3. Gain of the first stage (A ), second stage (A ), and overall amplifier
(A ) as a function of frequency. The results were obtained through simulation
of the circuit shown in Fig. 1(b) for the following parameters: R = R =
10 ; C = 1 pF; C = 0.5 pF; C = 0.1 pF; and g = g =
100 A=V.
close to the nondominant pole , estimated through exact analysis of the transfer function. Furthermore, the zero in the transfer
function of the first stage lies at the same frequency as pole
in the transfer function of the second stage, resulting in a
mutual cancellation of their effect on the overall transfer function. The poles and zeros in the transfer function of the first and
second stages can also be clearly seen in Fig. 3 which shows the
variation of gain with frequency for the first and second amplifier stages and also shows the frequency response of the overall
amplifier obtained through simulation of the circuit shown in
Fig. 1(b). The transfer function of the first stage has two poles
and a zero; the latter matches with the pole in the transfer function of the second stage. As a result, the overall transfer funcand
and a zero
at
tion reveals presence of only poles
higher frequencies. The physical reason for pole–zero cancellation is that when the gain of the second stage begins to reduce
, the effective capacitance
with frequency because of pole
seen by the first stage also begins to reduce. This
because of
reduction stops the 20-dB/decade decrease in gain of the first
stage and makes it constant. As a result, the overall gain is unaffected and continues to roll off at 20 dB/decade only as seen in
Fig. 3. The nondominant pole can also be understood in terms
) because of the compensation
of the effective impedance (
capacitor seen by the first stage, as follows:
(17)
At frequencies much higher than
duces to
To summarize, an estimate for both dominant and nondominant poles in an amplifier’s transfer function can be correctly
obtained from application of Miller’s theorem, provided that the
expression used for voltage gain in obtaining Miller’s capacitance includes frequency response of the gain as well. Conventional use of Miller’s theorem does not commit an error by incorrectly predicting the value of pole in the transfer function of
the second stage, but rather commits an error of omission by
failing to predict a zero at the exact location of this pole and an
additional nondominant pole in the transfer function of the input
stage. The analysis using Miller’s theorem provides the insight
that a process of pole–zero cancellation is involved in the well
known pole-splitting effect of compensation capacitor.
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, the above expression re-
(18)
Equation (18) shows that at high frequencies, the compensacontion capacitor appears to the input stage as a capacitor
B. Mazhari received the B.Tech. degree in electronics and electrical communication engineering from the Indian Institute of Technology (IIT) Kharagpur,
Kanpur, India, in 1987 and the Ph.D. degree in electrical engineering from the
University of Illinois at Urbana Champaign in 1993.
He is currently an Associate Professor with the Department of Electrical Engineering, IIT. His research interests include semiconductor device modeling,
analog circuit design, and organic electronics.