U.S. patent number RE35,261 [Application Number 08/355,080] was granted by the patent office on 1996-06-04 for differential input amplifier stage with frequency compensation.
This patent grant is currently assigned to Linear Technology Corporation. Invention is credited to Carl T. Nelson.
United States Patent |
RE35,261 |
Nelson |
June 4, 1996 |
Differential input amplifier stage with frequency compensation
Abstract
A differential input amplifier stage having improved frequency
compensation. Frequency compensation is achieved by cancelling
one-half of the signal output of a differential error amplifier in
the input stage, such that all error signals must pass through a
"current-mirror" type load circuit in which a resistor-capacitor
network is provided to roll of gain of the input stage.
Inventors: |
Nelson; Carl T. (San Jose,
CA) |
Assignee: |
Linear Technology Corporation
(Milpitas, CA)
|
Family
ID: |
24941942 |
Appl.
No.: |
08/355,080 |
Filed: |
December 12, 1994 |
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
732039 |
Jul 18, 1991 |
05182526 |
Jan 26, 1993 |
|
|
Current U.S.
Class: |
330/257; 323/280;
323/316 |
Current CPC
Class: |
H03F
1/083 (20130101); H03F 3/45071 (20130101) |
Current International
Class: |
H03F
1/08 (20060101); H03F 3/45 (20060101); H03F
003/45 () |
Field of
Search: |
;330/257
;323/265,273,280,315,316 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Data Sheet, "LM120 Series 3-Terminal Negative Regulators," National
Semiconductor Corporation 1982 Linear Databook, pp. 1-39 to 1-46.
.
Data Sheet, "LM747/LM741A/LM41C/LM741E Operational Amplifier,"
National Semiconductor Corporation 1982 Linear Databook, pp. 3-257
to 3-259. .
Data Sheet, "LM101A/LM301A/LM107/LM307 Operational Amplifiers,"
Linear Technology Corporation 1986 Linear Databook, pp. 2-177 to
2-182. .
James E. Solomon, "The Monolithic Op Amp: A Tutorial Study," IEEE
Journal of Solid-State Circuits, pp. 314-332, vol. SC-9, No. 6,
Dec. 1974..
|
Primary Examiner: Mullins; James B.
Attorney, Agent or Firm: Fish & Neave Rowland; Mark D.
Morris; Robert W.
Claims
I claim:
1. An improved differential input amplifier stage having first and
second input terminals between which a voltage difference can be
generated, the stage including:
an error amplifier circuit having a differential transistor pair
coupled to the first and second input terminals for generating
first and second differential currents responsive to a voltage
difference generated between the first and second input terminals,
at least a portion of each of the first and second differential
currents being respectively coupled to first and second output
terminals of the error amplifier circuit;
an active load coupled to the first and second output terminals of
the error amplifier for generating an error signal current at an
output terminal of the input stage by subtracting from the current
generated at the second error amplifier output terminal a current
conducted by the active load in response to the current generated
at the first error amplifier output terminal; and
a frequency compensation circuit coupled to the active load to
reduce the gain of the active load with respect to a range of
frequencies, as a result of which the output voltage of the active
load becomes less responsive to the current generated at the first
error amplifier terminal at frequencies within the range of reduced
gain;
wherein the improvement comprises:
means for dividing the first and second differential currents
generated by the differential transistor pair into portions;
means for providing a first portion of the first differential
current to the first output terminal of the error amplifier;
and
means for combining a second portion of the first differential
current with a portion of the second differential current to
generate a current at the second output terminal of the error
amplifier, whereby differential signals in the current generated at
the second error amplifier output terminal are substantially
cancelled.
2. The improved stage of claim 1, wherein the frequency
compensation circuit comprises a network of resistor and capacitor
elements, and wherein gain of the stage for signal frequencies
within a predetermined range can be reduced below 0.5, the
predetermined range being controllable by selectably varying values
of the resistor and capacitor elements.
3. The improved stage of claim 1, wherein the currents generated at
the first and second error amplifier output terminals are
substantially equal when no voltage difference is generated between
the first and second input terminals.
4. The improved stage of claim 1, wherein the differential
transistor pair of the error amplifier circuit includes first and
second transistors each having a first collector and a second
collector, the first collector of the first transistor being
coupled to the first output terminal of the error amplifier means,
the second collector of each of the first and second transistors
being commonly coupled to the second output terminal of the error
amplifier circuit, and the first collector of the second transistor
being coupled to a shunt point in the stage.
5. The improved stage of claim 4, wherein the second collectors of
the first and second transistors are of substantially equal
area.
6. The imnproved stage of claim 5, wherein the first and second
collectors of each of the first and second transistors have a 2:1
area ratio.
7. The improved stage of claim 1, wherein the active load means
comprises a current mirror circuit.
8. The improved stage of claim 1, wherein the stage is incorporated
in an integrated circuit.
9. The improved stage of claim 1, wherein the stage is incorporated
in a voltage regulator circuit.
10. A differential input amplifier stage having first and second
input terminals between which a voltage difference can be
generated, the stage including:
a differential error amplifier circuit for generating first and
second differential currents responsive to a voltage difference
generated between the first and second input terminals, said second
differential current being shunted;
a current source for providing a current substantially equal to a
non-signal component of the first differential current;
an active load coupled to receive the first differential current
and the current provided by the current source for generating an
error signal current by subtracting from the current provided by
the current source a current conducted by the active load in
response to the first differential current; and
a frequency compensation circuit coupled to the active load to
reduce the gain of the active load with respect to a range of high
frequencies, as a result of which the output voltage of the active
load becomes less responsive to the first differential current at
frequencies within the range of reduced gain.
11. In a negative voltage regulator circuit having a common-emitter
output stage including an output coupled to an output of the
regulator and an input, and having voltage reference circuitry for
generating a reference voltage, an input amplifier stage
comprising:
a differential error amplifier circuit having a first input coupled
to receive the reference voltage and a second input coupled to
receive a feedback signal responsive to the output of the
regulator, said differential error amplifier operating to compare
the reference voltage to a portion of the feedback signal to
generate first and second differential currents;
an active load coupled to receive the first differential current
and a second current substantially equal to a non-signal component
of the first differential current for generating an error signal by
subtracting the first differential current from the second current,
at least a portion of the error signal being coupled to the input
of the common-emitter output stage to control the output of the
voltage regulator; and
a frequency compensation circuit coupled to the active load to
reduce the gain of the active load with respect to a range of
frequencies, as a result of which the output voltage of the active
load becomes less responsive to the current generated by the error
amplifier circuit at frequencies within the range of reduced gain.
Description
BACKGROUND OF THE INVENTION
The present invention realtes to improved frequency compensation
for analog integrated circuits, especially integrated circuit
devices used in implementing voltage regulator circuits and other
negative feedback circuits requiring ripple rejection.
As is well known, the basic architecture of a series-type voltage
regulator circuit includes a pass element (e.g., a transistor)
connected in series between a load and an unregulated supply line,
an error amplifier which controls the pass element, a voltage
reference generator connected to one input of the error amplifer
and a negative feedback network connected between the other input
of the error amplifier and the output of the regulator circuit.
This type of architecture has been incorporated in many
commercially available integrated circuits.
Most applications of a boltage regulator circuit require the
circuit to be stable. Like any other circuit operating with
negative feedback, a series-type voltage regulator circuit will
tend to become unstable (i.e. break into oscillations) if total
phase shift across the circuit for a particular frequency
approaches 180.degree. and, simultaneously, the magnitude of the
gain of the circuit at that frequency approaches unity. In an
idealized voltage regulator circuit, stability is not a problem
because total phase shift is limited to 90.degree. and overall gain
exhibits a signale-pole rolloff characteristic corresponding to a
-6 dB/octave rolloff of dc open-loop gain for frequencies in excess
of a relatively low 3-dB bandwidth w.sub.0.
However, as a result of excess phase shifts associated with active
devices and stray capacitances inherent in practical voltage
regulator circuits, total phase shift across such circuits
typically exceeds 180.degree. at frequencies well within a desired
range of operation. Therefore, it is usually necessary to
compensate practical regulator circuits to provide stable
operation. Such compensation is typically requried in operational
amplifier circuits as well.
Commonly, capacitors are placed in voltage regulator and
operational amplifier circuits to provide frequency compensation.
However, the integration of a voltage regulator or operational
amplifier circuit complicates such frequency compensation by
limiting the points that an external capacitor can be connected,
and by imposing size constraints on internal capacitors.
One known frequency compensation technique associated with
integrated circuit voltage regulator devices is to use an internal
capacitor to reduce the gain of the differential input amplifier
stage of the regulator at high frequencies. This technique is used,
for example, in the LM120 integrated circuit negative voltage
regulator device available commercially from various manufactures.
The differential input amplifier stage of the LM120 includes a
differential amplifier coupled to a current mirror load. The
current mirror load acts as a gain stage, and performs
differential-to-single-ended signal conversion. The gain of the
differential input amplifier stage is rolled off by coupling a
bypass capacitor across the base-collector junction of the output
transistor of the current mirror load. This approach. however, has
shortcomings and disadvantages that limit the operating
characteristics of the circuit.
For example, even with the internal capacitor, a differential input
amplifier stage of the type in the LM120, which has npn-pnp
composite transistors at its inputs, exhibits a gain of at least
0.25, no matter what the value of the internal capacitor is. In
standard circuits that have a single transistor at each input
instead of a composite like the LM120, this minimum gain is
typically 0.5. Because the outpu stage of the device also
introduces gain into the regulator system looop, unstable circuit
conditions may still result. Also, regulator circuits, especially
when used in power supplies, typically include an external
capacitor coupled across the load to absorb transients. The
external capacitor introduces a pole into the regulator system loop
that, when combined with the pole created by the internal
capacitor, can cause excess phase shift tending to exacerbate
stability problems.
In most operational amplifier circuits, the voltage gain of the
input stage can be rolled-off by placing an internal of external
capacitor across the output stage, thus providing simple frequency
compensation. However, the capacitor tends to pass signals at the
amplifier supply line directly to the ouptut. In a regulator
circuit having a rectified ac voltage signal at is input, the
capacitor would create ripple in the output voltage. This
compensation technique therefore is not suitable for regulator
circuits, or even for some operational amplifier circuits, where it
is desired that th ecircuit have an ability to reject fluctuations
of voltage signals on the power supply lines (i.e. ripple
rejection).
It would therefore be desirable to be able to provide a
differential input amplifier stage for a voltage regulator or other
circuit that does not require a large capacitor for frequency
compensation, and that therefore can be implemented as part of an
integrated circuit. It would also be desirable to be able to
provide a differential input amplifier stage that cen be
compensated without degrading ripple rejection. Further, it would
be desirable to be able to provide a differential input amplifier
stage having a gain that cen be rolled substantially below 0.25
(for a composite npn-pnp type input) or 0.5 (for a single
transistor type input) over some range of frequencies and that can
be compensated without causing stability problems when load
conditions introduce an external pole approaching a 90.degree.
phase shift.
SUMMARY OF THE INVENTION
In view of the foregoing, it is an object of the present invention
to provide a differential input amplifier stage that can be
implemented in an integrated circuit without neef for a large value
internal capacitor.
It is another object of the present invention to provide a
differential input amplifier stage with a frequency compensation
circuit that can be implemented in a voltage regulator circuit
without degrading ripple rejection.
It is yet another object of the present invention to provide a
differential input amplifier stage whose gain can be rolled-off
over a range of frequencies to substantially less than the minimum
gain of a conventional stage having the same type of differential
error amplifier design.
It is a further object of the present invention to provide a
differential input amplifier stage whose gain can be rolled-off
independent of phase shift requirements.
These and other objects and advantages of the present invention are
accomplished by cancelling one-half of the signal output of the
differential error amplifier in the input stage of the voltage
regulator or operational amplifier, such that a single-ended error
signal is injected into a first transistor of a "current-mirror"
type load circuit. The injected signal causes a voltage change
across the first transistor, which signal is in turn fed into a
resitor-capacitor integrator network. Component values in the
integrator network can be chosen to cause gain of the error
amplifier to be rolled-off over a range of frequencies to a value
well below the minimum gain of a conventional amplifier of similar
type, while simultaneously making excess phase shift build-up
negligible over a wide range of load conditions.
In a preferred embodiment, the signal cancellation is effected by
spliting the collectors of transistors in the error amplifier to
combine currents havign offsetting differential signals at one
differential output of the error amplifier. In an alterantive
embodiment, cancellation is achieved by shunting one differential
output current of the error amplfier to a point which as negligible
influence on loop gain and replacing the shunted output current
with current from a DC current source.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects and advantages of the present invention
will be apparent upon consideration of the following detailed
description, taken in conjunction with the accompanying drawings,
in which like reference characters refer to like parts throughout,
and in which:
FIG. 1 is a simplified schematic diagram of a conventional voltage
regulator circuit;
FIG. 2 is a simplified schematic of a voltage regulator circuit
utilizing a preferred embodiment of the present invention; and
FIG. 3 is a simplified schematic of a voltage regulator circuit
utilizing an alterantive embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Referring to FIG. 1, a simplified schematic of a conventional
voltage regulator circuit 100 is shown. Regulator circuit 100
includes differential input amplifier stage 100a (hereafter
referred to simply as "input stage 100a") and output stage 100b.
Input stage 100a includes differential error amplifier 102 having a
non-inverting input 104 connected to a voltage reference generator
105, and inverting input 106 connected to receive a feedback
voltage from output terminal V.sub.OUT, and first and second
differetnial outpus 108 and 110. Voltage reference generator 105
generates a reference voltage level V.sub.REF, which is independent
of an unregulated supply voltage applied to input terminal
V.sub.IN. Error amplifier 102 compares V.sub.REF with the feedback
voltage and generates differential, corrective error signals to
modulate currents I.sub.1 and I.sub.2 at outputs 108 and 110
respectively. Each current is modulated to carry one-half of the
error signal output of the amplifier.
A current mirror circuit 115, including matching npn transistors
114 and 118, is connected to outputs 108 and 110 to provide active
loads for the error amplifier and to perform
differential-to-single-ended conversion of the error signals
imposed on currents I.sub.1 and I.sub.2. Current I.sub.1 is fed to
transistor 114, which sets the base-emitter voltage of transistor
118. Assuming transistors 114 and 118 have the same emitter areas,
and ignoring the effects of resistors 116 and 117 and capacitor
128, transistor 118 will conduct a collector current I.sub.3 equal
to current I.sub.1. The collector current of transitor 118
subtracts from current I.sub.2 at node 120 to provide an error
signal current to the base of output stage transistor 122. This
error signal current has a signal component contributed in part by
the error signal imposed on current I.sub.1, and in part by the
error signal imposed of current I.sub.2.
The collector of output stage transistor 122 (which represents the
gain of output stage 100b in simplified form) is connected to the
regulator output terminal V.sub.OUT, to which a typical output
load, represented by a variable resitor 124 having a resistance
value R.sub.L connected in parallel with a load capacitor 126
having capacitance C.sub.L and parasitic series resistance R.sub.C,
is shown connected. Output stage transistor 122, responsive to the
current applied to its base, regulates the voltage across the
output load to maintain the voltage equal to the voltage reference
level V.sub.REF. In many regulator circuits, the feedback signal
provided to the error amplifier is generated by scaling the output
voltage, such that the regulator will maintain the output voltage
at a level proportional to V.sub.REF.
In the aforementioned LM120 integrated circuit voltage regulator
device, which has a basic architecture of the type represented by
stages 100a and 100b of FIG. 1, internal frequency compensation was
provided by coupling a small capacitor across the base-collector
junction of the output transistor of the input stage current mirror
load circuit, as shown for example by capacitor 128 coupled across
transistor 118. Capacitor 128 adds a pole to the closed-loop
frequency response of regulator circuit 100 at a frequency
determined by its capacitance, meaning that at frequencies above
the pole the capacitor adds--6 dB/octave of gain rolloff to input
stage 100a. The added gain rolloff reduces the unity gain bandwidth
of the regulator circuit, and thus improves stability.
Although it would be desirable, from a stability perspective, to
choose a large capacitance for capacitor 128 such that the
capacitor pole is at a low frequency, this is not practical in an
integrated circuit because of the chip area that would be required
for the capacitor. However, by adding a resistor 117 between the
bases of transistors 114 and 118, as was done in the LM120, a
resistor-capacitor integrator network is formed having a pole at a
frequency substantially lower than could practically be obtained by
capacitor 128 alone. The addition of resistor 117 creates a dc
current imbalance in the current mirror. To compensate, a resistor
116 matching the resistance value of resistor 117 is connected
between the base and collector of transistor 114.
The integrator network of resistor 117 and capacitor 128 has two
disadvantages. First, the gain of input stage 100a cannot be rolled
down below a certain minimum gain that typically falls within a
range of 0.25-0.5. To explain this, consider error amplifier 102 as
comprising a pair of differentially-connected pnp transistors, with
the collector of one pnp transistor providing error amplifier
output 108 and the collector of the other pnp transistor providing
error amplifier output 110. The gain of input stage 100a for error
signals at error amplifier output 110 is approximately equal to the
output impedance of the current mirror load transistor 118 divided
by the emitter impedance of the error amplifier. In this case, the
emitter impedance of the error amplifier is equal to twice the
emitter resistance of the pnp transistor conducting current
I.sub.2. At frequencies where the capacitor effectively becomes a
short, capacitor 128 causes transistor 118 to appear to the error
amplifier as a diode, with an impedance equal to the emitter
resistance of the error amplifier transistors. Thus, the ratio of
the load impedance and error amplifier emitter impedance is equal
to 0.5 and the gain of the stage is 0.5. This is true even assuming
that resistor-117 and capacitor 128 are chosen to be of sufficient
value to severely attenuate high frequency gain of error amplifier
output 108, because the error signal path from output 110 still has
a gain of 0.5. If, as in some error amplified designs, each
differentially-connected pnp transistor is replaced by an npn-pnp
composite transistor, the minimum gain can be reduced to 0.25.
A gain of 0.25-0.5 in input stage 100a, when combined with the gain
of output stage 106b, creates an overall loop gain that can exceed
unity and destabilize the regulator circuit. In operational
amplifier circuits, it is known to provide a capacitor across a
second gain stage to add rolloff, as shown for example by capacitor
C.sub.x in FIG. 1. However, in regulator circuit 100, such a
capacitor would not be suitable because, at higher frequencies, the
capacitor would pass signals at terminal V.sub.IN directly to the
output terminal V.sub.OUT. This would significantly degrade the
circuit's ability to reject ripple on the unregulated input voltage
applied to terminal V.sub.IN.
A second disadvantage of the frequency compensation scheme of FIG.
1 is that the pole created by capacitor 128 will, when combined
with a pole created by external load capacitor 126, cause excess
phase shift to build up in the closed-loop response of the
regulator. Such build-up can be reduced by placing a resistor in
series with capacitor 128 between the base and collector of
transistor 118, but this then compromises the capacitor's ability
to provide gain rolloff.
The present invention overcomes the above disadvantages by
cancelling the error signal in the differential current output of
the error amplifier coupled directly to the output node of the
input stage, thus forcing the entire error signal to pass through
the current mirror load circuit of the input stage.
Regulator circuit 200 of FIG. 2 represents a simplified schematic
of a preferred embodiment of the present invention. Regulator
circuit 200 includes a differential input amplifier stage 200a
(hereafter "input stage 200a") and a second gain stage 200b. Input
stage 200a includes error amplifier 202 comprising a tail current
source 203 and a pair of differentially-connected pnp transistors
204 and 206. Tail current source 203 may be implemented in a
conventional manner or may simply be a resistor. The base of
transistor .[.204.]. .Iadd.206 .Iaddend.is the non-inverting input
of error amplifier 202, and is connected to voltage reference
generator 205 which generates a reference voltage level V.sub.REF.
The base of transistor .[.206.]. .Iadd.204 .Iaddend.is the
inverting input of error amplifier 202, and is connected to receive
a feedback signal indicative of the voltage at output terminal
V.sub.OUT. The connection between terminal V.sub.OUT and the base
of transistor .[.206.]. .Iadd.204 .Iaddend.is shown by a broken
line to indicate that circuitry such as a resistor divider network
may be interposed to scale the output voltage.
Each of transistors 204 and 206 has a pair of split collectors,
204a-b and 206a-b, respectively. The areas of the split collectors
are ratioed as follows: collectors 204a and .[.206b.]. .Iadd.206a
.Iaddend.have equal areas of x, and collectors 204b and 206b have
equal areas of 2x. Collector 204b forms a first output 208 of error
amplifier 202. Collectors 204a and .[.206b.]. .Iadd.206a
.Iaddend.are connected together to form a second output 210 of
error amplifier 202. Collector 206b is connected to a point which
has negligible influence on loop gain, such as terminal V.sub.IN
(as shown in FIG. 2). The area ratios of the split collectors are
chosen such that the zero-differential signal components of output
currents I1' and I2' of the error amplifier are balanced, and equal
but out of phase differential signal components are combined at
error amplifier output 210 to cancel the error signal at that
output.
Error amplifier outputs 208 and 210 are connected to current mirror
circuit 215, which includes diode-connected npn transistor 214 and
matching npn transistor 218. Transistors 214 and 218 operate as
active loads for error amplifier outputs 208 and 210, respectively.
In response to a voltage differential appearing across the inputs
of transistors 204 and 206 of error amplifier 202, error amplifier
202 modulates current I1' with a single-ended error signal
proportional to the input error signal voltage. The impedance of
diode-connected transistor 214 converts the single-ended error
signal into a voltage that sets the base-emitter voltage of
transistor 218, and causes transistor 218 to conduct a current I3',
which is subtracted at node 220 from error amplifier output current
I2'. The resulting error signal is provided to gain stage 200b
comprising npn transistors 234 and 236. Current I2' has no
differential signal component because the opposite phase signals
appearing at collectors 204a and .[.206b.]. .Iadd.206a
.Iaddend.cancel one another. Thus, the error signal gain of input
stage 200a is determined by the gain through current mirror circuit
215.
Although a mismatch between the collector currents combined at
error amplifier output 210 can result in a signal being conducted
by current I2', this signal will be relatively small for most cases
and therefore normally will not have enough gain to cause a
problem. For example, even if the mismatch is as large as 10%, the
gain of error amplifier output .[.110.]. .Iadd.210 .Iaddend.will
have been reduced approximately by a factor of 10. This reduction
in gain will still allow the overall gain of input stage 200a to be
rolled well below 0.5. Thus, the collector area ratio of collectors
204a and 206a need not be exactly one-to-one. Any area ratio that
results in substantial signal cancellation at output .[.110.].
.Iadd.210 .Iaddend.can suffice.
The higher frequency gain through current mirror circuit 215 is
controlled by the integrator network formed by resistor 217,
connected between the bases of transistors 214 and 218, and
capacitor 228 and resistor 230, connected in series with one
another between the base and collector of transistor 218.
Preferably, a resistor 216 of equal value to resistor 217 is
provided at the base of diode-connected transistor 214 to balance
DC currents within the current mirror. The values of resistor 217
and capacitor 228 can be chosen to roll off the gain of the error
signal path to well below 0.5. Because no signal is carried by
current I2', the overall gain of input stage 200a can be rolled-off
considerably below 0.5 thus overcoming the first-mentioned
disadvantage of the frequency compensation scheme of FIG. 1.
However, as previously discussed, excess phase shift across the
regulator must be considered simultaneously with gain. This phase
shift may vary widely depending on the nature of the load connected
to regulator output terminal V.sub.OUT. For example, assuming a
load consisting of a variable resistor 224 in parallel with a load
capacitor 226--a typical load for a regulator circuit--the
capacitance in the load will create a pole in the closed-loop
response of the regulator. Where load capacitance C.sub.L is large
(as is often the case in regulator circuit applications), the pole
will be created at a relatively low frequency. This pole may
superimpose on the phase response of the regulator a phase shift
approaching 90.degree. over a wide range of frequencies, especially
if load resistance R.sub.L is light. If the phase shift induced by
the load capacitor 226 overlaps to a large extent with that created
by capacitor 228, a destabilizing build-up of excess phase shift
may result at an undesirably low frequency.
To mitigate the degree of overlap, and thus to reduce the build-up
of excess phase shift, capacitor 228 should be prevented from
rolling off the gain of input stage 200a too far below 0.5. This is
accomplished by placing resistor 230 in series with capacitor 228.
Resistor 230 creates a system zero that stops the rolloff of gain
due to capacitor 228. In the frequency compensation scheme of FIG.
1, this result could not be achieved without increasing the minimum
gain of the input stage. Thus, phase and gain considerations had to
be offset. This disadvantage is overcome by the circuit of FIG. 2,
where gain can be reduced below 0.5 independent of phase shift
requirements.
The present invention has a broad range of applications, and can be
used in various regulator and operational circuit designs. The
optimum values of resistors 217 and 230, and of capacitor 228, are
dependent on the particular design of the circuit, as well as the
particular parameters of other devices in the circuit and the range
of expected load conditions. Selection of optimum values for a
particular application is well within the ordinary skill of a
person in the art of analog circuit design, and can be done
empirically. Desirable results in typical circuits and applications
should be achievable using values for resistors 217 and 230 and
capacitor 228 in the approximate ranges of 1-200K ohms, 0.1-10K
ohms, and 1-100pF, respectively. Thus, capacitor 228 can be made
sufficiently small to allow it to be economically incorporated in
an integrated circuit device.
An alternative embodiment of the present invention is illustrated
by the simplified schematic of regulator circuit 300 in FIG. 3.
Regulator circuit 300 includes a conventional error amplifier 302
with differential outputs 308 and 310. In this embodiment,
cancellation of the error signal in output 310 of error amplifier
302 is achieved, without the need for split collectors in the error
amplifier transistors, by shunting output 310 to a point in the
circuit having negligible effort on loop gain, such as terminal
V.sub.IN (as shown in FIG. 3). Current mirror 215 is balanced by
providng a dc current source 332 which conducts a non-signal
current 12" to node 220. Current source 332, which can be
implemented in a conventional manner, is designed such that current
I2" substantially matches the non-signal component of current I1"
conducted by error amplifier output 308. The remaining components
of regulator circuit 300 operate as previously described with
respect to FIG. 2. Likewise, optimum values for resistors 217 and
230 and capacitor 228 are chosen in accordance with the principles
of the present invention already discussed. Thus, a novel frequency
compensation technique for an analog circuit is provided. Persons
skilled in the art will appreciate that the present invention can
be practiced by other than the described embodiments, and that in
actual circuits various additional components and alternative
interconnections not shown in the figures may be used. For example,
although negative regulator circuits are shown and described, the
present invention can be implemented in positive regulators and
operational amplifier circuits as well. Also, the error amplifiers
of FIGS. 2 and 3 may be implemented with composite npn-pnp
transistors at the inputs, as in the circuitry of the LM120, or as
in the .mu.A-741 or LM101 type operational amplifiers commercially
available from various manufacturers. An error amplifier with
Darlington-connected transistor pairs also could be used. The
described embodiments are presented for the purposes of
illustration and not of limitation, and the present invention is
limited only by the claims which follow.
* * * * *