Amplifiers – With semiconductor amplifying device – Including signal feedback means
Reexamination Certificate
2000-02-29
2001-04-24
Pascal, Robert (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including signal feedback means
C330S098000
Reexamination Certificate
active
06222418
ABSTRACT:
TECHNICAL FIELD
This invention is related to an improved negative feedback op-amp circuit, such as a high performance integrator having an op-amp with low conductance for use in integrated circuits.
BACKGROUND OF THE INVENTION
Operational amplifiers in negative feedback arrangements are common circuit elements in analog integrated circuits. An ideal integrator circuit has infinite DC gain and a constant phase of −90°. However, due to non-idealities, these circuits have a finite gain and a phase shift which is different from −90° (henceforth called phase error). In particular, parasitic input and output capacitances introduce a extra poles in the transfer equation which produces unacceptable phase errors if the pole is too close to the unity gain frequency of the integrator.
This problem becomes particularly acute for high frequency applications which are implemented using MOS technologies. This is because the op-amps built using these technologies are usually single-stage circuits that are built with MOSFETS which have a limited transconductance. This limitation reduces the frequency of the parasitic pole.
A conventional feedback circuit is illustrated in
FIGS. 1
a
and
1
b
. The circuit
10
comprises an operational amplifier
12
having trans-conductance g
m
and a transconductance amplifier
14
having trans-conductance G
m
. Ideally, the transconductance amplifier
14
sources (or sinks) an output current equal to G
m
V
in
. A transconductance amplifier
14
is used instead of the more conventional resistor to ensure adequate DC gains for the integrator, which is the cascaded gains of the transconductor and the opamp. A feedback impedance
16
of magnitude Y is connected between the inputs and outputs of the op-amp
12
. Also illustrated are the parasitic input and output capacitances C
pi
18
and C
po
20
, respectively.
The frequency domain transfer function for this circuit
10
can be written as:
H
I
(
s
)
≈
[
1
s
(
Y
/
G
m
)
]
[
1
-
s
(
Y
g
m
)
1
+
s
(
C
po
+
C
pi
g
m
)
]
(
Equ
.
1
)
The first term in the equation represents the transfer function for an ideal op-amp
12
. The second term is a result of the non-ideal input and output capacitances combined with a non-infinite g
m
. Because of the difference in sign between the numerator and denominator of the non-ideal equation component and the non-infinite g
m
, the phase error terms of the pole and zero do not cancel and a net negative phase error is produced. The lower the value of g
m
, the more significant the error introduced by these terms, and thus the more significant the impact of the pole/zero on the performance of this circuit and other circuits which include a similar feedback circuit design.
Because of the feedback loop, the op-amp
12
must generate the same current as provided by the transconductance amplifier
14
. In addition, opamp
12
must also generate current to account for the current drawn by the parasitic capacitances. With reference to the current flows illustrated in
FIG. 1
b
, the op-amp
12
must source an output current I
O
=I
F
+I
PO
, where I
PO
is the current flow through the parasitic output capacitance C
PO
20
. Further, there is also an induced voltage V
PI
, at the input to the op-amp
12
, which produces an additional current I
PI
. Thus, I
F
=G
m
V
IN
+I
PI
. In other words, some of the output current is “stolen” to supply the parasitic input and output capacitances, This difference results in detriments, such as phase error, which impact the performance of the circuit.
Various techniques have been employed to reduce the errors caused by these non-idealities. In one variation, a resistance is introduced in series with the feedback impedance
16
. This provides some improvement at low frequencies, but is not particularly effective in high frequency situations. Alternative configurations make use of error detection devices which measure the output of the op-amp and adjust various circuit parameters by means of a control signal to compensate for the unwanted phase-shift. However, this technique can be cumbersome and requires relatively complex error detection and adaptive circuitry.
One particular solution for the case when the feedback impedance is a capacitor used for the purpose of Miller-compensating a transconductance stage has been implemented using a Multipath Miller Cancellation technique, such as described in U.S. Pat. No. 5,485,121 and discussed in R. Eschauzier and J. Huijsing, “An Operational Amplifier with Miller-Zero cancelation for RHP zero removal”, ESSCIRC'93, European Solid-state Circuits Conference 1993, pp.122-125. This technique provides a parallel current path which is configured to bypass the Miller-compensated transductance stage and provide a current which compensates for the current directly passing through the Miller capacitor. However, the solution presented is restricted to Miller-compensated amplifiers and does not generally address the problems created by non-ideal amplifiers in negative feedback configurations with non-capacitive impedances.
An alternative solution is to introduce a unity-gain buffer
22
in the feedback loop between the output of the op-amp
12
and the impedance
16
, such as shown in
FIG. 1
c
. The purpose of the buffer
22
is to supply the feedback current G
m
V
IN
instead of the op-amp
22
and thereby avoid introducing a voltage differential at the input of the op-amp
12
which results in a current drain into the parasitic input capacitance. However, the buffer
22
has a finite output impedance R
O
24
. Thus, the transfer function of this circuit is:
V
O
V
I
=
(
-
G
m
Y
)
(
1
-
YR
O
)
(
Equ
.
2
)
The first term in Equation 2 is the ideal behavior. The second term represents the error which results from the non-ideality of the buffer
22
. In particular, the current G
m
V
IN
produced by buffer
22
is forced to flow through the output impedance R
O
24
as well as the feedback impedance
16
. Thus, there is a voltage drop in the feedback path which degrades the performance of the circuit. Although the buffer
22
could be designed to have a very small output impedance, such a buffer would require substantially more power than is generally available for high-frequency, low power devices.
Accordingly, it would be advantageous to provide a generalized op-amp feedback circuit structure with compensation for input and output capacitances.
It would also be advantageous to provide an improved unity-gain buffered feedback circuit with compensation for the output resistance of the feedback buffer.
SUMMARY OF THE INVENTION
According to a first aspect of the invention, a feed-forward compensated negative feedback circuit is provided which comprises an operational amplifier with a conductance gm and having an inverting and a non-inverting input and an output. A non-capacitive impedance element is connected between the output of the operational amplifier and its inverting input to form a negative feedback loop. The inverting input of the op-amp is driven with a first transconductance amplifier having conductance Gm and which produces an output current proportional to an input voltage. A feed-forward transconductance amplifier with a conductance substantially equal to Gm receives the input voltage and produces an inverted output current proportional to the input voltage. The feed-forward current is injected at the output of the operational amplifier. By providing at the output of the op-amp the amount of current it would be required to carry over the feedback loop, a voltage differential at the op-amp inputs is avoided, thus eliminating parasitic current flows across the parasitic input capacitance and thereby improving the circuits overall performance.
In a second embodiment of the invention, the feed-forward current is injected into a unity-gain buffered feedback circuit at a point between the impedance element and the unity-gain feedback buffer. By providing the feedback current from an external source, the buffer does n
Gopinathan Venugopal
Prodanov Vladimir I.
Choe Henry
Darby & Darby
Lucent Technologies - Inc.
Pascal Robert
LandOfFree
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