Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2000-09-01
2002-04-30
Mottola, Steven J. (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S259000
Reexamination Certificate
active
06380806
ABSTRACT:
TECHNICAL FIELD
The present invention generally relates to operational amplifiers and, more particularly, to a switch capacitor common mode feedback (CMFB) circuit portion for a fully differential telescopic operational amplifier for reducing systematic current mirror error, the operational amplifier being particularly suited for use as a sample and hold stage or a residue amplifier stage in a high resolution pipeline analog to digital converter.
BACKGROUND ART
FIG. 1
illustrates a traditional switched capacitor common mode feedback (CMFB) circuit portion
10
for a differential telescopic operational amplifier. The operational amplifier has an input differential pair made up of MOSFETs M
3
and M
4
having gates respectively connected to input nodes In+ and In−, where In+ and In− are the non-inverting and inverting inputs of the operational amplifier. MOSFET M
1
is connected to mirror current to M
2
such that bias current I
3
is approximately equal to the sum of bias currents I
1
and I
2
. The current mirroring uses a switched current technique via switch
12
a
. Switch
12
a
is implemented with CMOS transmission gates enabled by a non-overlapping clock signal. In the context of a sample and hold amplifier, the clock signal defines a sample period and a hold period. More specifically, during the sample period, switches
12
a
,
12
b
and
12
c
close and capacitors C
CMFB+
and C
CMFB−
are charged to a voltage approximately equal to the output common mode voltage during the sample period (V
OCM, sample
) minus the gate to source voltage of the pair M
1
and M
2
(V
GS,M1/M2
). During the hold period, switches
12
a
,
12
b
and
12
c
open and M
2
along with the capacitors C
CMFB+
and C
CMFB−
form a common mode feedback loop for the telescopic amplifier.
The traditional switched capacitor CMFB portion
10
, however, suffers from the introduction of error in current mirroring (i.e., I
error
) such that the bias current I
3
plus l
error
no longer is approximately equal to I
1
plus I
2
. For no systematic current mirroring error due to channel length modulation, the drain to source voltage of M
1
(i.e., V
DS,M1
) should, theoretically, be equal to the drain to source voltage of M
2
(i.e, V
DS,M2
). However, in the traditional switched capacitor CMFB portion
10
, the drain to source voltage of M
1
is equal to the gate to source voltage of M
1
(i.e., V
DS,M1
=V
GS,M1
) and the drain to source voltage of M
2
is equal to the input common mode voltage (V
ICM
) of the differential input signal minus the gate to source voltage of differential pair M
3
/M
4
(i.e., V
DS,M2
=V
ICM
−V
GS,M3/M4
). If V
DS,M1
does not equal V
DS,M2
, a systematic error in current mirroring due to channel length modulation is to be expected, which can cause the output common mode voltage during the hold period (V
OCM hold
) to deviate significantly from to move significantly from V
OCM hold
. It is noted that V
OCM, hold
is equal to (V
OUT+
+V
OUT−
)/
2
. To reduce channel length modulation, it is known to cascode both M
1
and M
2
. However, to accommodate high output voltage swing (e.g., a swing of about 1.5 V where V
DD
equals 3.0 V), the M
1
/M
2
current mirror is not cascoded. It is also known to increase the lengths of M
1
and M
2
to decrease channel modulation effect. However, increases in length are limited due to a corresponding increase in the gate capacitance of M
2
which, if too great relative to the capacitances of C
CMFB+
and C
CMFB−
, can reduce the common mode feedback loop gain.
FIG. 2
illustrates another switched capacitor CMFB circuit portion
20
for a telescopic operation amplifier using a more elaborate current mirroring technique to accommodate high voltage swings. For a detailed discussion of the input stage
20
, attention is directed to Gulati, Kush and Lee, Hae-Seung,
A High
-
Swing CMOS Telescopic Operational Amplifier
, IEEE Journal of Solid-State Circuits, Vol. 33, No. 12, pp.2010-2019, December 1998.
Briefly, the current mirroring technique of the switched capacitor CMFB portion
20
uses a technique referred to as replica loop feedback. More specifically, the replica loop feedback replicates the differential pair M
3
/M
4
for M
1
as differential pair M
5
/M
6
. In addition, a negative feedback circuit using an operational amplifier
22
and a compensating capacitor (not shown) is provided to force the drain voltage of M
5
/M
6
to be equal to the bias voltage. The compensating capacitor for the operational amplifier
22
has a capacitance approximately equal to the gate capacitance of M
1
. Although the current mirroring for the switched capacitor CMFB portion
20
does provide good minimization of current mirror error, it suffers from the disadvantage that extra circuitry to implement the operational amplifier
22
is required.
SUMMARY OF THE INVENTION
The present invention provides a fully differential telescopic operational amplifier having a switch capacitor common mode feedback (CMFB) circuit portion. The switched capacitor CMFB circuit portion has a current mirror for mirroring current from a first transistor to a second transistor. The first and second transistors have gates which are coupled via a transmission gate switch. Drains of each of the first and second transistors are respectively coupled to a first and a second differential pair of transistors, each differential pair connected to receive a common mode voltage and drains of the first differential pair connected to the gate of the first transistor. The first and second differential pair are scaled with respect to each other such that the gate to source voltage of the first transistor is substantially equal to the gate to source voltage of the second transistor.
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Advanced Micro Devices , Inc.
Mottola Steven J.
Renner , Otto, Boisselle & Sklar, LLP
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