Amplifier slew rate boosting scheme

Details Amplifier slew rate boosting scheme Amplifier slew rate boosting scheme

2000-11-13

2001-10-16

Pascal, Robert (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including differential amplifier

C330S292000

Reexamination Certificate

active

06304143

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to operational amplifier (op-amp) slew rate boosting schemes, and more particularly to a technique to boost the slew rate of an op-amp which has a closed-loop gain that is equal to or very close to unity.
2. Description of the Prior Art
Folded cascode op-amps having a class ‘AB’ buffer output stage and a closed-loop gain that is equal to or very close to unity are well known in the prior art. One such op-amp
100
is illustrated in FIG.
1
. The folded cascode op-amp
100
can be seen to have a class ‘AB’ buffer
102
at its output. The equations which govern the operation of the op-amp
100
are also very well known and have been documented by Paul R. Gray and Robert G. Mayer in
Analysis and Design of Analog Integrated Circuits
, pp. 646-onward, John Wiley & Sons, Inc., 3
rd
Edition. It has been shown that if the op-amp
100
is to be made stable, one must control its unity gain crossing frequency. The conventional technique for controlling this unity gain crossing frequency is to couple a compensation capacitor
104
between a high impedance node
106
of the output buffer
102
and an AC ground
108
. The frequency where the open loop gain falls to unity for op-amp
100
is then defined as:
f
unity
=
G
m
C
comp
(
1
)
where G
m
is the transconductance of the input stage
110
and is determined by the tail current I
tail
shared by the input stage
110
transistors. Specifically, G
m
is defined as:
G
m
=
qI
tail
kT
(
2
)
The slew rate (maximum rate of change in output voltage for large input signals) for op-amp
100
is defined as:
Slewrate
=
I
tail
C
comp
(
3
)
where it is well known the compensation capacitor C
comp
104
must be set to a sufficiently large value in order to make the op-amp
100
stable. If more than two poles exist before the unity gain frequency f
unity
is reached, the op-amp
100
will be unstable. Specifically, the compensation capacitor C
comp
104
operates to push the first dominant pole down to a low enough frequency such that the op-amp
100
gain falls below unity before the second pole is reached. Setting the value of compensation capacitor C
comp
104
to achieve the above desired stability characteristics therefore also establishes the maximum slew rate for the op-amp
100
as set forth in equation (3) above. It is, of course, desirable to have a very high slew rate. In view of the foregoing, it can be appreciated that achieving a very high slew rate requires reducing the value of the compensation capacitor C
comp
104
. Reducing the value of the compensation capacitor C
comp
104
however, makes the op-amp
100
less stable since the first dominant pole will then be moved to a higher frequency as discussed herein before. The desired value of the compensation capacitor C
comp
104
and the desired high slew rate are therefore in direct conflict with one another.
One conventional technique used to address the direct conflict between the desired value of the compensation capacitor C
comp
104
and the desired high slew rate includes reducing the value of transconductance G
m
associated with the input stage
110
be inserting resistors into the emitter paths of the input stage
110
transistors. A lower value for the compensation capacitor C
comp
104
can then be used to achieve amplifier stability as seen by equation (1) above, which also then increases the slew rate as seen by equation (3) above. This technique however, is problematic in that the emitter resistors added to the emitter paths of the input stage
110
transistors introduce additional noise that cannot be tolerated in specific applications such as illustrated in
FIG. 2
that simply illustrates unity gain op-amp
100
driving a load
102
that is connected to ground in response to an input signal such as might be used to accommodate ADSL systems.
In view of the foregoing, a need exists for a technique to boost the slew rate of amplifiers having a closed-loop gain at or very near to unity without introducing additional noise into the system.
SUMMARY OF THE INVENTION
To meet the above and other objectives, the present invention provides a technique to increase the slew rate of a unity gain operational amplifier (op-amp) beyond that achievable using conventional techniques known in the art. According to one embodiment, an input signal is applied to the input of a unity gain buffer as well as the input of the input stage of the amplifier. The end of the compensation capacitor that is typically coupled to an AC ground is now coupled instead to the output of the unity gain buffer. The slew rate of the op-amp remains as defined by equation (3) above; however since the signal across the compensation capacitor is now much smaller, the slew rate of the op-amp is much higher.
Conventional op-amp compensation techniques provide a high small signal open loop voltage gain at a high output impedance at the op-amp gain node. This high output impedance in combination with the compensation capacitor then operates to place the dominant pole. When a conventional op-amp uses a compensation capacitor to achieve stability, all of the input stage tail current will flow into the compensation capacitor in response to a large step input voltage since one end of the compensation capacitor is coupled to the high impedance gain node while the opposite end of the compensation capacitor is coupled to an AC ground. The tail current will charge the compensation capacitor to achieve a slew rate defined by equation (3) above.
The structure described above in association with the present invention results in both ends of the compensation capacitor being driven by approximately the same voltage. Since approximately the same voltage is driving both ends of the compensation capacitor, the tail current will no longer flow substantially into the compensation capacitor. In other words, because the voltage appearing across the compensation capacitor is now very small, very little current is necessary to charge the compensation capacitor. The slew rate is therefore increased (without sacrificing noise performance by decreasing transconductance of the input stage) since the tail current is now available instead to charge up all of the parasitic capacitance within the op-amp (e.g. input capacitance of output stage, collector capacitance seen at node common to collectors of output stage PNP and NPN transistors).
In one aspect of the invention, a slew rate boosting technique increases the slew rate for amplifiers having a closed-loop gain equal to or very near unity.
In another aspect of the invention, a slew rate boosting technique increases the slew rate for amplifier having a closed-loop gain equal to or very near unity without noise degradation of the amplifier.
In yet another aspect of the invention, a slew rate boosting technique increases the slew rate for amplifiers having a closed-loop gain equal to or very near unity without sacrificing amplifier stability.


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