Amplifiers – With periodic switching input-output
Reexamination Certificate
2002-04-04
2003-06-17
Choe, Henry (Department: 2817)
Amplifiers
With periodic switching input-output
C330S011000, C330S051000, C327S307000
Reexamination Certificate
active
06580315
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to amplifiers, and more particularly, to a technique for maintaining an amplifier common-mode output voltage, regardless of whether the amplifier is on or off.
2. Description of the Prior Art
The common-mode output of an amplifier, in some applications, is required to remain at the same voltage, regardless of whether the amplifier is on or off. Two common solutions to achieve this requirement that are known in the art are shown in
FIGS. 1 and 2
. Each figure shows a basic amplifier in a common-emitter configuration with resistor R
1
as the load resistance.
Looking now at
FIG. 1
, one solution is illustrated for maintaining the output voltage V
out
at a constant voltage in which the current source I
high
is always ‘ON’. This solution however, is not a good low power solution, as I
high
can be a substantially large current to achieve a high gain. Further, when using this solution, steps need to be taken to ensure that the AC input signal does not pass through to the amplifier output. If there are multiple cascaded amplifier stages, this is usually accomplished by shutting off the preceding stages while leaving the output stage on to maintain the common-mode output voltage.
FIG. 2
illustrates another solution for maintaining the output voltage V
out
at a constant voltage that is well known in the art. The solution depicted in
FIG. 2
has the same basic amplifier as shown in
FIG. 1
with the addition of voltage reference circuitry (I
ref
and R
ref
) and two passgates or switches (M
0
and M
1
). The voltage reference circuitry is generally scaled to achieve a low-power solution. This is done by making I
ref
X-times smaller than I
high
and by making R
ref
X-times larger than R
1
such that the voltage drop across R
ref
is the same as the voltage drop across R
1
. As the amplifier is shut down (by turning off I
high
), the output V
out
is disconnected from the amplifier and is connected to the voltage reference by turning off M
1
and tuning on M
0
. This does provide a low-power solution to maintaining the amplifier common-mode voltage. This solution however, also degrades the performance of the amplifier when the amplifier is on. Since M
1
is in the signal path of the amplifier, it introduces another pole and reduces the overall amplifier bandwidth. The additional RC pole is due to the on-resistance of the MOS switch M
1
and the capacitance at the V
out
node. By increasing the size of M
1
, the on-resistance of M
1
can be reduced. Increasing the size of M
1
however, also increases the parasitic capacitance associated with M
1
, which then lowers the frequency of the pole at R
1
.
In view of the foregoing, it would be desirable and advantageous in the amplifier art to provide a low-power solution for maintaining an amplifier common-mode output voltage, regardless of whether the amplifier is on or off, that does not degrade the performance of the amplifier through the use of a passgate in the signal path.
SUMMARY OF THE INVENTION
The present invention is directed to a low-power solution for maintaining an amplifier common-mode output voltage, regardless of whether the amplifier is on or off, that does not degrade the performance of the amplifier through the use of a passgate in the signal path.
In one aspect of the invention, a system and method for maintaining an amplifier common-mode output voltage, regardless of whether the amplifier is on or off, is implemented without a passgate or switch in the signal path of the amplifier, such that the bandwidth of the amplifier is not degraded.
In another aspect of the invention, a system and method for maintaining an amplifier common-mode output voltage, regardless of whether the amplifier is on or off, is implemented without necessitating the continuous, uninterrupted flow from the current source (I
high
), such that a high gain can be achieved while simultaneously achieving a low-power solution to maintaining the amplifier common-mode output voltage.
REFERENCES:
patent: 4430622 (1984-02-01), Simoes
patent: 6310519 (2001-10-01), Ross
Brady W. James
Choe Henry
Swayze, Jr. W. Daniel
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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