Amplifiers – With semiconductor amplifying device – Including particular power supply circuitry
Reexamination Certificate
2003-07-07
2004-12-21
Mottola, Steven J. (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including particular power supply circuitry
C330S261000
Reexamination Certificate
active
06833760
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to differential amplifiers, and in particular, to differential amplifiers operating in a low power circuit environment.
2. Description of the Related Art
Referring to
FIGS. 1A and 1B
, switched capacitor circuits are well known in the art. Such circuits can be used to provide various signal filtering or integration functions and typically have two phases, or states, of operation: a sample phase Oa and a holding phase
10
b
. As is well known in the art, switches (not shown) driven by nonoverlapping clock signal phases (not shown) cause input and feedback capacitances coupled to the input terminals
11
a
,
11
b
of the differential amplifier
12
, to switch between reception of the positive Vinp and negative Vinn phases of the differential input signal Vin, and circuit ground GND, thereby feeding back the positive Voutp
13
a
and negative Voutn
13
b
signal phases of the output signal Vout. Load capacitances Cload couple the output signal terminals
13
a
,
13
b
, to circuit ground GND.
As is further well known, modern complementary metal oxide semiconductor (CMOS) processes are continuing to scale down in terms of power supply voltage magnitudes. Indeed, some circuits are now expected to operate at power supply voltages at or below one volt. Particularly for the amplifier
12
in a switched capacitor circuit, such low power supply voltages result in lost headroom for the analog amplifier circuitry.
Referring to
FIG. 2
, a typical implementation of the differential amplifier
12
includes two circuit branches
12
a
,
12
b
formed by the serial connections of transistors M
2
and M
4
and transistors M
1
and M
3
, powered by a power supply voltage VDD and driven by a tail current source
14
, which together produce a total amplifier bias current of 2*Islew. When used in the switched capacitor circuit
10
, the overall circuitry has one pole at a dominant pole position, and a feedback factor associated with the switched capacitors C. (It should be understood that this simplified analysis ignores the input capacitance of the differential amplifier circuit
12
.)
At low power supply VDD voltages, it is difficult to maintain the transistors M
1
, M
2
, M
3
, M
4
of the amplifier
12
in their respective operational states of saturation. This difficulty increases significantly when cascode devices are added to the P-MOS and N-MOS portions of the circuit
12
. Accordingly, high DC gain cannot be achieved. Moreover, simply increasing the power supply voltage VDD to improve headroom will significantly increase power dissipation of the circuit
12
.
Referring to
FIG. 3
, the aforementioned dominant pole causes the open loop gain bandwidth product GBW′ for the amplifier
12
to be a function of the transconductance gm of the input transistors M
1
, M
2
and the switched and load Cload capacitances. The feedback factor f, which is equal to 0.5 for equal valued switched capacitances C, affects the closed loop gain bandwidth product GBW accordingly (GBW=GBW′*f).
SUMMARY OF THE INVENTION
In accordance with the presently claimed invention, a low power differential amplifier is powered by a plurality of unequal power supply voltages. The input stage operates at a higher power supply voltage so as to maintain its transistors in operational states of saturation while providing a sufficient dynamic signal voltage range. The output stage operates at a lower power supply voltage while providing a sufficient dynamic signal current range.
In accordance with one embodiment of the presently claimed invention, a low power differential amplifier powered by a plurality of unequal power supply voltages includes power supply terminals, telescopic differential amplifier circuitry and voltage follower circuitry. A first power supply terminal conveys a first power supply voltage having a first voltage magnitude. A second power supply terminal conveys a second power supply voltage having a second voltage magnitude which is less than the first voltage magnitude. The telescopic differential amplifier circuitry, coupled to the first power supply terminal, responds to reception of the first power supply voltage and an input differential signal by providing an intermediate differential signal corresponding to the input differential signal. The voltage follower circuitry, coupled to the second power supply terminal and the telescopic differential amplifier circuitry, responds to reception of the second power supply voltage and the intermediate differential signal by providing an output differential signal corresponding to the intermediate differential signal.
In accordance with another embodiment of the presently claimed invention, a low power differential amplifier powered by a plurality of unequal power supply voltages includes power means, differential amplifier means and voltage follower means. A first power means is for conveying a first power supply voltage having a first voltage magnitude. A second power means is for conveying a second power supply voltage having a second voltage magnitude which is less than the first voltage magnitude. The differential amplifier means is for receiving the first power supply voltage and an input differential signal and responding thereto by generating an intermediate differential signal corresponding to the input differential signal. The voltage follower means is for receiving the second power supply voltage and the intermediate differential signal and responding thereto by generating an output differential signal corresponding to the intermediate differential signal.
In accordance with still another embodiment of the presently claimed invention, a low power differential amplifier powered by a plurality of unequal power supply voltages includes power supply terminals and amplifier circuitries. A first power supply terminal conveys a first power supply voltage having a first voltage magnitude and a first power supply current having a first current magnitude. A second power supply terminal conveys a second power supply voltage having a second voltage magnitude and a second power supply current having a second current magnitude, wherein the first voltage magnitude is greater than the second voltage magnitude and the first current magnitude is less than the second current magnitude. First amplifier circuitry, coupled to the first power supply terminal, responds to reception of the first power supply voltage, the first power supply current and an input differential signal by providing an intermediate differential signal corresponding to the input differential signal. Second amplifier circuitry, coupled to the second power supply terminal and the first amplifier circuitry, responds to reception of the second power supply voltage, the second power supply current and the intermediate differential signal by providing an output differential signal corresponding to the intermediate differential signal.
In accordance with yet another embodiment of the presently claimed invention, a low power differential amplifier powered by a plurality of unequal power supply voltages includes power means and amplifier means. A first power means is for conveying a first power supply voltage having a first voltage magnitude and a first power supply current having a first current magnitude. A second power means is for conveying a second power supply voltage having a second voltage magnitude and a second power supply current having a second current magnitude, wherein the first voltage magnitude is greater than the second voltage magnitude and the first current magnitude is less than the second current magnitude. A first amplifier means is for receiving the first power supply voltage, the first power supply current and an input differential signal and responding thereto by generating an intermediate differential signal corresponding to the input differential signal.
A second amplifier means is for receiving the second power supply voltage, the second power supply current and the intermediate differ
Mottola Steven J.
National Semiconductor Corporation
Vedder Price Kaufman & Kammholz P.C.
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