CMOS input stage with wide common-mode range

Details CMOS input stage with wide common-mode range CMOS input stage with wide common-mode range

2001-09-26

2003-01-21

Shingleton, Michael B (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including differential amplifier

C330S258000, C330S259000, C330S261000

Reexamination Certificate

active

06509795

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to rail-to-rail input common mode range differential amplifiers, and more particularly to a CMOS input stage with wide common-mode range.
BACKGROUND OF THE INVENTION
In many areas of the electronics industry, such as the portable electronics industry, designers are turning toward lower operating voltages. This enables electronic circuit designers to design systems with smaller power supplies, which in turn reduces product weight and size, and increases the life expectancy of the system DC power supply.
However, while reduced operating voltages are beneficial in reducing product size and extending useful battery life, the lower voltages typically adversely affect circuit operation. For example, as circuit supply voltages are reduced, the range of circuit signal voltages which are available is also reduced. The reduced range of operating voltages is especially a concern for designers of operational amplifiers, such as differential amplifiers having a differential input stage.
The design and operation of conventional differential amplifiers is well known. The input stage of the differential amplifier typically includes an upper and lower supply rail, where the input stage further comprises one or more differential transistor pairs with associated current source(s). In addition, the input stage has a common-mode input voltage which defines the voltage range within which the input stage of the amplifier operates. The common-mode input voltage may typically be measured between the upper or lower supply rail of the input stage and the gate or base of one of the transistors, and is sometimes called the rail-to-rail voltage.
It is the goal of differential amplifier designers to design an operational amplifier with an input stage capable of operating over the full range of the common mode voltage. As noted above, however, where the circuit supply voltages are reduced, the range of circuit signal voltages over which the input stage of amplifier may operate is also reduced, which deteriorates the overall effectiveness of the amplifier's operation.
In that regard, designers have attempted to design circuitry such that supply voltages will not be less than the transistor saturation voltage plus the transistor gate to source voltage (e.g., V
sat
+V
gs
). By manipulating the circuit supply voltage in this manner, the circuit transistors are provided sufficient “headroom” to operate. On the other hand, the effect of reducing the supply voltages is that the gate to source voltage V
gs
of the transistor which makes up the input stage of the amplifier will also be reduced.
As used herein, headroom may be defined as the capacity to accommodate an input signal swing without driving an amplifier into saturation or into a non-linear operating region. Where the headroom occupies a larger fraction of the voltage supply range, the available common-mode input voltage range and the operating range of the input stage decreases. That is, where a differential amplifier is forced to extend beyond this narrow range of common-mode voltages, the differential-mode gain may drop off sharply, sometimes to zero. As a result, the differential amplifier may typically operate only over a relatively narrow range of common-mode input voltages.
To extend the common mode operating range, conventional amplifiers are often designed with a rail-to-rail input stage using depletion type transistors. One such rail-to-rail input stage amplifier is currently being produced by ON Semiconductor, Semiconductor Components Industries, LLC as part number MC33502. The MC33502 amplifier uses a single pair of depletion transistors to form a differential input stage. The general operation of the MC33502 part may be understood with reference to
FIG. 1
, below.
FIG. 1
shows an example of a conventional rail-to-rail input common mode range differential amplifier
10
, as described in U.S. Pat. No. 5,808,513 issued Sep. 15, 1998 to Archer (hereinafter “Archer”). Archer purports to provide a rail-to-rail common mode range differential amplifier which operates on rail-to-rail voltages down to approximately one volt.
As shown in
FIG. 1
, amplifier
10
includes a current source
12
that outputs a tail current I
T
, a differential pair
14
having depletion-type transistors M
1
and M
2
which output first and second intermediate currents, I
M1
, and I
M2
, and an active or passive load that amplifies the difference in magnitude between the first and second intermediate currents I
M1
, and I
M2
. In this conventional construction, amplifier
10
may vary the magnitude of the first and second intermediate currents I
M1
, and I
M2
in response to the voltage difference between the differential inputs V
IN
+
and V
IN
−
.
One problem with utilizing depletion-type transistors M
1
and M
2
, however, is that as the supply voltage is reduced, the effective change in the threshold voltage due to the body effect becomes less and less. Thus, for example, where the threshold voltage effectively drops from 0.5 volts to −0.5 volts for a rail-to-rail voltage of 3 volts, the effective change for a rail-to-rail voltage of only 1 volt may only be seen as a change of 0.5 volts to 0.1 volts. This, in turn, widens the amplifier headroom and lowers t he common-mode range of the amplifier design.
Further, with respect to the construction of the ON Semiconductor part MC33502 which uses depletion-type transistors, a special production process is required which typically includes supplementing the circuit with additional implants. That is, there is a need to add additional diffuision ion-implanting steps to the process to provide a thin channel-type layer under the gate of the MOS transistor. Consequently, as should be understood, the reduced common-mode range and the added complexity of the special production process make the rail-to-rail differential amplifier constructions of the ON Semiconductor part MC33502 and the Archer patent less desirable to use.
Another well known rail-to-rail input stage differential amplifier construction is described in U.S. Pat. No. 4,555,673 issued Nov. 26, 1985 to Huijsing et al. (hereinafter “Huijusing”). Huijusing purports to describe an amplifier that operates between first and second supply voltages where the range for the power supply voltages, V
PS
, is divided into three sub-ranges. A first end sub-range of the invention is described as extending from the first supply voltage to a specified voltage between the supply voltages. The middle sub-range extends from the first specified voltage to a second specified voltage between the first specified voltage and the second supply voltage, and the second end sub-range extends from the second specified voltage to the second supply voltage.
To operate over the different ranges, Huijsing requires a switching of the operation currents depending on the differential portion used as the common mode voltage, V
CM
, enters the end range, where the input transistors in that differential portion are non-conductive. Further, the switching of the operating currents is purportedly done with one or more steering transistors differentially configured with respect to one or both pairs of input transistors. The supply lines for the other differential portion are then purportedly provided with current that is derived from current diverted away from the supply lines for one of the differential portions.
The switching involved in Huijsing, however, is undesirable in that it typically damages the common-mode CMRR. This occurs because the NMOS and PMOS transistors of the input stages have different offset voltages which require the switching point amplifier to rapidly change the offset voltage. The consequence of the rapid switching by the amplifier results in a low CMRR.
Accordingly, a need exist to provide a differential amplifier with a rail-to-rail input stage which provides a wide common-mode range, does not involve added complexities or require excessive switching, and additionally operates within very low voltage (e.g.,

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