Rail-to-rail input stage

Details Rail-to-rail input stage Rail-to-rail input stage

1999-11-30

2001-06-19

Pascal, Robert (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including differential amplifier

C330S255000

Reexamination Certificate

active

06249184

ABSTRACT:

TECHNICAL FIELD
The present invention is generally related to integrated amplifier circuits, and more particularly to operational amplifier circuits and input stages therefore.
BACKGROUND OF THE INVENTION
Operational amplifiers, commonly referred to as Op Amps, are integrated circuit amplifiers that have two inputs, one commonly referred to as the positive or non-inverting input, and a negative input commonly referred to as the inverting input. The operational amplifier has a single output. Operational amplifiers form a portion of numerous circuit designs, and are typically implemented into circuits to form functional circuits such as inverters, comparators, and voltage regulators just to name a few.
Operational amplifiers fabricated on a semiconductor wafer are formed by numerous subcircuits including input stages, buffering stages, output stages. In each of these various stages there are defined components forming current sources, current mirrors, amplification stages and voltage rails, one rail adapted to receive a voltage source having a potential greater than the other rail, which is typically tied to ground but may have other voltage potentials depending on the implementation.
With specific reference to input stages for Op Amps, one relatively simple prior art input stage is shown in FIG.
1
and depicted at
10
.
FIG. 1
shows a first pair of transistors Q
9
and Q
10
along with transistors Q
5
and Q
6
forming on differential input, and transistors Q
1
and Q
2
forming another differential input. When the differential input formed by Q
9
, Q
10
, Q
5
and Q
6
is on, the transconductance (Gm) is half of that produced by the pair of transistors Q
1
and Q
2
. This input stage is a good approach in that it can be easily implemented in a normal operational amplifier. The problem with this approach is that when the common mode range reaches the positive rail, the transconductance of the input stage halves.
There is desired improved input rail-to-rail input stage for operational amplifiers having the improved transconductance.
SUMMARY OF THE INVENTION
The present invention achieves technical advantages as a rail-to-rail input stage for an operational amplifier having a constant transconductance over a common mode range. Two transistors form a current mirror such that two transistors of one differential input run at the same current as two transistors forming another differential input when active.
The present invention achieves technical advantages by providing a cross-coupled quad in a rail-to-rail input stage of an operational amplifier establishing essentially an infinite transconductance, allowing the transconductance to be established and determined by a pair of transistors forming one differential input of the operational amplifier. Four transistors forming the cross-coupled quad, which can be MOSFETs or BJTs, do not contribute to the transconductance, therefore, the transconductance with either input stage active is the same.


REFERENCES:
patent: 5138276 (1992-08-01), Marie
patent: 5392002 (1995-02-01), Delano
patent: 5532644 (1996-07-01), Nakagawara
patent: 5621356 (1997-04-01), Phillippe
patent: 6054897 (2000-04-01), Sugawara

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