Bipolar rail-to-rail input stage with selectable transition...

Details Bipolar rail-to-rail input stage with selectable transition... Bipolar rail-to-rail input stage with selectable transition...

2002-06-07

2003-02-11

Nguyen, Patricia T. (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including differential amplifier

C330S252000, C330S261000

Reexamination Certificate

active

06518842

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of rail-to-rail amplifiers, and particularly to methods of establishing the transition threshold between the differential input pairs of a rail-to-rail amplifier.
2. Description of the Related Art
A rail-to-rail amplifier's input stage typically includes complementary differential pairs. One differential pair conducts when the common mode voltage V
cm
applied to the amplifier's inputs is above a particular threshold voltage V
th
, and the other pair conducts when V
cm
is less than V
th
.
A typical bipolar rail-to-rail input stage is shown in
FIG. 1. A
PNP differential pair Q
1
and Q
2
have their emitters connected to a common mode node
10
, and an NPN differential pair Q
3
and Q
4
have their emitters connected to a common mode node
12
. The bases of Q
1
and Q
3
are connected to receive one side (VN) of a differential input signal, and the bases of Q
2
and Q
4
are connected to receive the other side (VP) of the input signal. Input stage transistors Q
1
, Q
2
, Q
3
and Q
4
produce currents IPP, IPN, INP and INN, respectively, in response to a differential voltage applied at VN and VP. To form a complete rail-to-rail amplifier, currents IPP, IPN, INP and INN are routed to an output stage (not shown) which produces the amplifier's output voltage.
This type of input stage typically includes a tail current source
14
which provides a tail current I
tail
to the input stage transistors. The input stage is arranged such that I
tail
is provided to common mode node
10
when the common mode input voltage
V
cm
⁢
(
=
VN
+
VP
2
)
is less than V
th
, and to common mode node
12
when V
cm
>V
th
.
One possible mechanism for steering I
tail
to the proper node uses a steering transistor Q
S
, which is connected between node
10
and a pair of transistors Q
m1
and Q
m2
which form a current mirror that mirrors the current in Q
S
to common mode node
12
. Threshold voltage V
th
is applied to the base of Q
S
. When arranged in this way, when V
cm
<<V
th
, Q
S
is off, I
tail
is provided to common mode node
10
, and Q
1
and Q
2
are active. When V
cm
>>V
th
, Q
S
is on, I
tail
is provided to common mode node
12
, and Q
3
and Q
4
are active.
Unfortunately, as the common-mode input voltage crosses V
th
, a discontinuity appears in the output due to crossover distortion. The threshold voltage is generally skewed toward one rail or the other in order to yield the widest distortion-free input range. However, unless the amplifier receives a input voltage which operates over a known, limited range, crossover distortion will inevitably occur.
SUMMARY OF THE INVENTION
A bipolar rail-to-rail input stage is presented which overcomes the problem noted above. A user is allowed to select a desired transition threshold voltage via a “select” signal, thereby providing a selectable crossover-distortion-free common mode input voltage range.
The input stage includes complementary differential input pairs, and a switching circuit which makes one or the other of the input pairs active depending on the relationship between a transition threshold voltage V
th
and the common mode input voltage V
cm
. The present input stage also includes a transition threshold voltage selection circuit which provides a selectable one of at least two different V
th
voltages to the switching circuit in response to a select signal.
The present input stage is preferably arranged such that it can be turned off in response to a disable signal. In this embodiment, the select signal has first, second, and third states which correspond to logic “high”, logic “low”, and “floating” states. The transition threshold voltage selection circuit is arranged to provide a first V
th
voltage to the switching circuit when the select signal is in a first state, to provide a second V
th
voltage when the select signal is in a second state, and to assert the disable signal and thereby turn the input stage off when the select signal is in a third state.
Further features and advantages of the invention will be apparent to those skilled in the art from the following detailed description, taken together with the accompanying drawings.


REFERENCES:
patent: 5371474 (1994-12-01), Wassenaar et al.
patent: 5714906 (1998-02-01), Motamed et al.
patent: 6462619 (2002-10-01), Ivanov et al.

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