Voltage to current converter

Details Voltage to current converter Voltage to current converter

2000-01-10

2001-08-21

Cunningham, Terry D. (Department: 2816)

Miscellaneous active electrical nonlinear devices, circuits, and

Signal converting, shaping, or generating

Converting input voltage to output current or vice versa

C327S065000

Reexamination Certificate

active

06278299

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a voltage to current converter. Such a converter has many applications, for example in low noise amplifier (LNA), mixer and automatic gain control (AGC) applications.
2. Description of the Prior Art
FIG. 1
of the accompanying drawings illustrates a known type of voltage to current converter or “transconductor amplifier” which is based on a long tail pair of active devices shown as first and second npn transistors T
1
and T
2
. The emitters of the transistors T
1
and T
2
are connected via resistors R
1
and R
2
, respectively, to one terminal of a tail current source in the form of a constant current source CC
1
whose other terminal is connected to ground gnd. The collectors of the transistors T
1
and T
2
form differential current outputs OUT
1
and OUT
2
, respectively, of the converter. The bases of the transistors T
1
and T
2
are connected to differential input terminals IN
1
and IN
2
, respectively, and via resistors R
5
and R
6
to a bias voltage source BS.
For relatively low differential input voltages to the converter shown in
FIG. 1
, the transconductance is substantially linear so that the differential output current is substantially proportional to the differential input voltage. However, the transconductance is not in fact linear but includes non-linear terms which give rise to distortion. The principal non-linearity in the transconductance is of third order or cubic form and the transconductance gm may be written as:
gm=gml.v+&agr;.v
3
  (1)
where gml is the small signal transconductance, v is the differential input voltage and &agr; is a constant.
BASIC CONCEPT OF THE INVENTION
According to the invention, there is provided a voltage to current converter comprising: a first long tail pair comprising first and second active devices of a first conduction type and a first tail current source, each of the first and second active devices having an output terminal, a control terminal and a common terminal; and a second long tail pair comprising third and fourth active devices of a second conduction type opposite the first conduction type and a second tail current source, the third active device having a control terminal connected to the control terminal of the first active device and an output terminal connected to the common terminal of the second active device, the fourth active device having a control terminal connected to the control terminal of the second active device and an output terminal connected to the common terminal of the first active device.
The common terminals of the first and second active devices may be connected via first and second resistors, respectively, of substantially equal first value to the first tail current source.
The first tail current source may comprise a first constant current source.
The third and fourth active devices may have common terminals connected via third and fourth resistors, respectively, of substantially equal second value to the second tail current source. The second value may be greater than the first value. The second value may be substantially between four and five times the first value.
The second tail current source may comprise a second constant current source.
The tail current of the first long tail pair may be greater than the tail current of the second long tail pair. The tail current of the first long tail pair may be substantially ten times the tail current of the second long tail pair.
Each of the first to fourth active devices may comprise a bipolar transistor whose output, control and common terminals comprise collector, base and emitter electrodes, respectively.
Each of the first to fourth active devices may comprise a field effect transistor whose output, control and common terminals comprise drain, gate and source electrodes, respectively.
It has been found that the non-linear terms of the transconductance, such as the cubic term, can be substantially cancelled or greatly reduced by connecting a second transconductance stage in anti-phase with a first main stage. The second or “corrector” transconductance stage can be made to dissipate substantially less power than the main stage and, from a noise point of view, is connected in parallel with the main stage. The transconductance of the main stage is substantially greater than that of the corrector stage so that the main transconductance stage remains the dominant noise source and the noise performance is not, therefore, substantially degraded.
It is therefore possible to provide a voltage to current converter having substantially improved linearity without substantially or unacceptably degrading other aspects of performance such as power dissipation and noise performance. This can be achieved by the addition of only a relatively small number of components to the converter so that the cost penalty is insignificant.


REFERENCES:
patent: 4379345 (1983-04-01), Uri
patent: 5523717 (1996-06-01), Kimura
patent: 5552729 (1996-09-01), Deguchi
patent: 5714906 (1998-02-01), Motamed et al.
patent: 5804994 (1998-09-01), Marlow et al.
patent: 5892382 (1999-04-01), Ueda et al.
patent: 5963064 (1999-10-01), Toyota et al.

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