Voltage-current converter

Details Voltage-current converter Voltage-current converter

2002-08-15

2003-07-01

Berhane, Adolf Denske (Department: 2838)

Electricity: power supply or regulation systems

Self-regulating

Using a three or more terminal semiconductive device as the...

C327S103000

Reexamination Certificate

active

06586919

ABSTRACT:

BACKGROUND OF THE INVENTION
Field of the Invention
The invention concerns a voltage-current converter having a first current mirror containing two transistors that are designed such that under identical drive conditions the current flowing through the first transistor is greater than the current flowing through the second transistor, which constitutes the output current of the voltage-current converter, by a predetermined factor.
Voltage-current converters are well-known in the prior art, and are used for converting an input voltage into a proportional output current. This is required, for example, for the voltage-controlled oscillator (VCO) in a phase-locked loop (PLL).
The voltage-current converter that is known in the art and that has been mentioned above is shown in FIG.
2
. It contains a current mirror
10
having two normally-off n-channel MOSFETs
12
,
14
(metal-oxide-semiconductor field-effect transistors). The current mirror
10
is programmed using a series resistor
16
that is connected in series with the drain of the first transistor
12
to the input voltage U
E
. The series resistor
16
determines the drain current I
12
of the first transistor
12
, and this drain current I
12
constitutes the input current I
E
of the current mirror
10
.
The gates of the two transistors
12
,
14
are connected together and are also connected to the drain of the first transistor
12
, so that both transistors
12
,
14
are driven under the same conditions. The source of the first transistor
12
is connected to ground. The source of the second transistor
14
is connected to ground, and the output current I
A
of the voltage-current converter is taken from the drain of the second transistor
14
.
The current mirror
10
is disclosed in FIG. 6.21 in the book SEIFART, MANFRED,
Analoge Schaltungen
-5.
Auflage
(
Analog circuits
-5
th Edition,
Verlag Technik GmbH, Berlin, 1996, DE (ISBN 3-341-01175-7). The circuit shown in
FIG. 2
is different from the voltage-current converter that is known from Seifart in that the input voltage U
E
is connected to the series resistor
16
instead of to the supply voltage U
DD
. Consequently, the input voltage U
E
is proportional to the input current I
E
in accordance with the resistance value of the series resistor
16
.
Since the transistors
12
,
14
are operated in the saturation region, their respective drain currents I
12
, I
14
are proportional to each other. Provided the remaining parameters, such as the surface mobility of the charge carriers in the channel &mgr;
0
, the gate capacitance per surface area C
0x
and the threshold voltage U
T
, are identical for the transistors
12
,
14
, then this proportionality can be set simply by selecting the geometrical dimensions of the transistors
12
,
14
. In this case the following equation holds for the two drain currents I
12
and I
14
:
I
14
/I
12
=&bgr;
14
/&bgr;
12
,
where &bgr;=W/L is the geometrical quotient of a transistor of channel width W and channel length L.
If the layout of the first transistor
12
and the second transistor
14
on the chip is such that the geometrical dimensions result in the equation &bgr;
12
=10·&bgr;
14
, for instance, by the channel of the first transistor
12
being made the same length but ten times wider than the channel of the second transistor
14
, then one accordingly obtains the relationship I
12
=10·I
14
.
Thus in this case, because of the aforementioned proportionality between the input voltage U
E
and the input current I
E
≡I
12
, the drain current I
14
of the second transistor
14
, which constitutes the output current I
A
of the known voltage-current converter, is proportional to the input voltage U
E
.
Since in the cited applications of the phase-locked loop, the input voltage U
E
normally lies in the range of 2 to 5 volts, and the required output current intensity I
A
is meant to lie in the region of a few nanoamps, the series resistor
16
must have a resistance value in the region of several megaohms (M&OHgr;). Resistances of this order of magnitude, however, require a very large area in integrated circuits, which is a major disadvantage because the costs of integrated circuits are mainly determined by the area requirement.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a voltage-current converter which overcomes the above-mentioned disadvantages of the prior art apparatus of this general type.
With the foregoing and other objects in view there is provided, in accordance with the invention, a voltage-current converter with a first current mirror including a first transistor and a second transistor each being designed such that under identical drive conditions a current flowing through the first transistor is greater than a current flowing through the second transistor by a predetermined factor; a second current mirror including a first transistor and a second transistor; and a MOSFET connected in series with the first transistor of the first current mirror. The MOSFET has a gate connected to an input voltage. The current flowing through the second transistor is an output current of the voltage-current converter. The first transistor of the first current mirror and the first transistor of the second current mirror are connected in series to a supply voltage. The second transistor of the first current mirror and the second transistor of the second current mirror are connected in series to the supply voltage.
In accordance with an added feature of the invention, a current flowing through the first transistor of the second current mirror is equal to a current flowing through the second transistor of the second current mirror.
In accordance with an additional feature of the invention, the first transistor of the first current mirror and the second transistor of the first current mirror are operated in weak inversion.
In accordance with another feature of the invention, the MOSFET has a threshold voltage such that the voltage-current characteristic starts at 0.
In particular, it is an object of the invention to provide a voltage-current converter that requires less area that that required by known voltage-current converters.
In the voltage-current converter, the series resistor
16
previously required in the voltage-current converter known in the art is dispensed with, and since the MOSFET that is now provided occupies a considerably smaller area in an IC compared with a resistor, a considerable area savings is obtained, even though more components are provided compared with the voltage-current converter known in the art.
In order to simplify the explanation of how this voltage-current converter works, it is assumed below that in the second current mirror the two transistors are identical, which here implies that currents of equal magnitude flow through them under identical drive conditions. In addition it is assumed that the factor equals ten.
If the first current mirror were considered on its own, currents of different magnitudes would flow through its two transistors under the same drive conditions, or more precisely the current through the first transistor would equal ten times the current through the second transistor in accordance with the factor. In other words, the first transistor has a conductance that is ten times the conductance of the second transistor in accordance with the factor.
This first current mirror is not on its own, however, but is connected in series with the second current mirror to the supply voltage, which, like the input voltage, lies normally in the range 2 to 5 volts. The two first transistors are connected in series and form the input-current path of the voltage-current converter. The two second transistors are connected in series and form the output-current path of the voltage-current converter. The two identical transistors of the second current mirror ensure that currents of equal magnitude also flow through the two non-identical transistors of the first current mirror. Since this has no effect on their conductances, howeve

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