Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
2000-06-06
2002-04-23
Zweizig, Jeffrey (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
Reexamination Certificate
active
06377114
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to current generation circuitry and, in particular, to a current generating circuit which does not rely upon resistors to control the current magnitude and which has a temperature coefficient which is advantageous in many applications.
2. Background Art
Referring to the drawings,
FIG. 1
is a schematic diagram of a conventional current generator circuit which utilizes both MOS and bipolar circuit components. Circuit
6
includes a pair of PMOS transistors
10
A and
10
B connected as a current mirror. Since transistors
10
A and
10
B are the same size, the drain-source currents for the two transistors are the same. A second pair of cascode-connected NMOS transistors
12
A and
12
B are connected in series with transistors
10
A and
10
B, respectively. Transistors
12
A and
12
B operate to maintain their respective source voltages, VA and VB, at the same level.
PNP transistors
14
A and
14
B are connected in series with transistors
12
A and
12
B and thus conduct equal currents. Typically, transistors
14
A and
14
B are parasitic substrate transistors that are present in many circuits fabricated using conventional MOS processes. Transistor
14
B has an emitter area A
2
which is larger than the emitter area A
1
of transistor so that the base-emitter voltage
14
B is smaller than the base-emitter voltage of
14
A. The difference in base-emitter voltages &Dgr;VBE is given by the following equation:
&Dgr;
VBE=
(
kT/q
) ln (
A
2
/
A
1
) (1)
where k is Boltzmann's constant, q is electronic charge and T is temperature in Kelvin.
Since &Dgr;VBE is the voltage drop across resistor R, the current flow I through resistor R is as follows:
I=[
(
kT/q
) ln (
A
2
/
A
1
)]/
R
(2)
The output Iout of current generator
6
is provided by a third PMOS transistor
10
C connected to have the same gate-source voltage as transistors
10
A and
10
B. Iout can be made to differ from I by adjusting the channel width of transistor
10
C relative to the channel width of transistors
10
A and
10
B.
One shortcoming of the
FIG. 1
biasing circuit is due to the fact that the value of resistor R, which determines the output current Iout, is not well controlled. In a typical CMOS process, resistor R is made of diffusion or poly silicon. Neither of these materials provides a tight control on the resistor value, which could vary ±30% from the nominal value.
If a circuit being biased by the
FIG. 1
current generator circuit requires a certain amount of minimum current, Inom, the current generator must be capable of providing 1.3 (Inom) to ensure that current Inom will be provided where the resistance is 30% larger than the nominal value. At the same time, if the resistance turns out to be 30% less than the nominal value, then the current generator will provide (1.3)(1.3) Inom or 1.7 Inom. This is 70% more current than the current generator was nominally required to provide.
The present invention addresses the above-noted shortcomings of the prior art by providing a current generator circuit with an output current which is more precisely controlled. Thus, unnecessary power consumption is substantially reduced. In addition, as will be explained, the current generator circuit disclosed herein is capable of enhancing the settling time of amplifier circuits which are biased by the circuit. These and other advantages of the present invention will become apparent to those skilled in the art upon a reading of the following Detailed Description of the Invention together with the drawings.
SUMMARY OF THE INVENTION
A current generator circuit which provides an output current having a stable absolute value and a temperature coefficient which, when used to bias an amplifier, provides reduced settling time and optimum power consumption. A first MOS transistor conducts a current related to the output current and is biased to operate in the linear region. A second MOS transistor, having gate and source electrodes which are connected to the gate and source electrodes of the first MOS transistor, is biased for saturation region operation. The second MOS transistor also conducts a current related to the output current and is typically equal to the current of the first MOS transistor.
The current generator circuit preferably further includes a pair of bipolar transistors operating at different current densities to as provide different base-emitter voltages. The bipolar transistors are connected relative to the first MOS transistor so that the drain-source voltage of the first MOS transistor is equal to the difference between the base-emitter voltages. The output current is more stable than that provided by prior art current generator circuits and provides a current which is approximately proportional to TD
0.5
, with T being temperature in degrees Kelvin.
REFERENCES:
patent: 4450367 (1984-05-01), Whatley
patent: 4792750 (1988-12-01), Yan
patent: 4890052 (1989-12-01), Hellums
patent: 4935690 (1990-06-01), Yan
patent: 5144223 (1992-09-01), Gillingham
patent: 5432432 (1995-07-01), Kimura
patent: 5559425 (1996-09-01), Allman
patent: 5619160 (1997-04-01), Sirito-Olivier et al.
patent: 5818292 (1998-10-01), Slemmer
patent: 5892388 (1999-04-01), Chiu
Girard & Equitz LLP
National Semiconductor Corporation
Zweizig Jeffrey
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