Miscellaneous active electrical nonlinear devices – circuits – and – Specific identifiable device – circuit – or system – With specific source of supply or bias voltage
Reexamination Certificate
2001-04-09
2002-08-13
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific identifiable device, circuit, or system
With specific source of supply or bias voltage
C327S539000, C327S542000, C323S315000
Reexamination Certificate
active
06433621
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to current source as it relates to an integrated circuit. More particularly, the present invention relates to current sources designed to provide rejection of high power supply voltages.
BACKGROUND OF THE INVENTION
Portable and low power consumption electronic devices such as, e.g., laptop computers, personal digital assistants (PDAs), cellular telephones, and the like, are requiring less and less power to operate. This is due, in part, to the continued advancement in computer and electronic technology, with attention to the design and manufacturing of the components used in the construction of the low power consuming devices. These new low power devices have become more and more powerful such that they are now able to perform processing tasks previously associated with desktop and workstation computers.
However, the circuitry and components implemented in these low power consuming devices is of a design usually implemented in high power consumption devices such as, e.g., engineering workstations and desktop computer systems. In a low power consuming device, this amount of voltage would cause serious damage to the components and circuits contained therein.
Therefore, because the circuitry and components are not particularly well suited for implementation in low power consuming devices, modification of the existing components and circuits or entirely new designs are necessary. Additionally, it is advantageous to provide a current source that is relatively constant and highly insensitive to changes in supply.
One such component associated with most devices, whether high power consumption devices or low power consumption devices like those mentioned above, is the current source. The current source is an important component in circuits.
FIG. 1
shows a conventional current source
1000
commonly used in many of today's circuits. Transistor
400
is the bias transistor and has an emitter coupled to resistor
200
, a base coupled to voltage bias node
800
, and a collector coupled to bias current output
600
. Current flow through transistor
400
is indicated by arrow
401
. Resistors
100
and
200
are coupled to the power source, Vdd,
900
. Diode
300
has its anode side coupled to resistor
100
and its cathode side coupled to voltage bias node
800
. The voltage bias node
800
is coupled to the base of transistor
400
. Current source
500
is coupled to output node
800
and ground
700
. Current flow at current source
500
is indicated by arrow
501
.
Still referring to the circuit
FIG. 1
, it is known that the most dominant error source in circuit
1000
was that when the supply changed, transistor
4
reacted to that change. In this figure, when power supply
900
changes, the collector emitter voltage of transistor
400
will vary with the change in supply. The voltage at node
800
is established through resistor
100
and diode
300
to bias transistor
400
. Because the power supply
900
may vary, and circuit
1000
is susceptible to change in the supply, this circuit does not provide the relative stability needed for use with low power consuming devices. This is reflected in the following equations.
Where Ic
1
=f(Vce
1
), Vbe=constant, Vt=kT/Q are given, and
where,
Ic
1
is the collector current of transistor
400
Vbe is the base emitter voltage of transistor
400
Vt is the result of the diode equation from above
Vce
1
is the collector emitter voltage of transistor
400
V
1
is the load voltage at bias voltage
600
Va is the early voltage of transistor
400
Vdd is power supply
900
R
1
is emitter resistor
100
R
2
is emitter resistor
200
Is is the current source
500
Isat is the saturation current of diode
300
and
transistor
400
The collector current of transistor
400
is:
Ic
1
=
Isat
exp(
Vbe/Vt
)(1
+Vce
1
/Va
) 1)
where Vce=Vdd−V
1
−(Ic
1
*R
2
)
Inserting Vce into equation 1, we obtain
Ic
1
=Isat
exp(
Vbe/Vt
)[1+((
Vdd−V
1
−Ic
1
*
R
2
)/
Va
) 2)
Assuming Va=20, V
1
=1V, and Vdd changed from 2 to 6 volts, current Ic will approximately change 20%.
Therefore, as shown in the equations above, current source
1000
, designed originally for use in a high power consuming device, is not particularly well suited for implementation in a low power consuming device which requires a relatively stable current source. This is due, in part, to the fact that the current of traditional current sources has low power supply rejection because of early voltage. While advancement in BJT (bi-polar junction transistor) technology has been directed toward shallower
3
unctions, which therefore has lower Va (early voltage) result, it is still susceptible to a change in supply. In attempt to provide a circuit more insensitive to a change in supply than circuit
1000
, a cascode current device was introduced.
A cascode current device, historically, is a commonly used cure all to increase a circuit's immunity to change in power supply voltage. A cascode current device is a series transistor, and, in the prior art
FIG. 1
, would be disposed between transistor
400
and bias current output
600
. The cascode current device would have the effect of reducing the dependency of the circuit on power supply
900
, Vdd, such that the reaction of transistor
400
to those changes in Vdd is reduced, thereby becoming more stable.
However, the cascode device is not without drawbacks. Because the cascode current device is a series transistor, and as such requires power, additional voltage must be added to the supply voltage to account for the voltage dropped across the cascode current device. Because the cascode current device requires the minimum power supply voltage to be increased, this is contradictory to the requirements of a low power consuming device or circuit.
Additionally, another drawback is that the cascode current device requires additional real estate within the component or device in which it will be implemented. This might not be possible due to the component's or device's limited amount of available physical space which could therefore require a complete redesign of the device or component.
Thus, a need exists for a current source circuit that can operate at low power supply voltage levels, such as those down to one volt. An additional need exists for a current source circuit that is highly immune to changes in power supply voltage levels. A need also exists for a current source circuit that can compensate for and reject high supply voltages. A further need exists for a current source circuit that requires negligible additional power for proper operation.
SUMMARY OF THE INVENTION
Accordingly, the present invention provides a bias current source for providing high power supply rejection. The present invention further provides a bias current source for use in low power consumption markets. The present invention additionally provides a highly stable bias current source.
The present invention provides a bias current source circuit with very high power supply rejection even with very low Va (early voltage). In one embodiment, the present invention is comprised of a bias current source circuit. The bias current source circuit is comprised of a primary current source coupled to a power supply, a secondary current source for biasing the primary current source and also coupled to the power supply, and a simple gain stage amplifier. In the present embodiment, the primary current source includes a first transistor having a first region coupled to a second resistor and a second transistor having a first region coupled to the second resistor. In the present embodiment, the gain stage amplifier includes a first input coupled to a second region of the first transistor. The gain stage amplifier further includes a second input coupled to a second region of the second transistor. The gain stage amplifier also includes an output coupled to the first regions of the first transistor and the second transistor.
Chen Yinming
Smith Gregory J.
Cunningham Terry D.
National Semiconductor Corporation
Tra Quan
Wagner , Murabito & Hao LLP
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