Electricity: power supply or regulation systems – Self-regulating – Using a three or more terminal semiconductive device as the...
Reexamination Certificate
1998-12-22
2001-07-24
Berhane, Adolf Deneke (Department: 2838)
Electricity: power supply or regulation systems
Self-regulating
Using a three or more terminal semiconductive device as the...
C323S907000
Reexamination Certificate
active
06265857
ABSTRACT:
This invention relates generally to temperature compensation of electronic circuits and more particularly to a circuit having selectable temperature coefficients to provide constant current to stabilize the performance of a load. The load may be an optoelectronic circuit, such as a parallel array of vertical cavity surface emitting lasers (VCSELs), affected by temperature.
BACKGROUND OF THE INVENTION
It is well known that electronic circuits generate thermal energy which increases the temperature of the circuit and affects its performance; for example, the output of current sources and current mirrors vary with temperature. The output current of one of these current sources, moreover, may drive or bias loads located on an integrated circuit or chip other than the current source and which may also have either unpredictable or unknown responses to changes of temperature. Such an off chip load having an unpredictable or unknown temperature coefficient is the vertical cavity surface emitting laser (VCSEL), a semiconductor laser which emits light parallel to the direction of the optical cavity.
Predicting the necessary bias currents in a VCSEL has been difficult because VCSELs have not been thoroughly or consistently characterized for industrial purposes, and manufacturing processes are also variable. It is known that a VCSEL operates at a higher frequency if the current is modulated between a level just above its light emitting threshold current and a level which results in emission of maximum optical power, instead of the VCSEL current being turned off and on. It is also known that both the light emitting threshold current and the differential quantum efficiency which determines the maximum optical power of a VCSEL drift with temperature. Therefore, a tunable means to provide a current which can compensate for temperature variation of either or both the threshold current or the current for maximum emission of the VCSEL is required.
To compensate for temperature variations of a constant current source or mirror, a bandgap reference may be used to obtain a zero temperature coefficient. Also, a constant current source having either a negative temperature coefficient or a positive temperature coefficient may be used for compensation. In any of the three former cases, once the temperature coefficient is set by choosing semiconductor device dimensions, i.e., emitter widths, resistor values, or MOSFET device dimensions, and the circuit is manufactured, the temperature coefficient cannot be changed. Any of these methods, moreover, do not compensate for changes of the temperature coefficient in the load. A known technique to moderate a load to control its performance that changes with temperature variations is to provide feedback to the current source driving the load. For instance, the optical output power of a VCSEL can be monitored and when the optical power requires adjustment, current driving the VCSEL is increased or decreased, as needed, to maintain constant optical output. When VCSELs or other optical devices are placed for parallel optical transmission, monitoring the output of each optical device becomes impracticable.
It is thus an object of the invention to provide an analog version and a digital version of a constant current source with a range of adjustable temperature coefficients to compensate for temperature effects. The range of temperature coefficients can be from a positive value to a negative value including flat temperature response. The current output which is temperature-compensated can be used to drive parallel loads.
SUMMARY OF THE INVENTION
Thus, a constant current source circuit is provided wherein the current sources comprises a first current source having a positive coefficient of temperature compensation to generate a first bias voltage, and a second current source having a negative coefficient of temperature compensation to generate a second bias voltage, a first current selector connected to the first bias voltage and a second current selector connected to the second bias voltage, and an output current derived from selectively combining current from the first and second current selector. Each of the first and second current selectors may comprise two transistors, one transistor connected to a variable control voltage and the second transistor connected to a reference voltage. The transistors may be bipolar transistors, npn bipolar transistors, p-channel enhancement MOSFETs, n-channel enhancement MOSFETs, p-channel depletion MOSFETs, n-channel depletion MOSFETs, GASFETs, or JFETs. In an embodiment, as the variable control voltage increases, the partial derivative of the output current with respect to temperature decreases. In another embodiment of the invention, as the variable control voltage decreases, the partial derivative of the output current with respect to temperature increases. Yet another manifestation of the invention, as the variable control voltage increases, the partial derivative of the output current with respect to temperature increases. And yet another embodiment of the invention permits the partial derivative of the output current with respect to temperature to decrease as the variable control voltage decreases.
The invention is further embodied in a constant current source circuit, comprising a first current source having a positive coefficient of temperature compensation to generate a first bias voltage, a second current source having a negative coefficient of temperature compensation to generate a second bias voltage, at least one first transistor connected to the first bias voltage and at least one second transistor connected to the second bias voltage. The circuit further comprises a first programmable enable switch connected to the first transistor to enable the first transistor to conduct current having a positive coefficient of temperature compensation and a second programmable enable switch connected to the second transistor to enable the second transistor to conduct current having a negative coefficient of temperature compensation so that output current is a combined current from those transistors which have been enabled to conduct current. An inverter between and connecting the first programmable enable switch and the second programmable enable switch may be provided and the first and second transistors may have the same physical dimensions, such that only one of the first or second transistor is on at any one time. The first transistor and first programmable enable switch, and the second transistor and second programmable enable switch may be configured into an integrated complementary unit cell.
An embodiment of a constant current source circuit is provided which comprises a first n-bit digital-to-analog converter electrically connected to a first current source having a positive coefficient of temperature compensation, where n≧1, and a second m-bit digital-to-analog converter electrically connected to a second current source having a negative coefficient of temperature compensation, where m≧1. The constant current source circuit of this embodiment further comprises at least n first programmable enable lines connected to the first n-bit digital-to-analog converter and at least m second programmable enable lines connected to the second m-bit digital-to-analog converter so that a mixed output of a first current output of the first digital-to-analog converter is added to a second current output of the second digital-to-analog converter having a net temperature coefficient determined by the number of the first and the second programmable enable lines that are on. When n=m, the first n-bit digital-to-analog converter and the second m-bit digital-to-analog converter may further comprise
∑
n
=
0
n
-
1
2
n
integrated complimentary unit cells in a common centroid arrangement.
A constant current source circuit is also provided comprising means to generate a first bias voltage having a positive temperature coefficient, means to generate a second bias voltage having a negative temperature coefficient,
Demsky Kevin Paul
Ewen John Farley
Paschal Matthew James
Berhane Adolf Deneke
International Business Machines - Corporation
Ojanen Karuna
Truelson Roy W.
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