Method and device for control of avalanche photo-diode characteristics for high speed and high gain applications

A device that may include a DC power supply coupled to a fixed current source; an APD; a DC voltage regulator that comprises a regulating transistor, arranged to maintain a regulated voltage at a fixed value over different APD currents; a temperature control module that is arranged to maintain a portion of the temperature control module at a fixed temperature; and compensation circuit that comprises a compensation component that is thermally coupled to the APD. A voltage drop over the compensation component is smaller than a voltage drop over the APD. A sum of (a) a current that pass through the APD and (b) a current that passes through the compensation component is fixed. The portion of the temperature control module is thermally coupled to the compensation component and to the APD.

BACKGROUND

When light signal is applied to APD, it generates current (I) that equals a product of a multiplication of the power (P) of light that impinges on the APD, the photo-sensitivity (S) of the APD and the gain (M) of the APD. The Gain (M) is also referred to as an internal gain of the APD.

The power (P) may be measured in Watts [W], the photo-sensitivity (S) may be measured in Ampere per Watts [A/W] and the gain (M) may be measured in Ampere per Ampere [A/A].

The Value of the gain depends on a value of a voltage applied to the APD (hereinafter APD voltage or VAPD) and the APD junction temperature. This dependence is especially strong for high values of the gain. For simplicity of explanation it is assumed that the APD temperature is the temperature of the APD junction on which light impinges. The APD junction temperature is referred to as APD temperature. It is known in the art that the ability to use an APD in high gain applications directly depends on the stability of the APD voltage and voltage noise as well as on the stability of the APD temperature. For example, highly sensitive optical systems may aim for using APD at a gain of 500 and at APD current (IAPD) of 300 microAmpere. Under such requirements, APD junction temperature variation of about 0.05 Celcius may cause an APD current variation (which, for some application, represent an error) of 1/256=0.004=0.4%. A 0.4% error is high enough in order to interfer the correct work of highly sensitive optical systems. Known APD-based systems support APD temperature variations of about 1.0 Celcius, which increases error level above and beyond the requirements of certain highly sensitive optical systems.

It is noted that APD temperature is influenced by the APD average current (static current) and by fast APD current changes (dynamic current). Furthermore, in order to prevent APD damage the level of the average APD current should be limited. For example, the average APD current may be limited to a level of few tens of micro-Amperes till few hundreds micro-Amperes (for example, 500 micro Ampere). An APD current dynamic range in practice may be five decades, i.e. 100,000, or even more, and APD current's frequency range may be in the range of few tens of GHz.

The electrical power dissipated on an APD (P) equals IAPD*VAPD. This electrical power is directly converted to heat. If the APD is maintained at a fixed gain then the value of the APD voltage is constant. Therefore, heat dissipated on the APD may change, for example in the range of 100,000 times.

It has been found that the APD temperature changes over time and this induces changes in the gain of the APD. Thus, static or/and dynamic non-linearity of APD response are experienced and this is undesired for certain applications.

Typically, The APD voltage may be set to values between 0V and 500V, depending on required APD gain (the higher limit may be between 5V and few thousand of volts 3,000V for different technologies of APD). Together with wide APD current dynamic range (static and dynamic) it sets significant challenge to designer of bias voltage supply system.

The APD gain may be between 1 A/A and few thousands A/A. For APD gain in the range of few hundreds and for an allowed error of not more than 1/256=0.4%, the required stability of the APD voltage is in range of few tens of mV peak-to-peak (voltage domain) and the required stability of the APD temperature is in range of few tens of milli-degrees (temperature domain). For gains of one thousand and more above requirements are even tighter.

FIG. 1is a schematic diagram of a prior art device201that includes: (a) Controller19. (b) Direct current to direct current (DC-DC) converter11that serves as a high voltage supply module for providing the APD voltage (VAPD102). DC-DC converter11is controlled by a control signal APD HV set101that is supplied by controller19. (c) APD13. (d) First capacitor C112that filters the voltage supplied to APD13. (e) Trans-impedance amplifier (TIA)14that includes amplifier U111and a feedback resistor R1115. TIA14is arranged to output via output port16an output voltage VOUT104, wherein VOUT=IAPD*R11.FIG. 1also shows load resistor such as Rload17that is connected to output port16.

In voltage domain device201may suffer from the following problems: DC-DC converter11usually has a slow load regulation response (in the range of DC-DC switching frequency, which is about 100 KHz); The time response of the DC-DC converter's output current limiting circuit is slow (in the range of DC-DC switching frequency, which is about 100 KHz); DC-DC converter11usually has a high output ripple and noise.

In the temperature domain—the prior art device201may suffer from the following problems: There are no special means for APD junction temperature stabilization; therefore, APD13may be used with relatively low gains (up to few tens) without to sacrifice APD gain linearity.

FIG. 2illustrates prior art device202. Device202is connected to a load that is represented by Rload17.

Prior art device202includes: (a) Controller19. (b) DC-DC converter11. DC-DC converter11is controlled by (i) control signal APD HV set101that is supplied by controller19and by (ii) an offset signal105provided from temperature feedback module22. (c) APD13. (d) First capacitor C112that filters the voltage supplied to APD13. (e) TIA14. (f) Temperature sensor (TS)30for sensing the temperature of APD13. (g) Temperature feedback module22that receives temperature readings from TS30and outputs temperature offset signal105for compensating for changes in the temperature of the APD13. This circuit may be included in controller19or be separated from the controller19.

Prior art device202allows at least a limited amount of compensation for temperature changes. In voltage domain and for certain applications, this configuration may show the following disadvantages: DC-DC converter11usually has slow load regulation response (in the range of tens KHz). The time response of DC-DC converter's output current limiting circuit is slow (in the range of tens KHz). The DC-DC converter11usually has high output ripple and noise.

In temperature domain and for certain applications this configuration has following disadvantages: The function, realized by temperature feedback module22is complicated (APD gain M depends on both HV and APD temperature), and may be realized properly only in microcontroller with multi-dimensional look-up table (LUT). The time response of the temperature feedback module22is slow (in the range of tens KHz).

FIG. 3illustrates prior art device203. Device203is connected to a load that is represented by Rload17.

Device203includes: (a) Controller19. (b) DC-DC converter11. DC-DC converter11is controlled by control signal APD HV set101that is supplied by controller19. (c) APD13. (d) First capacitor C112that filters the voltage supplied to the APD13. (e) TIA14. (f) Thermoelectric cooler (TEC)40that includes cold plate41, hot plate42and solid state devices43. Solid state devices43transfer heat from cold plate41to hot plate42under the control a TEC controller44. TEC40includes TS30for sensing the temperature of APD13or of cold plate41. TS30provides its temperature readings to TEC controller44. (g) TEC controller44. TEC controller44is also controlled by a temperature set signal106from controller19.

Device203allows at least a limited amount of APD temperature control. TEC controller44controls the temperature applied by TEC40in order to determine the APD temperature and compensate for changes in the APD temperature.

In voltage domain and for certain applications this configuration has following disadvantages: The DC-DC converter11usually has slow load regulation response (in the range of tens KHz). The time response of DC-DC converter's output current limiting circuit is slow (in the range of tens KHz). The DC-DC converter11usually has high output ripple and noise.

In temperature domain and for certain applications this configuration has following disadvantages: The time response of such temperature compensation is slow (in the range of hundred Hz), which allow APD application with low gains (in the range of 50) with limited APD currents (about 50 uA). The temperature stabilization performance is limited by finite thermal resistance between APD die and cold plate41.

FIG. 4is a cross sectional view of a portion211of a prior art device. Portion211includes: (a) controller19, (b) TEC40, (c) TEC controller44, (d) intermediate plate50, (e) APD die71that is located within a package that is illustrated as having base61, housing63and window64. APD die71includes APD junction72that is light sensitive and faces window64and is positioned above electrical insulator62.

Electrical insulator62is electrically insulating but thermally conductive. Electrical insulator62is supported by base61. TEC40includes cold plate41, hot plate42, solid state devices43and TEC controller44. TEC controller44is fed by a control signal from controller19and by temperature reading from TS30that measures the temperature of the cold plate41or of intermediate plate50. Intermediate plate50is connected between cold plate41and base61. Intermediate plate50is more massive than cold plate41and is used for stabilizing the temperature due to its greater mass. It is noted that if cold plate41is big enough then intermediate plate50may be omitted.

FIG. 5is a cross sectional view of a portion212of a prior art device.

Portion212differs from portion211by the location of TEC40and by using the intermediate plate50as a hot plate—instead of being used as a cold plate. TEC40is located within the package that surrounds APD die71. Portion212includes controller19, TEC40, intermediate plate50and APD die71. APD die71is located within a package that includes base61, housing63and window64. TEC controller44may be included inside the package or outside the package.

TEC40is positioned between electrical insulator62and base61so that cold plate41may contact electrical insulator62and hot plate42may contact base61. TEC40also includes TS30and solid state devices43. Intermediate plate50is more massive than hot plate42. Intermediate plate50is used for conducting the heat to an external air or fluid.FIG. 5also illustrates heat flux402that is generated by APD die71and propagates through electrical insulator62and cold plate41.

There is a growing need to provide a device that facilitates the APD at high gain values.

SUMMARY

According to an embodiment of the invention there may be provided a device that may include a direct current (DC) power supply coupled to a fixed current source; an avalanche photo-diode (APD); a DC voltage regulator that comprises a regulating transistor, arranged to maintain a regulated voltage at a fixed value over different APD currents; a temperature control module that is arranged to maintain a portion of the temperature control module at a fixed temperature; a compensation circuit that comprises a compensation component that is thermally coupled to the APD; wherein a voltage drop over the compensation component is smaller than a voltage drop over the APD; wherein a sum of (a) a current that pass through the APD and (b) a current that passes through the compensation component is fixed; and wherein the portion of the temperature control module is thermally coupled to the compensation component and to the APD.

According to an embodiment of the invention there may be provided a method that may includes maintaining, by a direct current (DC) voltage regulator, a regulated voltage at a fixed value; wherein the DC voltage regulator comprises a regulating transistor; outputting the regulated voltage to an avalanche photo-diode (APD) that is coupled to the regulating transistor; maintaining, at a fixed value, a sum of (a) a current that pass through the APD and (b) a current that passes through a compensation component that is thermally coupled to the APD; wherein the compensation component belongs to compensation circuit; wherein a voltage drop over the compensation component is smaller than a voltage drop over the APD; and maintaining, by a temperature control module, a portion of the temperature control module at a fixed temperature; and wherein the portion of the temperature control module is thermally coupled to the compensation component and to the APD.

The voltage drop over the compensation component may be smaller than one fourth of the voltage drop over the APD.

The voltage drop over the compensation component may be smaller than one fifth of the voltage drop over the APD.

The voltage drop over the compensation component may be smaller than one tenth of the voltage drop over the APD.

The APD and the compensation component belong to a thermally homogenous region of the device.

The APD and the compensation component are formed in a same die.

The APD and the compensation component are positioned at opposite sides of a thermally conductive electrical insulating layer.

The APD may include an APD die, the compensation component may include a compensation component die; wherein the device may include a thermally coupling element; and wherein the compensation component die and the APD die are spaced apart from each other and are thermally coupled to each other by the thermally coupling element.

The portion of the temperature control module may be a cooling plate of the temperature control module.

The compensation component may be a transistor.

The compensation circuit may include (i) a trans-impedance amplifier that may be coupled between an anode of the APD and an output port of the device, (ii) a configurable voltage amplifier, (iii) a trans-conductance amplifier that may be coupled between the configurable voltage amplifier and the in parallel to the compensation component.

The device may include (a) a ratio between the voltage drop over the APD and the voltage drop over the compensation component equals (b) a product of a trans-impedance of the second trans-impedance amplifier, a gain of the configurable voltage amplifier and a transimpedance of the first trans-impedance amplifier.

The device may include may include a first current source for feeding, with a first fixed current, a junction that may be coupled to the APD and regulating transistor

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 6illustrates a device204according to an embodiment of the invention.

Device204is connected to a load that is represented by Rload17. Device204includes: (a) Controller19. (b) DC-DC converter11. DC-DC converter11is controlled by control signal fixed HV set109that is supplied by controller19. Control signal fixed HV set109controls the output voltage of the DC-DC converter11. (c) APD13. (d) First capacitor C112that filters the voltage supplied to the APD13. (e) TIA14. (f) TEC controller44g. (g) A temperature control module such as TEC40. TEC40includes cold plate41, hot plate42, TS30and solid state devices43. Solid state devices43transfer heat from cold plate41to hot plate42under the control of TEC controller44.

TS30is for sensing the temperature of APD13or of cold plate41. TS30provides its temperature readings to TEC controller44. TEC controller44is also controlled by a temperature set signal106from controller19.

Device204also includes (h) A DC voltage regulator90that may be a shunt DC voltage regulator. DC voltage regulator90includes regulating transistor Q92and operational amplifier91. Operational amplifier91is fed by control signal APD HV set101(from controller19) and by a feedback signal provided from an output port of the DC voltage regulator90. The collector of transistor Q2is connected to the output port of the DC voltage regulator90. (h) Fixed current source CS80and (i) Second capacitor C218for filtering the output voltage of the DC-DC converter11. Either one of C112and C218can be replaced by any filtering circuit.

The positive node of DC-DC converter11, a first end of second capacitor C218, and a first port of fixed current source CS80are connected to first junction81. The second end of capacitor C218is grounded.

The output terminal of fixed current source CS80, a first end of first capacitor C112, a cathode of APD13and an output port of DC voltage regulator90are connected to second junction82.

The anode of APD13is connected to an input port of TIA14. Cold plate41(or an intermediate plate that is not shown inFIG. 6) is connected to the APD13, to the packaging of the APD or to an electrical insulator connected to an APD die.

Fixed current source CS80drives a fixed current ICS107into the second junction82. DC voltage regulator90maintains the potential of the second junction82to a regulated voltage of a fixed value.

The regulated voltage equals the voltage of the APD-VAPD102.

Fixed current ICS107is split between APD (IAPD103) and Ireg108. Ireg108flows through DC voltage regulator90. Ireg108flows through regulating transistor Q92.

IAPD103and Ireg108may equal fixed current ICS107. It is noted that the currents that flow through a feedback loop and into operational amplifier91and through first capacitor C112are insignificant and can be ignored of. Accordingly, it may be assumed that ICS=Ireg+IAPD.

The DC voltage regulator90may have a relatively low response time and may respond to changes in VAPD very quickly. Furthermore, the regulated voltage outputted by the DC voltage regulator90is much smoother and exhibits much less noise that the output voltage of DC-DC converter11. Device204also exhibits an inherent fast APD current limiting function.

FIG. 7is schematic diagram of a portion213of device204according to an embodiment of the invention.

Integrated circuit111includes both APD and the regulating transistor and the cross sectional view illustrates it as including the transistor body93and the APD body112and a shared substrate1181. The APD and the regulating transistor share the same thermal environment and thus are kept at the same temperature. Accordingly—the integrated circuit111may form a thermally homogenous region601.

The implementation of the APD and the regulating transistor on the same integrated circuit111and in proximity to each other causes both regulating transistor to be kept at the same temperature.

FIG. 7also illustrates the heat flux411caused by power dissipation on APD and the heat flux421caused by power dissipation on the regulating transistor. The sum of those heat fluxes is constant, therefore the temperature of the integrated circuit111is constant.

The sum of both heat fluxes is maintained constant as they are both produced by a constant electrical power supplied to the APD13and the regulating transistor Q92. The constant electrical power equals a product of a multiplication of fixed current ICS107by the regulated voltage of a fixed value VAPD.

Because the sum of both heat fluxes is constant there is a constant temperature difference between the temperature of the APD (and the regulating transistor) and the temperature of the cold plate41. Accordingly—the response period of the TEC is of less importance.

Because the cold plate41is maintained at a constant temperature and because the sum of heat dissipated from the APD13and the regulating transistor is constant—the temperature of the APD is maintained constant and the APD may operate at high gain.

The gain of the APD is determined by the value of the regulated voltage.

FIG. 8is a simplified thermal circuit diagram500of portion213according to an embodiment of the invention.

The following elements of the simplified thermal circuit diagram500illustrate the following: TQ501is the temperature of the regulating transistor. TAPD502is the APD temperature. TCP510is the temperature of cold plate41of TEC40. RTDV503is die vertical thermal resistance related to the regulating transistor. RAPDDV505is die vertical thermal resistance related to APD. RDL504is die lateral thermal resistance between the regulating transistor and the APD. RTIV507is electrical insulator vertical thermal resistance related to the regulating transistor. RAPDIV508is an electrical insulator vertical thermal resistance related to the APD. RIL506is an electrical insulator lateral thermal resistance between the regulating transistor and the APD. RTBASE509is thermal resistance of the package base and of the intermediate plate.

A second end of RTDV503is connected to a second end of RDL504and outputs TQ501. A second end of RTIV507is connected to a second end of RIL506and to a first end of RTDV503. A second end of RAPDDV505is connected to a first end of RDL504and outputs TAPD502. A second end of RAPDIV508is connected to a first end of RIL506and to a first end of RAPDDV505. A second end of RTBASE509is connected to first ends of RTIV507and RAPDIV508. TCP510is an input to a first end of RTBASE509.

These thermal resistances should support good thermal coupling of the regulating transistor and the APD.

In voltage domain this configuration has following advantages: a. DC voltage regulator time response may be relatively fast (in the range of MHz). b. Relatively low ripple and noise may be achieved at the output of the DC voltage regulator c Inherent fast APD current limiting function. d. The first capacitor C112may provide charge for tracking after fast IAPD changes even before the DC voltage regulator responds to these changes.

In temperature domain this configuration has following advantages: a. Time response of temperature compensation is very fast. b. Temperature stabilization performance is limited only by mutual thermal resistance between APD and the regulating transistor that are fabricated on the same integrated circuit and in close proximity to each other.

FIG. 9is schematic diagram of a portion214of device204according to an embodiment of the invention. Portion214differs from portion213ofFIG. 7by having two separate dies—regulating transistor die113and APD die114instead of (single) integrated circuit111. Such a system may be preferred from the reason of practical implementation.

The APD die114and the regulating transistor die113are supported by the electrical insulator62and are proximate to each other.

FIG. 9also shows an APD heat flux432caused by power dissipation on APD, a heat flux431caused by power dissipation on the regulating transistor and a sum heat flux433that is a total heat flux caused by power dissipation on APD and on the regulating transistor, which is constant.

The sum of both heat fluxes is maintained constant as it equals a constant electrical power supplied to the APD13and the regulating transistor Q92. The constant electrical power equals a product of a multiplication of fixed current ICS107by the regulated voltage of a fixed value VAPD.

Because the sum of both heat fluxes is constant there is a constant temperature difference between the temperature of the APD (and the regulating transistor) and the temperature of the cold plate41.

FIG. 10is a simplified thermal circuit diagram550of portion214according to an embodiment of the invention. The following elements of the simplified thermal circuit diagram550illustrate the following: TQ501is the temperature of the regulating transistor. TAPD502is the APD temperature. TD519is a temperature at the bottom boundary of electrical insulator62. Electrical insulator62is highly thermally conductive.

TCP510is the temperature of cold plate41. RTQ511is the thermal resistance of the regulating transistor die113of the regulating transistor. RIL506is lateral thermal resistance of highly thermally conductive electrical insulator62RTDIQ517is a vertical thermal resistance of the highly thermally conductive electrical insulator, related to the regulating transistor. RTAPD515is a thermal resistance of the die of the APD. RTDIAPD518is a vertical thermal resistance of the highly thermally conductive electrical insulator related to the APD. RTBASE509is a thermal resistance of a base61and of the intermediate plate50together

A second end of RTQ511outputs TQ501. A second end of RTDIQ517is connected to a second end of RIL506and to a first end of RTQ511. A second end of RTAPD515outputs TAPD502. A second end of RTDIAPD518508is connected to a first end of RIL506and to a first end of RTAPD515. A second end of RTBASE509is connected to first ends of RTDIQ517and RTDIAPD518. TCP510is an input to a first end of RTBASE509.

FIG. 11is schematic diagram of a portion215of device204according to an embodiment of the invention.

Portion215differs from portion214by having an highly thermally conductive electrically insulating element118that is positioned between the regulating transistor die113and the APD die114and above the electrical insulator62.

For example—thermal conductivities (a property of material) of Alumina Al2O3 ceramics is 30 Wm-1·K-1, of Silicon is 149 Wm-1K-1 and of Diamond—up to 3320 Wm-1K-1. The selection of the material from which the highly thermally conductive material element should depend upon the thermal resistance, which is a property of specific mechanical device.

The thermal resistance is Rphi=x/(A*k), wherein Rphi is is the absolute thermal resistance (across the length of the material) (K/W), x is the length of the material (measured on a path parallel to the heat flow) (m), k is the thermal conductivity of the material (W/(Km)) and A is the cross-sectional area (perpendicular to the path of heat flow) (m 2).

The invention is not limited by the type of APD, APD size and shape, APD die materials, APD manufacturing methods and more. For example, the invention can be implemented with APD may be formed on Si, GaAs, InGaAs and more; the invention is not limited to a single pixel APD. For example the invention can be implemented in a multi-pixel APD camera. The invention is further not limited by the type of insulating material. For example, electrical insulator62may be made of Al2O3 (Alumina), BeO, CVD diamond, natural diamond, as well as other electrical insulating material with high thermal conductivity. The invention is further not limited by the type, design and method of operation of the thermoelectric cooler (TEC)40and its components.

In at least some of the previous figures it was assumed that (a) the APD is coupled to a trans-impedance amplifier (TIA14), (b) the power supply module is a DC to DC converter, (c) the APD is coupled to a first capacitor C112, and (d) intermediate plate50interfaces between base61and TEC40. It is noted that these assumptions are only made for brevity of explanation. The invention is not limited by the specific electrical configuration and many modifications and variations can be implemented. For example (a) the invention is not limited by the type and specific implementation of trans-impedance amplifier; (b) various types of power supply modules other than DC-DC converters may be provided; (c) filtering circuits other than the first capacitor C112may be used; (d) intermediate elements other than a plate may interface between base61and TEC40(e.g. various massive bodies with significant thermal capacity, made in various shapes and of various materials).

It is further noted that any control signals sent by controller19is aimed to set a working point of the TEC40, DC-DC converter11and DC voltage regulator respectively.

FIG. 12illustrates method400according to an embodiment of the invention.

Method400may start by stages410,430and440. Stage410includes maintaining, by a direct current (DC) voltage regulator, a regulated voltage at a fixed value. The DC voltage regulator includes a regulating transistor. Stage410may be followed by stage420of outputting the regulated voltage to an avalanche photo-diode (APD) that is coupled in parallel to the regulating transistor.

The APD and the regulating transistor may belong to a thermally homogenous region of the device. The APD and the regulating transistor may be formed in a same die. The APD and the regulating transistor may be positioned above an electrical insulator that is electrically insulating and thermally conductive. The APD may include an APD die, the regulating transistor may include a transistor die. The transistor die and the APD die may be spaced apart from each other and are thermally coupled to each other by a thermally coupling element.

Stage430includes providing, by a fixed current source, a fixed current to the APD and the regulating transistor so that a sum of currents that flow through the APD and the regulating transistor equals the fixed current.

Stage440may include maintaining by a temperature control module a portion of the temperature control module at a fixed temperature. The portion of the temperature control module is thermally coupled to the DC voltage regulator and to the APD. The portion may be a cold plate.

Method400may also include stage450of outputting by the APD an output current that is responsive to light impinged on the APD and to a gain of the APD. Stage450may be followed by stage460of amplifying the output current generated by the APD by a trans-impedance amplifier that is coupled to an anode of the APD. Stages450and460may be executed in parallel to stages410,420,430and440.FIG. 13illustrates a device600according to an embodiment of the invention.

Device600is connected to a load that is represented by Rload17. Device204includes: (a) Controller19; (b) DC-DC converter11. DC-DC converter11is controlled by control signal fixed HV set109that is supplied by controller19. Control signal fixed HV set109controls the output voltage of the DC-DC converter11; (c) APD13; (d) First capacitor C112that filters the voltage supplied to the APD13; (e) TIA14. TIA14includes an operational amplifier U7142and a resistor R7141that form a transimpedance amplifier having a transimpedance (gain) that is denoted GTIA6; (f) TEC controller44; (g) A temperature control module such as TEC40. TEC40includes cold plate41, hot plate42, temperature sensor (TS)30and solid state devices43. Solid state devices43transfer heat from cold plate41to hot plate42under the control of TEC controller44. Temperature sensor (TS)30is for sensing the temperature of APD13or of cold plate41. TS30provides its temperature readings to TEC controller44. TEC controller44is also controlled by a temperature set signal106from controller19; (h) A DC voltage regulator90that may be a shunt DC voltage regulator. DC voltage regulator90includes regulating transistor Q92and operational amplifier91. Operational amplifier91is fed by a control signal APD HV set101(from controller19) and by a feedback signal provided from an output port of the DC voltage regulator90. The collector of transistor Q2is connected to the output port of the DC voltage regulator90; (i) First fixed current source CS80; (j) Second capacitor C218for filtering the output voltage of the DC-DC converter11. Either one of C112and C218can be replaced by any filtering circuit. (k) Compensation circuit that includes, in addition to TIA14, configurable voltage amplifier AMP6628, transconductance amplifier TC6622that is coupled between the configurable voltage amplifier AMP6628and second DC voltage regulator660that includes amplifier602and a compensation component such as transistor Q7603. Transistor Q7603is fed by a second current source CS2610. InFIG. 13the base of transistor Q7603is connected to amplifier602that has one input connected between transistor Q7603to second fixed current source CS2610. A second input of amplifier is fed with voltage Vcc set.

Transistor Q7603is subjected to a voltage drop (VQ7614) that is smaller (for example less than 10%. 20%, 25%, 33%, 50%) of the voltage drop across APD13.

Second fixed current source CS2610supplied second fixed current Ics2612to a node that is connected to transconductance amplifier TC6622, transistor Q7603and amplifier602. Current IQ7618flows through transistor Q7603and current Itc7616flows towards transconductance amplifier TC6622.

The positive node of DC-DC converter11, a first end of second capacitor C218, and a first port of fixed current source CS80are connected to first junction81. The second end of capacitor C218is grounded.

The output terminal of fixed current source CS80, a first end of first capacitor C112, a cathode of APD13and an output port of DC voltage regulator90are connected to second junction82.

The anode of APD13is connected to an input port of TIA14. Cold plate41is connected to the APD13, to the packaging of the APD or to an electrical insulator connected to an APD die.

First fixed current source CS80drives a first fixed current ICS107into the second junction82. DC voltage regulator90maintains the potential of the second junction82to a regulated voltage of a fixed value.

The regulated voltage equals the voltage of the APD—VAPD102.

The fixed current ICS107is split between APD (IAPD103) and Ireg108that flows through DC voltage regulator90. Ireg108flows through regulating transistor Q92.

IAPD103and Ireg108may equal fixed current ICS107. It is noted that the currents that flow through a feedback loop and into operational amplifier91and through first capacitor C112are insignificant and can be ignored of. Accordingly, it may be assumed that ICS=Ireg+IAPD.

The DC voltage regulator90may have a relatively low response time and may respond to changes in VAPD very quickly. Furthermore, the regulated voltage outputted by the DC voltage regulator90is much smoother and exhibits much less noise that the output voltage of DC-DC converter11. Device204also exhibits an inherent fast APD current limiting function.

The compensation circuit, and especially the compensation component (Q7603ofFIG. 13) are subjected to a smaller voltage drop and thus may have a response time that is even lower than the response time of DC voltage regulator90.

The compensation circuit (including TIA14, AMP6628, TC6622, CS2610and second DC voltage regulator660) performs voltage scaling down from high value of VAPD102to relatively low VQ7614.

The low value of VQ7614allows use of fast transistor Q7603which provides a base for implementation of high speed thermal compensation with low transient artifacts.

In order to keep power dissipation on both APD and Q7603together to be constant for each pre-defined VAPD, we need following Psum=PAPDPQ7=|VAPD×IAPD|+VQ7×IQ7|=const.

Therefore an

IQ⁢⁢7=constVQ⁢⁢7-VAPD×IAPDVQ⁢⁢7=constVQ⁢⁢7-VAPDVQ⁢⁢7×ITC⁢⁢7GTC⁢⁢6×GAmp⁢⁢6×GTIA,
where the second term is a scaled up APD current, allowing the corresponding scale down of the transistor working voltage, and a first term is a constant bias current (a value of Ics2612), which is higher than a maximum value of the scaled up APD current.

The voltage scaling down factor is

(VAPDVQ⁢⁢7),
and the current scaling up factor is (GTC6×GAmp6×GTIA)

For correct system operation, both scaling factors should be equal, i.e.

The practical implementation may include a high speed trans-conductor TC6, transforming a voltage into the current.

A value of VAPDsets the APD internal gain and it may be set to different values according to system application need.

This will change a scaling factor, which will distort an operation of the thermal compensation loop. In order to eliminate such a possible negative influence, an amplifier gain GAmp6or a trans-conductor gain GTC6may be adjusted by the way, which will keep both scaling factors to be equal.

All components, used in the thermal control loop, i.e. TIA14, AMP6628, TC6622, Q7603and amplifier602, may be selected to be very fast, since they are intended to work at low supply voltage, therefore they support an implementation of the wide bandwidth thermal compensation.

For example, for

GTC⁢⁢6=4505×12×6000=0.0075⁢AV=7.5⁢⁢mS
(milli siemens) and the scaling factor is

For IAPD=320 uA this will results with IQ7=320e−6×90=0.0288=28.8 mA.

Changing to VAPD=225 V will require to change to so the

GAmp⁢⁢6=1⁢VV,
scaling factors will become

This circuit may allow a tighter control of the APD junction temperature than in original circuit, and as result a better APD linearity at high output currents and/or higher APD gains.

FIG. 14is schematic diagram of a portion700of device600according to an embodiment of the invention.

Highly thermally conductive insulator722is highly thermally conductive but is electrically insulating.

Transistor body723belongs to transistor Q7603ofFIG. 13. Transistor body723interfaces with transistor die724. Transistor die724is positioned within cavity725that is formed in base61. Base61can be made of a material that has a low thermal conductivity and a low electrical conductivity.

The upper surface of the transistor die724and the cavity725interface with a lower surface of highly thermally conductive insulator722.

APD die114interfaces with an upper surface of highly thermally conductive insulator722. APD die114includes APD junction72and APD body112. Base61is positioned above intermediate plate50.

The cold plate41of TEC40cools intermediate plate50. TS30is arranged to sense the temperature of cold plate41. TEC40also includes hot plate42, solid state devices43and TS30. TEC40is controlled by TEC controller44. The APD has an APD junction72that is light sensitive.

The APD and the transistor Q7share the same thermal environment, dissipate together a constant power and thus are kept at the same temperature. Accordingly—APD die114, highly thermally conductive insulator722, transistor body723and transistor die724may form a thermally homogenous region666.

FIG. 14also illustrates the following heat fluxes:a. Transistor heat flux732that is generated by transistor body723and flows upwards towards the highly thermally conductive insulator722.b. APD heat flux731that is generated by APD body112and flows downwards towards the highly thermally conductive insulator722.c. Right heat flux734and left heat flux733that propagate through the highly thermally conductive insulator722from the right and the left to the cavity725, propagate through base61and intermediate plate50.

The sum of a current that pass through the APD and a current that passes through the compensation component is fixed.

Accordingly—the sum of heat fluxes733and734is constant because they produced by the sum of power dissipated on APD and transistor Q7together.

Because the sum of both heat fluxes is constant there is a constant temperature difference between the temperature of the APD (and the Q7) and the temperature of the cold plate41. Accordingly—the response period of the TEC is of less importance.

Because the cold plate41is maintained at a constant temperature and because the sum of currents that pass through the APD13and Q7is constant—the temperature of the APD is maintained constant and the APD may operate at high gain.

The gain of the APD is determined by the value of the regulated voltage.

FIG. 15is schematic diagram of a portion701of device600according to an embodiment of the invention.

FIG. 16illustrates method800according to an embodiment of the invention.

Method800includes step810of maintaining, by a direct current (DC) voltage regulator, a regulated voltage at a fixed value; wherein the DC voltage regulator comprises a regulating transistor.

Step810is followed by step820of outputting the regulated voltage to an avalanche photo-diode (APD) that is coupled to the regulating transistor;

Method800also includes step830of maintaining, at a fixed value, a sum of (a) a current that pass through the APD and (b) a current that passes through a compensation component that is thermally coupled to the APD; wherein the compensation component belongs to compensation circuit; wherein a voltage drop over the compensation component is smaller than a voltage drop over the APD.

Step830may include using a compensation circuit that monitors and maintains a fixed relationship between the voltage drop over the APD and the voltage drop over the compensation component and a fixed and the opposite relationship between the current through the APD and the current through the compensation element.

Method800also includes step840of maintaining, by a temperature control module, a portion of the temperature control module at a fixed temperature; and wherein the portion of the temperature control module is thermally coupled to the compensation component and to the APD.

Method800may also include stage450of outputting by the APD an output current that is responsive to light impinged on the APD and to a gain of the APD.

Stage450may be followed by stage460of amplifying the output current generated by the APD by a trans-impedance amplifier that is coupled to an anode of the APD.

Stages450and460may be executed in parallel to stages410,420,430and440.