TECHNICAL FIELD

The present invention relates to a compensator device for biasing a gate of a Monolithic Microwave Integrated Circuit (MMIC) High-Electron Mobility Transistor (HEMT) amplifier. The present invention also relates to a MMIC amplifier device comprising such a compensator device, and to a method of compensating a gate voltage of a MMIC HEMT amplifier, wherein the method is carried out with such a compensator device.

BACKGROUND

Conventional MMIC HEMT amplifiers, which are usually designed and manufactured in the GaAs material system, typically suffer from significant performance reductions, which are due to process variations and operating temperatures. In particular, gain, dissipated power, nonlinearity, and yield of an amplifier based, for example, on GaAs HEMT technology is very sensitive to temperature and process variations, when a fixed gate-to- source bias voltage is adopted.

This can be explained by the fact that a drain-to-source current (IDS) of such a GaAs HEMT is related to the gate-to-source voltage (VGS) according to the following equation:

Eqn.l

¾S = gm ( ^v GS - ^y T)

In the above equation, g _m is the transconductance and VT is the threshold voltage of the HEMT. Both parameters g _m and VT are dependent on the operating temperature of the HEMT, and process variations occurring during manufacturing of the HEMT. These variations change the operating IDS for a fixed VGS. Since the main features of GaAs HEMTs, being gain-isolation-input/output impedances, are strictly related to the actual IDS, also the overall performance of an amplifier based on GaAs HEMT technology is very sensitive to variations of the IDS caused by the temperature and process variations.

The sensitivity of a conventional HEMT amplifier to temperature and process variations can be reduced by tuning the gate-to-source bias voltage, in order to obtain a fixed drain- to-source bias current. This method is conventionally implemented with two different approaches.

For a first approach, as is shown in Fig. 7, an external circuit is adopted, in order to read the drain-to-source bias current, and to adjust the gate-to-source bias voltage accordingly. For a second approach, a gate bias circuit showing an opposite sensitivity to temperature and process variations than the HEMT amplifier is integrated in the biasing network of the HEMT amplifier within the MMIC Compared to the first, external feedback approach, the second, integrated compensator approach has the following advantages. External components between DC power suppliers and the MMIC amplifier are not required. Further, the size of the board, in which the MMIC is mounted, can be made smaller than for the first approach. Also, a bias routing complexity is minimized for the second approach. The same is true for instability issues, which are reduced for the second approach. Finally, the costs for the second approach are considerably lower.

However, the second, integrated compensator approach has also some significant disadvantages. To explain these disadvantages, a scheme of the most advanced conventional compensator device based on the GaAs HEMT technology is shown in Fig. 8. The compensator device is composed of two HEMTs (Ql and Q2) and three resistors (Rl, R2 and R3). The input of the compensator device is Vs-, which is a voltage coming from an external gate power supplier. The output of the compensator device is VG, which is also a gate bias voltage applied to the MMIC amplifier, wherein the amplifier is represented by QAMP in Fig. 8.

The compensator device shown in Fig. 8 has two major disadvantages. Firstly, it is designed to compensate only variations of the threshold voltage of the GaAs HEMT amplifier caused by temperature and process variations. However, GaAs HEMT amplifiers are also sensitive to transconductance variations caused likewise by temperature and process variations. These transconductance variations cannot be compensated with the compensator device shown in Fig. 8. Secondly, the compensator device of Fig. 8 dissipates a considerable amount of DC power due to the current flowing across the two HEMTs and the three resistors. The power dissipation reduces the performance of the GaAs HEMT amplifier in terms of efficiency. Others conventional compensator devices and schemes have been reported as well, but all suffer from the same disadvantages as the compensator device shown in Fig. 8.

SUMMARY In view of the above-mentioned problems and disadvantages, the present invention aims to improve conventional compensator devices. The present invention has specifically the object to provide a compensator device for biasing a MMIC HEMT amplifier, wherein the compensator device can compensate variations of both a threshold voltage and a transconductance of the HEMT amplifier, the variations being caused by temperature and process variations. Further, the proposed compensator device should also minimize dissipated DC power. Accordingly, it is an aim to improve, in particular maximize, the efficiency of the HEMT amplifier.

The object of the present invention is achieved by the solution provided in the enclosed independent claims. Advantageous implementations of the present invention are further defined in the dependent claims.

In particular, the present invention proposes a new compensator device, which is composed of a series-connection of two or more HEMTs and two resistors.

A HEMT is the field-effect transistor most commonly used in, for instance, GaAs technology. The HEMT in the present invention can, however, be substituted or replaced with any component that can be integrated in the same MMIC of an amplifier, and that has the same sensitivity of the amplifier with respect to temperature and process variations.

A resistor is a two terminal component, the constitutive behavior of which can be described with the following equation / = k-V. In this equation, / is the current flowing across the component, and V is the voltage drop between its two terminals. Any component used to realize the resistor for the present invention should show optionally a low sensitivity to temperature and process variations. In particular, the component for realizing the resistor for the present invention, can be a single resistive element, or can be a lump or a circuit of elements, which follows in total the above equation, and provides a well-defined voltage drop.

A first aspect of the present invention provides a compensator device for biasing a gate of a MMIC HEMT amplifier, the compensator device comprising two resistors, and at least two HEMTs connected in series with and between the two resistors, wherein the resistors and HEMTs are selected such that in operation of the compensator device a bias point of at least one first HEMT is in a saturation region, and a bias point of at least one second HEMT is in an ohmic region.

Specifically, in operation of the compensator device, a voltage can be applied to the opposite ends of the series-connection of the at least two HEMTs and two resistors. A part of this voltage naturally drops over each of the series-connected components. Another voltage can be applied to bias a gate of each of the at least two HEMTs. The resistor values and transistor characteristics of the individual components can be carefully selected, namely such that at the desired applied voltages, one transistor is biased in the saturation region, and the other resistor in the ohmic region.

The compensator device of the first aspect allows compensating both variations of a threshold voltage and of a transconductance of the MMIC HEMT amplifier, which are caused by temperature and process variations. Additionally, the compensator device of the first aspect dissipates a significantly lower amount of DC power compared with conventional compensator devices.

In a first implementation form of the compensator device according to the first aspect as such, a threshold voltage and/or a transconductance of the at least two HEMTs shows the same sensitivity to temperature and/or process variations than a threshold voltage and/or transconductance of the HEMT amplifier, respectively.

That is, the HEMTs should be selected to resemble a close as possible the HEMT of the amplifier. The same sensitivity can, for instance, be achieved, when the individual HEMTs of the compensator device are taken from the same wafer or at least from the same lot as the HEMT of the amplifier. However, it is of course also possible to test the HEMTs for their transconductance and threshold voltage before putting together the compensator device, and further to test them for their temperature sensitivity with respect to these parameters. Thereby, a most precise compensation of the threshold voltage and the transconductance of the HEMT amplifier can be achieved.

In a second implementation form of the compensator device according to the first aspect as such or according to the first implementation form of the first aspect, at least one of the two resistors is a thin film resistor.

Thin film resistors provide the advantage of a low sensitivity against temperature and process variations, so that the compensation of the HEMT amplifier by means of the at least two HEMTs can be made more easily and precisely. In a third implementation form of the compensator device according to the first aspect as such or according to the first or second implementation form of the first aspect, at least one of the two resistors is a mesa resistor.

With mesa resistors, higher absolute values, which may be needed for compensating specific HEMT amplifiers, are possible than with thin film resistors. Additionally, for the same given resistor value, mesa resistors require a lower area than thin film resistors.

Both resistors can be thin film resistors or both resistors can be mesa resistors. However also one resistor can be a thin film resistor, and the other resistor a mesa resistor.

In a fourth implementation form of the compensator device according to the first aspect as such or according to any of the previous implementation forms of the first aspect, the at least two HEMTs are GaAs HEMTs. The compensator device of the first aspect was particularly designed for the GaAs material system, and thus the best results are achieved.

In a fifth implementation form of the compensator device according to the first aspect as such or according to any of the previous implementation forms of the first aspect, an input port, optionally a boding pad, connected to a gate of each HEMT, wherein the input port is further connectable to an external gate power supplier.

Thus, via the compensator device, the gate voltage of the HEMT amplifier can be provided, in particular with a simple compensator device structure and simple wiring.

In a sixth implementation form of the compensator device according to the first aspect as such or according to any of the previous implementation forms of the first aspect, a ground- connected via hole is connected to a first resistor.

In a seventh implementation form of the compensator device according to the first aspect as such or according to any of the previous implementation forms of the first aspect, a bonding pad is connected to a first resistor. In both of the above implementation forms, a reference voltage can be provided. A via hole is the simplest implementation form and provides a fixed ground reference. A bonding pad allows applying different reference voltages, and allows removing the compensating effect of the compensator device (namely by applying the same voltage to both the input bonding pad and the reference bonding pad). This is, for instance, useful for test scenarios of the HEMT amplifier.

In an eighth implementation form of the compensator device according to the first aspect as such or according to the sixth or seventh implementation form of the first aspect, an output port connected between the first transistor and the at least two HEMTs.

On the output port of the compensator device, a gate voltage can be provided to a HEMT amplifier that is inversely affected by temperature and process variations than the HEMT amplifier. A second aspect of the present invention provides a MMIC amplifier device comprising a HEMT amplifier and a compensator device according to the first aspect as such or according to any implementation form of the first aspect for biasing a gate of the HEMT amplifier. In a first implementation form of the MMIC amplifier device according to the second aspect, an input port of the compensator device is connected to a gate power supplier, and an output port of the compensator device is connected to the gate of the HEMT amplifier. The MMIC amplifier device of the second aspect enjoys all advantages described above for the compensator device. In particular, the performance variations of the MMIC HEMT amplifier device caused by process variations and operating temperature are significantly reduced if not completely compensated by means of the compensator device. Thereby, these variations are compensated with respect to both transconductance and threshold voltage.

A third aspect of the present invention provides a communications device comprising at least one MMIC amplifier device according to the second aspect as such or the first implementation form of the second aspect.

The communications device of the third aspect enjoys all advantages described above for the compensator device. In particular, the one or more MMIC amplifier devices of the communications device show reduced performance variations due to process variations and operating temperature.

A fourth aspect of the present invention provides a method of compensating a gate voltage provided to a MMIC HEMT amplifier device with a compensator device, wherein the compensator device comprises two resistors and at least two HEMTs connected in series with and between the two resistors, and wherein the method comprises setting a bias point of at least one first HEMT in a saturation region, and a bias point of at least one second HEMT in an ohmic region.

The method of the fourth aspect of the present invention enjoys all advantages described above for the compensator device.

It has to be noted that all devices, elements, units and means described in the present application could be implemented in the software or hardware elements or any kind of combination thereof. All steps which are performed by the various entities described in the present application as well as the functionalities described to be performed by the various entities are intended to mean that the respective entity is adapted to or configured to perform the respective steps and functionalities. Even if, in the following description of specific embodiments, a specific functionality or step to be full formed by external entities is not reflected in the description of a specific detailed element of that entity which performs that specific step or functionality, it should be clear for a skilled person that these methods and functionalities can be implemented in respective software or hardware elements, or any kind of combination thereof.

BRIEF DESCRIPTION OF DRAWINGS

The above described aspects and implementation forms of the present invention will be explained in the following description of specific embodiments in relation to the enclosed drawings, in which Fig. 1 shows a compensator device according to an embodiment of the present invention.

Fig. 2 shows operating regions of HEMTs of a compensator device according to an embodiment of the present invention.

Fig. 3 shows a compensator device according to an embodiment of the present invention.

Fig. 4 shows a compensator device according to an embodiment of the present invention.

Fig. 5 shows a compensator device according to an embodiment of the present invention. Fig. 6 shows a MMIC amplifier device according to an embodiment of the present invention.

Fig. 7 shows a conventional MMIC amplifier. Fig. 8 shows a conventional compensator device.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 shows a compensator device 100 according to an embodiment of the present invention. The compensator device 100 can be used particularly for biasing a gate of a MMIC HEMT amplifier (see e.g. in Fig. 6, the amplifier 602). The compensator device 100 comprises two resistors Rl, R2, and at least two HEMTs Ql, Q2...QN, which are connected in series with each other, in series with the two resistors Rl, R2, and between the two resistors Rl, R2. Further, the compensator device 100 may comprise an input port VS, which may advantageously be connected to a gate of each HEMT Ql, Q2...QN, and an output port VG, which may be connected between a first resistor Rl and the at least two HEMTs Q 1 , Q2... QN, respectively.

The input VS is also connectable to an external gate power supplier (see e.g. in Fig. 6, the power supplier 601). The output VG is connectable to the gate bias terminal of a HEMT amplifier (see e.g. in Fig. 6 the amplifier 602). Thus, the compensator device 100 compensates a voltage of the external gate power supplied and supplies it to the gate of the HEMT amplifier. The current IRI flowing across the resistor Rl is the same as flowing across the HEMTs Ql, Q2...QN, and the resistor R2, respectively.

The functionality of the proposed compensator device 100 is based on the same assumptions that are also valid for conventional compensator devices. That is, a threshold voltage VT of the HEMTs Ql, Q2...QN of the compensator device 100 of Fig. 1 has the same sensitivity to temperature and process variations than a threshold voltage of the biased HEMT amplifier. Further, a transconductance g _m of the HEMTs Ql, Q2...QN of the compensator device 100 in Fig. 1 has the same sensitivity to temperature and process variations as the biased HEMT amplifier.

The HEMTs Ql, Q2...QN of the compensator device 100 are selected such that with the voltage VS at least two different bias points can be set (e.g. for Ql and Q2) when the compensator device 100 is operated. Also the resistors Rl, R2 are purposefully selected to match the HEMTs Ql, Q2...QN such that with the current IRI flowing through the circuit, the different bias points can be set. That is, the resistors Rl, R2 and the HEMTs Ql, Q2...QN are selected such that in operation of the compensator device 100, a bias point of at least one first HEMT Ql is in a saturation region, and a bias point of at least one second HEMT Q2 is in an ohmic region. In other words, at least Q 1 is in operation in a saturation region, and at least Q2 is in operation in an ohmic region. Ql is in operation in the saturation region, and Q2...QN are in operation in the ohmic region.

A practical example of a compensator device 100 according to Fig. 1 with an advantageous selection of the resistors Rl, R2 and at least two HEMTs Ql, Q2...QN, respectively, is explained in the following. It is noted initially that in a system, in which a MMIC amplifier is adopted, a bias voltage of -5V is typically available. Further, power dissipated for compensation with a compensator device 100 is typically limited to lmW, in order to avoid a detriment of system efficiency. Further, the nominal gate bias voltage of a typical MMIC amplifier is -IV.

In the practical example, the compensator device 100 includes two HEMTs Ql, Q2, i.e. the minimum number of the at least two HEMTS Ql, Q2...QN. The available bias voltage (VS) of -5V is applied at the input VS. Considering the above-mentioned maximum power dissipation (Pdiss), the current IRI flowing in the compensator device 100 has to be

IRI = Pdiss / |VS| = lmW / 5V = 0.2mA

Consequently, the value of the resistor Rl is

Rl = |VG|/ IRI = 1 V/0.2mA = 5kQ

In order to have a HEMT Ql with a similar gate bias voltage as the MMIC amplifier, which optimizes the threshold voltage compensation of the compensator device 100, the gate-to- source nominal voltage (vgs) of Ql is selected to be -IV. As a consequence, the drain-to- source nominal voltage (vds) of Ql is 3V. The size of Ql is selected looking at the IV- curve as the transistor that has a drain-to-source current (ids) of 0.2mA at vgs = - IV and at vds = 3V. The transconductance compensation of the compensator device 100 is most powerful, if the compensation is applied on the transconductance curve at a vgs of -0.5V. Thus, the vgs of the HEMT Q2 is selected to be -0.5V. In this case, the vds of Q2 is selected to be 0.5V, in order to maintain Q2 in the ohmic operation region. Finally, the size of Q2 is selected looking to the IV-curve as the transistor that has an ids of 0.2mA at vgs = -0.5V and at vds = 0.5V.

In order to obtain the selected operating point for Q2, with VSQ2 being the voltage at the source of Q2, the value of the resistor R2 has to be

R2 = (VsQ2 - VS)/ IRI = 2.5 kQ

The practical example described above shows specifically how the resistors Rl, R2 and HEMTs Ql, Q2 may be selected such that in operation of the compensator device 100 a bias point of at least one first HEMT Ql is in a saturation region, and a bias point of at least one second HEMT Q2 is in an ohmic region.

Fig. 2 is a graphical representation of the operating bias points of the HEMTs Q 1 , Q2... QN shown in Fig. 1. The graphical representation is given through the output IV-curves of the respective HEMTs.

Due to the selected bias points for the HEMTs Ql, Q2...QN of the compensator device 100, the output voltage VG is

gmr ^R i

v _G = - •V·

1 ^ T_Q

1 + ¾nl R _{2 +} + ... + Eqn.2

¼n2 J As a consequence, the drain-to-source bias current of the HEMT amplifier is

gmr ^R i

DS_amp ~ ¾n amp •V. T_Q1 ^{" v} T_amp

1 + ¾nl - R ₂ + — + +— Eqn.3

¼n2 Finally, the DC power dissipated by the compensator device 100 is

^P DISS ^{= I} R1 ^V S

The above Eqn. 2 and Eqn. 3 demonstrate the functionality of the compensator device 100 according to Fig. 1, and its advantages with respect to conventional compensator devices.

Firstly, the compensator device 100 has some identical functionality as conventional compensator devices. In particular, from Eqn. 2 it is possible to note that the proposed compensator device 100 allows the compensation of threshold voltage VT variations of the amplifier, which are due to temperature and process variations. In fact, the drain-to-source current of a HEMT amplifier is proportional to VT QI - VT amp, which are parameters that both have the same sensitivity to such temperature and process variations.

Secondly, the compensator device 100 has an important first advantage over conventional compensator devices. From Eqn. 2 it is possible to note that the proposed compensator device 100 allows also the compensation of transconductance g _m variations of the amplifier, due to temperature and process variations. In fact, the transconductance variations of the amplifier (g _m amp in the numerator in Eqn. 2) is compensated by the terms (l/gm2 + ... + 1/gmN) in the denominator in Eqn. 2, which are parameters having the same sensitivity to said temperature and process variations. The use of more than two HEMTs Ql, Q2...QN for a transconductance compensation allows for a higher degree of compensation, since the transconductance has a nonlinear sensitivity to the temperature and process variations. The transconductance of HEMT Ql (gmi) is self-compensated in Eqn. 2, being both at the numerator and denominator. Conventional compensator devices do not allow the compensation of the transconductance variations of an amplifier at all.

Thirdly, the compensator device 100 has an important second advantage over conventional compensator devices. As highlighted in Eqn. 3, the dissipated DC power from the proposed compensator device 100 is lower with respect to all conventional compensator devices. As an example, the power dissipated in the compensator device of Fig. 8 is VS (IR1 + IR3), that is higher than the power dissipated by the compensator device 100 according to Eqn. 3. Fig. 3 shows a compensator device 100 according to an embodiment of the present invention, which bases on the compensator device 100 shown in Fig. 1. The compensator device 100 of Fig. 3 has N HEMTs, where N>2 is an integer number. The compensator device 100 comprises two thin film resistors TFR-1 and TFR-2 as the resistors Rl, R2 of Fig. 1, respectively, which are commonly available e.g. in the GaAs technology. The thin film resistors TFR-1 and TFR-2 show advantageously very low sensitivity to process and temperature variations.

Further, the compensator device of Fig. 3 has one via hole 301 that is connected with ground, and which is used as a reference voltage. Also, the compensator device 100 has one bonding pad 302 as the input port VS shown in Fig. 1. The bonding pad 302 is used to connect an external gate power supplier. The via hole 301 is the simplest alternative to provide a reference voltage, but the reference voltage cannot be changed. Fig. 4 shows an alternative compensator device 100 according to an embodiment of the present invention, which bases on the compensator device 100 shown in Fig. 1. The compensator device 100 of Fig. 4 has a second bonding pad 401 instead of the via hole 301 of the compensator device 100 of Fig. 3. The second bonding pad 401 is used to provide the reference voltage. The bonding pad 401 allows applying different reference voltages. If the same voltage is applied to the bonding pad 401 and the bonding pad 302 described above, the effect of the compensator device 100 on the gate voltage of the HEMT amplifier is removed. That is, no compensation is carried out to the gate voltage provided to the HEMT amplifier. This is particularly useful for scenarios, in which the HEMT amplifier itself is to be tested or calibrated.

Fig. 5 shows another alternative compensator device 100 according to an embodiment of the present invention, which bases on the compensator device 100 shown in Fig. 1. The compensator device 100 of Fig. 5 adopts mesa resistors MESA-1 and MESA-2 as the resistors Rl and R2 of Fig. 1, i.e. instead of thin film resistors TFR-1 and TFR-2 as in the Figs. 3 and 4. The mesa resistors MESA-1 and MESA-2 can achieve higher resistance values than thin film resistors, and require less area for a given resistance value to be used. As mentioned previously, also one resistor, for instance Rl, could be a thin film resistor TFR-1, while the other resistor, for instance R2, could be a mesa resistor MESA-2, or vice versa. Fig. 6 shows a MMIC amplifier device 600 according to an embodiment of the present invention. The MMIC amplifier device 600 includes a HEMT amplifier 602 and a compensator device 100 according to one of the above-described embodiments of Figs. 1, 3, 4 and 5. The compensator device 100 is for biasing a gate of the HEMT amplifier 602. To this end, an input port VS of the compensator device 100 may be connected via a gate pad 603 to a gate power supplier 601. An output port VG of the compensator device 100 may be connected to the gate of the HEMT amplifier 602. Further, a drain power supply 605 may be connected via a drain pad 604 to the HEMT amplifier 602.

According to a further embodiment of the present invention, a communications device comprising at least one MMIC amplifier device 600 according to Fig. 6 is provided.

The present invention provides also a method of compensating a gate voltage provided to a MMIC HEMT amplifier 602 shown e.g. in Fig. 6, wherein the compensator device 100 is designed according to one of the embodiments shown in the Figs . 1 , 3 , 4 or 5. The method comprises setting a bias point of at least one first HEMT Ql of the compensator device 100 in a saturation region, and a bias point of at least one second HEMT Q2 of the compensator device 100 in an ohmic region.

The present invention has been described in conjunction with various embodiments as examples as well as implementations. However, other variations can be understood and effected by those persons skilled in the art and practicing the claimed invention, from the studies of the drawings, this disclosure and the independent claims. In the claims as well as in the description the word "comprising" does not exclude other elements or steps and the indefinite article "a" or "an" does not exclude a plurality. A single element or other unit may fulfill the functions of several entities or items recited in the claims. The mere fact that certain measures are recited in the mutual different dependent claims does not indicate that a combination of these measures cannot be used in an advantageous implementation.