Power resistor with clamping element for improved cooling

Abstract

A resistor assembly, in particular for a power combiner, comprising a resistor chip and a clamping element for clamping the resistor chip to a cooling element, where the clamping element has a clamping arm for applying pressure to the resistor chip.

Description

Technical Field

[0001] The present invention relates to a resistor assembly, in particular a power combiner for an RF generator. Furthermore, the present invention relates to such a power combiner and an RF generator. Background Art

[0002] The handling of reflected power is a particular challenge in radio frequency (RF) power applications used for plasma generation (e.g., for etching and depositing thin films on silicon wafers during the manufacture of integrated circuits). RF plasmas are highly dynamic loads that can change rapidly. The use of impedance matching networks between the RF power generator and the plasma can compensate for impedance mismatches in steady-state plasma operations. However, such matching networks are typically too slow to immediately adapt to changes in load impedance. For example, during the ignition phase of the plasma, during power level changes, and in pulsed mode operation, the RF generator is exposed to transient reflected power that must be dissipated within the generator.

[0003] The RF power stage of the RF generator includes the following circuits:

[0004] Power Amplifier

[0005] Hybrid Combiner

[0006] High power combiner (if you want to combine multiple PAs to get higher output power),

[0007] And operate at frequencies in the range of 0.3-300 MHz and power levels of 500 W or more. For power levels up to about 2 kW, air cooling is sufficient, higher power levels above 2 kW generally require water cooling.

[0008] In a combiner, termination resistors are often used to absorb and dissipate reflected power. Since reflected power levels can get very high depending on the situation, the termination resistors must be properly cooled so that the generated heat is quickly transferred to a cold plate or heat sink.

[0009] The term "heat sink" refers to a heat exchanger that transfers heat from electronic circuits to a fluid medium (e.g., air or liquid coolant). A portion of a heat sink typically maximizes contact with, and thus heat transfer to, the fluid medium through an enlarged surface area (e.g., by means of cooling fins or tips). The term "cold plate" refers to a heat exchanger through which a liquid coolant circulates, typically a thick plate with embedded tubes through which the coolant flows. Heat sinks and cold plates are typically made of metallic materials such as aluminum, copper, or metal alloys, but ceramics and other materials with high thermal conductivity are also possible.

[0010] A heat sink is a device with good thermal conductivity, usually a plate, that distributes heat generated in a small area to a larger area. Heat sinks are often used in conjunction with a heat sink or cold plate that transfers the heat to a fluid medium. Heat distribution in a heat sink can be enhanced by thermoelectric cooling (e.g., Peltier elements) or by using heat pipes.

[0011] In the following, the term "cooling element" is used for all types of air-cooled heat exchangers with fins or similar enlarged surface area, for liquid-cooled cold plates and for combinations of heat sinks with air-cooled heat sinks or liquid-cooled cold plates, including possible enhancements like thermoelectric cooling or heat pipes.

[0012] A typical power resistor (which allows it to be clamped to a heat sink with screws) consists of a heat sink and a resistor chip soldered to this heat sink. However, clamping such a resistor from both ends (i.e., clamping only the heat sink) results in reduced pressure and a weakened thermal connection in the center of the device where heat is generated by the resistor chip. In addition, the area required for clamping cannot be used by the resistor chip, thereby reducing the maximum possible size of the resistor chip, which increases the total heat load in the center of the resistor.

[0013] A typical RF generator must fit into an industry standard electronics rack, therefore, the width of the generator is usually specified as 19 / 2 or 19 inches and its height is specified as 3-5 rack units, where 1 rack unit is 1.75 inches. As a result, it is difficult to arrange all the circuits and components so that they fit into the available dimensions, therefore, the terminating resistor must be placed so that it does not interfere with other circuits or components and uses the minimum space.

[0014] Especially for frequencies below 30 MHz, the size of the circuit components, capacitors and inductors must increase in value as the frequency decreases, and thus in size. As a result, for example, for the important ISM frequencies of 13.56 and 27.12 MHz, it is difficult to fit all the required components of an RF generator into a 19 / 2 inch housing. Therefore, most RF generators with more than 1 kW are forced to use a full 19 inch size housing.

[0015] In modern RF power circuit designs for mass production, in order to achieve small variations between units, three-dimensional arrangements of wires and ferrites for inductors are avoided because they are difficult to reproduce with the required geometric accuracy. Instead, fully planar structures are preferred, which can be manufactured with high precision in industrial printed circuit board (PCB) production. Although the size of the planar circuit is a challenge when a 19 / 2-inch wide housing is to be used, the planar combiner advantageously allows the terminating resistor to be placed in the center of the inductor structure.

[0016] For optimal heat transfer, a PCB with such an inductor will have a cutout in its center so that the power resistor can be clamped directly to the cooling element that serves as the ground connection. To avoid coupling of the inductor's magnetic field into the cooling element, which is usually made of metal, the PCB with the inductor cannot be placed directly onto the cooling element. Instead, a ceramic heat sink with a thickness of several millimeters must be used to extend the distance.

[0017] Furthermore, for the resistor component, non-magnetic material must be used. Otherwise, eddy currents will be induced by the magnetic field of the inductor, which will cause heating of the resistor component.

[0018] As a result, the flange of the power resistor used to establish the terminating ground connection is located within a cutout in the ceramic heat sink and in direct contact with the cooling element, while the resistor-to-inductor connection must bridge a considerable gap across the PCB on top of the ceramic heat sink. In addition, the coefficient of thermal expansion (c TE ) are all different. Therefore, in cycling operation when the RF is frequently switched on and off or the RF power level is frequently changed, for example on a scale of seconds, the connection between the top contact of the termination resistor and the inductor is subjected to considerable thermomechanical stress in cycling operation, which is common for short semiconductor plasma processes and during pulsed operation. These brazed or welded joints may deteriorate rapidly and eventually break at high operating temperatures.

[0019] The flange to cooling element connection is made in flange material (e.g. having a c similar to that of the resistor material). TE Copper-molybdenum alloy) and cooling elements usually made of copper or aluminum or alloys TE Subject to thermomechanical stress when mismatched. Summary of the invention

[0020] It is therefore an object of the present invention to provide a resistor assembly which is reliably connected to a cooling element in a compact manner.

[0021] The resistor assembly according to claim 1, the power combiner according to claim 14 and the RF generator according to claim 16 solve the problems of the prior art.

[0022] In a first aspect, a resistor assembly is provided, in particular for use in a power combiner. The resistor assembly comprises a resistor chip. Preferably, the resistor chip comprises a resistor chip having a width of 50 mm. 2 and 500mm 2. Preferably, the size of the resistor chip is between 12 mm×6 mm and 30 mm×14 mm. Preferably, the rated power of the resistor chip is above 600 W. Preferably, the resistor chip is constructed as a thick film resistor, in particular based on aluminum nitride (AlN). In particular, the operating temperature of the resistor chip is up to 100°C or more. Preferably, the resistor chip has one terminal on its upper surface or side surface and a second terminal on its lower surface. The lower surface can be completely covered with a metal layer so that the second terminal can be attached to a conductive passive heat sink, which transfers heat to a cooling element serving as a ground connection.

[0023] According to the present invention, the resistor assembly includes a clamping element for clamping the resistor chip to the cooling element. Wherein, the cooling element can be a heat sink, which is configured to distribute the heat generated by the resistor chip and transfer it to the cooling element. Due to the clamping element, contact is established between the resistor chip and the cooling element so as to dissipate the heat generated by the resistor chip to the cooling element. Wherein, the clamping element has a clamping arm for applying pressure to the resistor chip. Therefore, by applying pressure directly to the resistor chip, the contact between the resistor chip and the cooling element is improved. Therefore, in contrast to the prior art, the resistor chip itself is clamped, wherein the clamping force is applied directly to the resistor chip instead of the heat dissipation element, for example, on the flange of the resistor of the prior art as described above. Therefore, the space required for fixing the resistor chip to the cooling element is reduced, and therefore, the area of ​​the resistor chip can be increased, resulting in improved heat transfer away from the resistor chip. In addition, by clamping (in contrast to fixing the resistor by screws in the prior art), thermal expansion of the resistor chip is allowed, thereby reducing thermally induced stress in the resistor chip. Therefore, heat dissipation from the resistor chip to the cooling element is improved, and the reliability and life of the resistor are increased due to the reduced operating temperature.

[0024] Preferably, the resistance value of the resistor chip is 50 ohms.

[0025] Preferably, the clamping element comprises a base element which is connected to the clamping arm in an L-shape. Therein, in particular, the base element has a first end connected to the clamping arm and an opposite second end which is directly connected to the cooling element or in contact with the cooling element in the mounted state. In particular, the base element and the clamping arm are one-piece and are constructed integrally. Therein, in particular, the angle between the base element and the clamping arm can be 90° or less. Thus, a contact point / line is generated by the base element, wherein the clamping element pivots around this contact point / line so that the clamping arm can apply pressure to the resistor chip.

[0026] Preferably, the second end of the base element is inclined to allow rotational movement of the clamping element around an edge in a direction away from the resistor chip. Therefore, the edge of the second end of the base element that is at a greater distance away from the resistor chip becomes the pivot point / line of the clamping element. Therefore, the pressure applied by the clamping element can be increased by up to 60% compared to the situation where the pivot axis is at an edge closer to the resistor chip (corresponding to the flat second end of the base element). In the case of a flat second end of the base element, the clamping element will rotate around the pivot axis on the second end of the base element close to the resistor chip. As an alternative to an inclined second end, the second end may include a protrusion that provides the pivot axis at a position of the second end away from the resistor chip.

[0027] Preferably, the clamping arms and the base element are made of a rigid, non-magnetic material, such as an aluminium alloy, brass or austenitic steel.

[0028] Preferably, a pressure distribution plate is arranged between the clamping arm and the resistor chip. By means of the pressure distribution plate, the pressure applied by the clamping arm is distributed over the entire area or at least a part of the area of ​​the resistor chip. Preferably, the pressure distribution plate is a rigid element constructed of metal or plastic. Preferably, the pressure distribution plate is a separate element or can be constructed integrally with the clamping arm. Preferably, the thickness of the pressure distribution plate is between 2 mm and 5 mm, wherein the area of ​​the pressure distribution plate is preferably 50 mm. 2 and 400mm 2 between.

[0029] Preferably, the pressure distribution plate is made of a rigid, non-magnetic material, such as an aluminum alloy, brass or austenitic steel. In particular, the pressure distribution plate can be made of the same material as the clamping arms and / or the base element. Alternatively, the pressure distribution plate is made of plastic, which inherently can provide isolation between the clamping elements and the resistor chip.

[0030] Preferably, the lower side of the clamping arm connected to the pressure distribution plate includes an alignment structure, and the upper surface of the pressure distribution plate includes a corresponding alignment structure, which engages the alignment structure of the clamping arm and is configured to align the positions of the clamping arm and the pressure distribution plate. Wherein, the alignment structure can be a ball, a pawl, a hemispherical protrusion, etc. Wherein, the alignment structure of the clamping arm can be constructed integrally with the clamping arm, or can be an additional element. Wherein, the hemispherical alignment structure provides a beneficial contact surface so that the pressure of the clamping arm is evenly transmitted to the pressure distribution plate. Wherein, the corresponding alignment structure of the pressure distribution plate has a shape corresponding to the alignment structure of the clamping arm so as to engage or receive the alignment structure of the clamping arm. Wherein, the corresponding alignment structure can be configured as a groove, a recess, a dent, etc. Of course, the shapes of the alignment structure of the clamping arm and the corresponding alignment structure of the pressure distribution plate can be freely selected. The orientation of the pressure distribution plate is maintained by the alignment structure of the clamping arm and the corresponding alignment structure of the pressure distribution plate.

[0031] Preferably, an insulating layer is arranged between the resistor chip and the clamping arm, and the insulating layer is preferably arranged directly between the resistor chip and the pressure distribution plate. Wherein, the insulating layer can be arranged at the upper surface of the resistor chip. The insulating layer is electrically insulating. Preferably, the insulating layer is made of a high-temperature elastic material, such as a perfluoroelastomer, a polyimide or a polyamide. By the insulating layer, a short circuit, in particular between the metal pressure distribution plate and the resistor chip, is avoided. Wherein, if the pressure distribution plate is constructed of a plastic material, the insulating layer can be constructed integrally with the pressure distribution plate. Thus, the insulating effect is inherently provided by the pressure distribution plate itself, and therefore the elements of the pressure distribution plate and the insulating layer are combined. Alternatively, the insulating layer and the pressure distribution plate are separate elements. By constructing the insulating layer as an elastic layer, the pressure distribution from the pressure distribution plate to the resistor chip is improved, and the elastic insulating layer can compensate for slight manufacturing defects of the resistor chip and / or the pressure distribution plate without producing a short circuit.

[0032] Preferably, the resistor assembly comprises a pressure device which is connected to the clamping arm and can be connected to the cooling element and is configured to generate a pressure force of the clamping arm.Preferably, the pressure device is configured as a screw.

[0033] In particular, the pressure device extends through the clamping arm and preferably through the pressure distribution plate. In particular, the through hole of the clamping arm can correspond to the outer diameter of the pressure device, wherein the through hole of the pressure distribution plate can be larger than the pressure device in order to allow the movability of the pressure distribution plate relative to the pressure device. In addition, the orientation of the pressure distribution plate is maintained by the pressure device. In combination with the alignment structure of the clamping arm and the corresponding alignment structure of the pressure distribution plate, the position of the pressure distribution plate is determined and fixed.

[0034] Preferably, the pressure device does not extend through the resistor chip. In particular, the pressure device is away from the resistor chip. By avoiding contact between the pressure device and the resistor chip, the possibility of a short circuit is prevented.

[0035] Preferably, the pressure device does not extend through the insulating layer. Thus, the insulating layer can be arranged only between the pressure distribution plate and the resistor chip. This may be particularly applicable when the pressure distribution plate is made of an electrically conductive material such as metal. Alternatively, the pressure device may extend through the insulating layer, in particular if the insulating layer is constructed, for example, integrally with the pressure distribution plate.

[0036] Preferably, a conductive layer is arranged at the lower surface of the resistor chip opposite to the clamping arm, wherein preferably, the conductive layer is thermally and / or electrically conductive. Preferably, the thermal conductivity is greater than 200 W / m / K, more preferably greater than 300 W / m / K, most preferably greater than 400 W / m / K, thereby exceeding the thermal conductivity of a heat sink made of aluminum, for example, with a thermal conductivity of 240 W / m / K, and / or the thermal conductivity of a heat sink made of Al2O3, for example, with a thermal conductivity of 30 W / m / K.

[0037] Preferably, the conductive layer is compressible. Preferably, the compressibility of the conductive layer is 10% or more, more preferably 20% or more. Preferably, the bulk modulus of the conductive layer is less than 1000 kPa, preferably 600 kPa. By the compressibility of the conductive layer, small manufacturing imperfections of the resistor chip and / or the lower surface provided by the cooling element can be compensated, thereby providing complete contact between the respective surfaces in order to achieve optimal heat dissipation from the resistor chip to the cooling element.

[0038] Preferably, the conductive layer is provided by a thermally conductive paste or a thermally conductive pad or a compressible graphite thermal interface material, preferably having a compressibility of 10% or more, more preferably 20% or more.

[0039] Preferably, the conductive layer provides electrical contact between ground and the resistor chip.

[0040] Preferably, the resistor assembly comprises a connecting element, which is preferably configured as a strip, which is connected to the resistor chip and is particularly connected to the side of the resistor chip opposite to the base element of the clamping element. Thus, in particular, the connecting element is connected to the short side of the resistor chip. Wherein, the connecting element can be connected to the resistor chip on the top surface of the resistor chip. Thus, via the connecting element, the resistor is electrically connected to the corresponding circuit. As described above, the second terminal of the resistor chip can be provided by the lower surface of the resistor chip by means of a metal layer.

[0041] Preferably, the connecting element is S-shaped. Thermal expansion can be compensated by the S-shape, so that thermal expansion cannot lead to damage of the connecting element.

[0042] On the other hand, a power combiner is provided. The power combiner comprises a heat sink or a heat sink (i.e., a cooling element), and at least one inductor connected to the cooling element, wherein the at least one inductor can be a planar inductor. In particular, the power combiner comprises at least two input terminals and an output terminal. By using at least one inductor, the power combiner is configured to combine two input RF signals into a single RF signal. In particular, the resistor assembly as described above is connected to at least one inductor as a terminating resistor. Typically, the resistance value of the resistor chip is 50 ohms.

[0043] Preferably, at least one inductor comprises a central recess, preferably down to the cooling element, wherein the resistor assembly is completely arranged in the recess and mounted directly onto the cooling element. Thereby, a compact arrangement can be achieved. The recess is in the central area of ​​the inductor and can extend through the PCB and the ceramic heat sink of the power combiner. The resistor assembly and in particular the resistor chip is in direct contact with the cooling element, i.e. the cold plate, of the RF generator.

[0044] In another aspect of the invention, there is provided an RF generator comprising at least two power amplifier stages and a power combiner as described above. The combiner is connected to the at least two power amplifier stages and combines the output power of the at least two power amplifier stages. Preferably, the RF generator is provided for plasma applications. The components of the RF generator are assembled on a common cooling element.

[0045] Preferably, the RF generator provides an output power of 5 KW or more. In particular, at least two power amplifier stages each provide up to 2.5 KW or more.

[0046] Preferably, the RF generator is water-cooled. In particular, the cooling element comprises active cooling configured as water cooling.

[0047] Preferably, the RF generator is arranged in a housing of size 19 / 2 inches. Thus, the RF generator is compact in size. Due to the resistor according to the invention, high output power levels become feasible due to improved heat dissipation from the resistor to the cooling element of the RF generator. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] The present invention is described in more detail with reference to the accompanying drawings.

[0049] The accompanying drawings show:

[0050] FIG. 1 is an exploded view of a resistor according to the present invention,

[0051] FIG. 2 is a side view of the resistor of FIG. 1 ,

[0052] FIG3 is a cross-sectional view of the resistor according to FIG1 ,

[0053] FIG. 4 is a diagram of the resistor according to FIG. 1 in an assembled state of the power combiner according to the present invention, and

[0054] FIG5 is a schematic diagram of an RF generator. DETAILED DESCRIPTION

[0055] In the following, reference is made to FIGS. 1 to 3 . The resistor assembly according to the present invention includes a resistor chip 12 having an upper surface 16 and a lower surface 18. The resistor chip 12 includes a connecting element 14 configured as a strip. The connecting element 14 can be bent into an S-shape. The second terminal of the resistor chip 12 is provided by the lower surface 18, which is usually grounded. The second terminal can be a metal layer 13 covering the lower surface 18. In addition, the resistor assembly includes a clamping element 20, which has a clamping arm 22 and a base element 24. Among them, the base element 24 includes a first end 26 connected to the clamping arm 22 and a second end 28 directly connected to a cooling element 50 of a radio frequency (RF) generator. The clamping element 20 is connected to the cooling element 50 by a screw 30. As shown in FIG. 2 , the second end 28 of the base element 24 includes an inclined surface 44 having an angle α. By the inclined surface 44, a pivot axis 46 is constructed at the outer edge of the base element 24 (i.e., the edge away from the resistor chip 12). By this angle α provided by the inclined surface 44, the clamping force generated by the clamping element 20 can be increased by 60% compared to a flat surface that will rotate around a pivot axis closer to the edge of the resistor chip 12. When the screw 30 is turned, pressure is applied to the pressure distribution plate 32 by the clamping arm 22. Wherein, the screw 30 can extend through the through hole 38 of the pressure distribution plate 32. In addition, the clamping arm 22 includes a hemispherical alignment structure 34, wherein the pressure distribution plate 32 includes a hemispherical recess 36 to receive the alignment structure 34 of the clamping arm 22. Thus, the position of the pressure distribution plate 32 is defined by the alignment structure 34 and the corresponding recess 36 together with the through hole 38 receiving the screw. The pressure distribution plate 32 distributes the pressure applied by the clamping arm 22 and is constructed of a rigid material such as a rigid plastic or metal. An insulating layer 40 is provided between the pressure distribution plate 32 and the upper surface 16 of the resistor chip 12 to electrically insulate the resistor chip 12 from the pressure distribution plate 32. Wherein, the screw 30 can extend through the through hole of the insulating layer 40. The insulating layer 40 may be compressible to evenly distribute the pressure provided by the pressure distribution plate 32 to the resistor chip 12 and to allow thermal expansion without introducing thermal stress into the resistor chip 12. The insulating layer 40 may be made of a high temperature elastic material such as perfluoroelastomer, polyimide or polyamide.

[0056] At the lower surface 18 of the resistor chip 12, a thermally and electrically conductive conductive layer is arranged between the cooling element 50 and the resistor chip 12. Thus, a contact of the lower surface 18 of the resistor chip 12 to ground is established through the conductive layer. At the same time, the heat generated by the resistor chip is transferred to the cooling element 50.

[0057] 4, there is shown a power combiner 102 according to the invention which may be part of a high power RF generator. The power combiner comprises at least one inductor 52, wherein a resistor assembly according to the invention is connected to the inductor 52 via its connecting element 14 as a terminating resistor.

[0058] Preferably, the operating temperature of the resistor assembly according to the present invention is 80° C. or higher, preferably 100° C. or higher.

[0059] Fig. 5 schematically shows an RF generator 100 having at least two power amplifier stages 104A, 104B. The outputs of the individual power amplifier stages are combined in a power combiner as described above with reference to Fig. 4 .

[0060] Wherein, in the prior art, the flange resistor connected to the cooling element by two screws allows to handle a maximum absorption power of 400W, the present invention increases the resistor chip area and improves the thermal contact between the resistor chip and the cooling element, thereby allowing an absorption power of up to 700W or more. At the same time, the operating temperature of the resistor can be reduced by 15°C or more.

[0061] In addition, the life of the soldered or brazed connection from the top contact of the resistor to the combiner PCB is significantly increased due to lower operating temperatures and less thermo-mechanical stress during power cycling. In addition, thermal expansion and compression of the clamped components is facilitated by means of a conductive, compressible thermal interface material, which results in reduced thermo-mechanical stress on the resistor chip.

Claims

1. A resistor assembly, in particular for a power combiner, wherein: Resistor chips, and a clamping element for clamping the resistor chip to a cooling element, The clamping element has a clamping arm for applying pressure to the resistor chip.

2. The resistor assembly according to claim 1, characterized in that The clamping element comprises a base element connected to the clamping arm in an L-shape, wherein preferably the base element has a first end connected to the clamping arm and an opposite second end in contact with the cooling element.

3. The resistor assembly according to claim 2, characterized in that The second end of the base element is inclined to allow rotational movement of the clamping element about an edge in a direction away from the resistor chip.

4. The resistor assembly according to any one of claims 1 to 3, characterized in that A pressure distribution plate is arranged between the clamping arm and the resistor chip.

5. The resistor assembly according to claim 4, characterized in that The underside of the clamp arm connected to the pressure distribution plate includes an alignment structure, and the upper surface of the pressure distribution plate includes a corresponding alignment structure that engages with the alignment structure of the clamp arm and is configured to align the positions of the clamp arm and the pressure distribution plate.

6. The resistor assembly according to any one of claims 1 to 5, characterized in that An insulating layer is arranged between the resistor chip and the clamping arm, preferably directly between the resistor chip and the pressure distribution plate.

7. The resistor assembly according to any one of claims 1 to 6, characterized in that A pressure device is connected to the clamping arm and is connectable to the cooling element to generate pressure of the clamping arm.

8. The resistor assembly according to claim 7, characterized in that The pressure device extends through the clamping arm and preferably through the pressure distribution plate.

9. The resistor assembly according to claim 7 or 8, characterized in that The pressure device does not extend through the resistor.

10. The resistor assembly according to any one of claims 1 to 9, characterized in that A conductive layer is arranged at a lower surface of the resistor chip opposite the clamping arms, wherein preferably the conductive layer is thermally and / or electrically conductive.

11. The resistor assembly according to claim 10, wherein: The conductive layer is compressible.

12. The resistor assembly according to claim 10 or 11, wherein: The conductive layer is provided by a thermally conductive paste or a thermally conductive pad or a compressible graphite thermal interface material.

13. The resistor assembly according to any one of claims 1 to 12, characterized in that A connecting element is preferably configured as a strip, which is connected to the resistor chip and in particular to a side of the resistor chip opposite to the base element of the clamping element.

14. A power combiner comprising a cooling element, at least one inductor arranged on the cooling element, and a resistor assembly according to any one of claims 1 to 13 connected to at least one of the inductors.

15. The power combiner of claim 13, wherein: At least one of the inductors includes a central recess, wherein the resistor assembly is disposed entirely within the recess.

16. A radio frequency generator comprising at least two power amplifier stages and a power combiner according to claim 14 or 15 connected to the at least two power amplifier stages, the power combiner combining output powers of the at least two power amplifier stages.

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