Wideband amplifier using fet

Abstract: The present invention concerns with a wideband amplifier concurrently receiving multi-channel signals in a predetermined frequency range and amplifying the received signals. In particular, the wideband amplifier is characterized in that as a semiconductor element employed in the amplifier, a field-effect transistors (11) is used, the transistor (11) having a substantially flat characteristic of mutual conductance to a voltage (V GS ) between a gate and a source with respect to the center of an operation point for suppressing a second order distortion of an output of the amplifier. And, there is also provided a wideband amplifier wherein in case of inspecting a field-effect transistor, the second order distortion in a high frequency is guaranteed to be a constant value or less by accepting only the wideband amplifier having a predetermined or lower value of the difference between the mutual conductances to the applied voltages at an input terminal having a constant difference therebetween.

Description: BACKGROUND OF THE INVENTION The present invention concerns with a wideband amplifier which serves to reduce a second order distortion particularly in a UHF and a VHF bands. In receiving multi-channel signals in a wide range, these signals are subject to mutual interference, thereby disadvantageously causing distortions of the signals. Those distortions result from nonlinearity of an active device in a signal processing circuit. Though a number of high degree distortions may be brought about, the second degree and third degree distortions are the most significant ones caused in the active device used for signal processing. In particular, in a system for receiving signals in a VHF to UHF band such as a front-end amplifier of a cable television converter or a RF (Noise Frequency) modulator connected with a booster used in a video tape recorder, the second order distortion has to be severely considered. For wideband signal processing, a wideband bipolar transistor device or an FET (Field-Effect Transistor) consisting of a compound semiconductor may be employed. The bipolar transistor, however, has so inferior a noise figure that it gives rise to a large distortion of a signal. In recent days, hence, the system for processing signals in the VHF or UHF band often employs a compound semiconductor device such as a GaAs FET. For describing such a wideband signal processing circuit, there can be referred a wideband amplifier having a territory of a VHF to UHF band, in particular, a negative feedback amplifier consisting of a compound semiconductor such as a GaAs FET. Fig. 5 is an equivalent circuit of the GaAs wideband amplifier. Fig. 6 shows an example of a frequency dependence of gain in this amplifier. It is assumed that a feedback resistance R f 12 is 300 Ω, a gate bias resistance R G 13 is 10 kΩ, FET 11 has a gate width of 800 µm, a gate length of 1 µm and a mutual conductance of 80 mS, and the consumed current I DD is about 30 mA. 14 denotes a choke coil C f for supplying bias current and 15 denotes a feedback capacitor for d.c. blocking. When the wideband amplifier receives at an input terminal IN multi-channel signals over 64 to 80 channels (50 MH b to 500 MH b ) in the VHF to UHF band, nonlinearity entailed in an element itself gives rise to a second order and a third order distortions of a signal. As noted above, the wideband amplifier used for the cable television converter requires to have such distortions strictly suppressed. The conventional GaAs FET, in essence, has a feature that only a small third order distortion is brought about. The present inventors, however, have researched that a large second order distortion caused by the GaAs FET kills the superior feature of lowering a noise, thereby being unable to meet the satisfactory efficiency for the use. For selecting an FET used for the amplifier, traditionally, high-frequency inspection has been manually done for a lot of manufactured GaAs FETs one by one for selecting an FET having a low noise and a small second order distortion, because it has been so difficult to obtain a GaAs FET having a small second order distortion and a low noise. The inspected result was that just one satisfactory GaAs FET could be found among 70 FETs. This result indicates that it has been difficult to obtain a proper GaAs FET and the inspection has required a lot of labor and high cost. It means that the GaAs FET amplifier has not been so easily manufactured. It has been recognised that second order distribution in an FET can be improved by flattening the characteristic of g m in relation to V as by selection of an appropriate doping profile in the active layer of the FET. This is described for example in IEEE Transactions on Microwave Theory and Techniques, Vol. 36, No. 6 Jan 1988, page 1023-1032, "A low-distortion-band GaAs power FET, and in IEEE MTT-S International Microwave Symposium Digest, 9th-11th June 1987, Vol. 2 pages 569-572: "A GaAs Microwave MESFET with extremely low distribution". Neither of these documents deal with wideband amplifiers. SUMMARY OF THE INVENTION It is an object of the present invention to provide a wideband amplifier which can provide an improved second order distortion characteristic. It is another object of the present invention to provide an automatic inspecting process employing a low frequency or direct current which permits easy selection of an FET having a superior second order distortion characteristic in a VHF to VHF band. According to one aspect of the present invention there is provided a wideband amplifier for receiving concurrently multi-channel signal in a predetermined frequency range and for amplifying the receiver signals, said wideband amplifier comprising a field effect transistor which has a gate, a drain and a source and is arranged to amplify the received signals, said transistor having a negative feedback circuit for feeding back a signal at said drain to said gate, and a gate bias circuit between said gate and said source for setting a gate bias of the field effect transistor, characterised in that said feedback circuit includes a feedback capacitor arranged to cut off a direct component of current between the gate and the drain and a feedback resistance, said gate bias circuit includes a gate bias resistance, and wherein said field effect transistor formed by ion implantation, has a substantially flat characteristic of mutual conductance versus voltage between the gate and the source with respect to the center of an operation region to thereby suppress a second order intermodulation distortion of an output of said wideband amplifier. According to a second aspect of the present invention there is provided a method of inspecting a field effect transistor for the wideband amplifier according to the first aspect of the invention in order to ensure a sufficient low level of second order distortion in a high frequency range, said method comprising the steps of: applying two different voltages on an input terminal of a field effect transistor; measuring a difference between mutual conductances relating to each of said predetermined voltages; comparing said difference with a predetermined value; and determining whether said field effect transistor is acceptable for the wideband amplifier according to whether said difference is lower than said predetermined value. The present invention can offer a wideband amplifier having a superior second order distortion characteristic, because it employs a field-effect transistor having a flat mutual conductance to a voltage V GS between a gate and a source (referred to as gate-source voltage) is made flat with respect to the center of an operation point. That is, in an amplifier which serves to concurrently receive multi-channel signals in a wide frequency range such as a UHF and a VHF band and amplify them, the present invention can offer a wideband amplifier which employs as an amplifying semiconductor element a field-effect transistor having a substantially flat characteristic of mutual conductance top the gate-source voltage with respect to the center of the operation point for suppressing second order distortion components in an output. Further, the invention can offer a wideband amplifier which ensures suppression a second order distortion at a high frequency by inspecting the field-effect transistor and accepting it only if the difference between mutual conductances to applied voltages at an input terminal having a constant difference therebetween which is equal to or less than a constant value. The invention will be described now by way of example only, with particular reference to the accompanying drawings. In the drawings: BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a characteristic curve of g m to V GS in a GaAs FET according to one embodiment of the invention; Fig. 2 is a curve showing a doping carrier profile of the FET according to the embodiment of the invention; Fig. 3 is a section of the FET; Fig. 4 is a graph showing Δg m - IM₂ relation of the FET; Fig. 5 is an equivalent circuit diagram of a GaAs wideband amplifier; Fig. 6 is a graph showing frequency dependence of a gain in the wideband amplifier; Fig. 7 is a curve of g m - V GS relation of a conventional GaAs FET; and Fig. 8 is a schematic view showing the manufacturing step of the GaAs FET. DESCRIPTION OF THE PREFERRED EMBODIMENTS The principle of the invention is based on the fact experimentally found out by the present inventors, who have analyzed the characteristic of an FET composing a wideband amplifier and a second order distortion characteristic. Traditionally, the second order distortion has resulted from nonlinearity of g m of an FET and gate capacitance. However, as a result of studying the relation between the proper characteristic of the FET and the second order distortion, the present inventors have found out that there exists large correlation between a difference Δg m between both mutual conductances g m to voltages applied at an input terminal and a second order distortion caused in the cable television converter. Fig. 7 shows the relation between the mutual conductance g m and the gate-source voltage V GS of a compound semiconductor normally used in a wideband amplifier, for example, a GaAs field-effect transistor (FET). A normal GaAs FET has a gradient of g m as shown in Fig. 7. The FET is allowed to operate by setting an operation point on the curve with the gradient and applying a signal positioned between - V G to V G to a gate. The present inventors have researched the relation between the difference Δg m between g m of V G = ±0.3 V and a second order intermodulation distortion (IM₂) caused in the cable television converter. Fig. 4 shows the researched result. The GaAs FET is normally manufactured by the process shown in Fig. 8. The process comprises the steps of forming a mask 21 on a semi-insulated GaAs substrate; implanting Si-ion 20 as an impurity (A) for forming an active layer 24 in which the FET is formed; etching, that is, adjustably etching a surface portion 23a for realizing a proper characteristic (B); and forming a Schottky gate electrode 25, a source electrode 26, and a drain electrode 27 for forming an FET (C). In addition, the Si-ion implantation condition except No. 41-4 is indicated in the below TABLE. Fig. 4 shows the relation between Δg m and a second order intermodulation distortion (IM₂) in a wideband amplifier employing an FET (trial lot No. 41-4 shown in Fig. 4) whose active layer 24 is formed by implanting Si-ion under the condition of 70 KeV accelerating voltage and 7 × 10¹² dose amount or an FET (any one except the trial lot No. 41-4) manufactured in such a trial and error manner as varying the Si-ion implantation condition. As shown in Fig. 4, it is to be understood that as Δg m becomes smaller, IM₂ becomes more excellent. This result indicates the fact that as Δg m becomes smaller, that is, as the used FET has a more flat mutual conductance characteristic with respect to the center of the operation point, the second order intermodulation distortion (IM₂) becomes superior. This also indicates that the smallest value of the first order differential coefficient σg m /σV GS to the gate voltage V GS of g m is important to reducing Im₂. That is, it is found out the fact that the FET having a smaller gradient of g m , in other words, a flat g m on the operation point has an effect on reduction of the IM₂. This fact has been found out by the present inventors. Hereinafter, a description will be directed to a wideband amplifier according to one embodiment of the invention employing a GaAs FET. The circuit diagram of the amplifier is identical to that shown in Fig. 5. In this diagram, V GS = 0 V is denoted as an operation point. The present invention has the following greatly different point from the conventional one, wherein the used FET has a substantially flat area of g m with respect to the center of the operation point of V GS = 0v in the characteristic of g m (mutual conductance) to V GS (voltage between gate and source) as shown in Fig. 1. The value of Δg m is suppressed to be 8 mS or less in the range of V GS = 0V and 20% or less value of g m0 = 70 mS given in case of V GS = 0v. The present invention employs the FET having the foregoing condition for its wideband amplifier. The FET used in the invention is shown in Fig. 3. The FET shown in Fig. 3 is identical to that shown in Fig. 8 except the characteristic shown in Fig. 1. The carrier doping profile, as shown in Fig. 2, is smaller than the conventional one near the surface of the Schottky gate electrode 2 and is higher in a deep portion of the electrode 2. 100 denotes an FET carrier doping profile for the invention, 200 denotes a carrier doping profile for a conventional FET, 3 denotes a semi-insulated GaAs substrate, 4 denotes a source electrode, and 5 denotes a drain electrode. The carrier doping profile 100 can be obtained by properly selecting any one except the LOT No. 41-4. For example, it could be implemented by using the Si-ion implantation condition of an accelerating voltage of 120 KeV and a dose amount of 3 × 10¹² cm⁻³ and an FET gate width of 800 µm and adjustably etching the thickness of an active layer 1 so as to have a drain saturation current I pss of 30 to 50 mA. This FET gives a larger accelerating voltage and a smaller dose amount than the conventional FET for reducing the adjustably etched amount. The foregoing ion implantation condition was obtained by doing a trial and error for searching the most proper condition among several conditions that Δg m shown in Fig. 4 becomes smaller. In general, by lowering the accelerating voltage, pushing a peak value into the depth of the active layer, reducing the dose amount, and reducing the adjustably etched amount, it is possible to leave a doping carrier profile near the surface, that is, the profile where a carrier density drops in the active layer of the device intact, resulting in being able to suppressing Δg m to be a small value. This type of FET can be also implemented by providing a larger source resistance than the conventional FET. However, as the source resistance is made larger than a predetermined value, a value of g m is made smaller, thereby making the other characteristics such as a noise characteristic and a gain inferior. Hence, the source resistance has to be an optimal value. It is well known that the relation between the source resistance and the g m of the device can be represented by the following equation: g m = g m0 /(1 + R s g m0 ) where g m0 denotes a g m value of an intrinsic FET from which the source resistance is excluded. The equation indicates that when R s has a large value, g m can be suppressed to be a small value as g m0 becomes larger. As a result, as shown in Fig. 1, the region where Δg m is small can be formed near the portion of V GS = 0v where g m0 is large. An excellent IM₂ characteristic was implemented when the wideband amplifier designed above (which is identical to the design shown in Fig. 5) is used as a front-end amplifier of the cable television converter. In this example, when the input signal level is -5dBm and the cable television converter receives 87-channel signals over its band, IM₂ is guaranteed to be 60 dB or more. And, in order to make the IM₂ characteristic of the wideband amplifier more stable and excellent, traditionally, it has been necessary to follow the troublesome steps of performing d.c. inspection of the wideband amplifiers, applying-multi-channel signals to each successful wideband amplifier like the cable television converter, and actually measuring an IM₂ value of each wideband amplifier for guaranteeing a proper IM₂ characteristic of the wideband amplifier, because the correlation between the Im₂ and the d.c. characteristic has been made unobvious. The aforementioned researched result, however, indicates that the IM₂ characteristic has strong correlation with Δg m to be easily measured by the d.c. automatic inspection. Hence, for guaranteeing the IM₂ characteristic, the acceptance quality level was assumed as follows: the difference Δg m between a g m in case of V GS = 0.15 V and a g m in case of V GS = -0.15 V has to reside within 30% of g m0 , that is, a g m in case of V GS = 0v. Concretely, the difference Δg m has to be 8 ms or lower. Since the inspection can be automated by using a low frequency or a direct current, it is possible to guarantee the IM₂ characteristic to be a constant value or more merely by doing IM₂ spot check using a high frequency, resulting in greatly lowering the cost of the inspecting process. This method is found out by the present inventors and makes great contribution to easy implementation of a wideband amplifier designed for suppressing a second order distortion. As has been obvious from the above description, the present invention allows implementation of a low-distortion and low-noise wideband amplifier consisting of a GaAs FET. And, since the invention is capable of guaranteeing a second order distortion with the low-frequency automatic inspection, the inspecting cost can be greatly reduced, so that it is possible to easily implement the wideband amplifier which is excellent in a second order characteristic at low cost, resulting in making great contribution to implementation of a high-efficient wideband amplifier.

Claims: A wideband amplifier for receiving concurrently multi-channel signal in a predetermined frequency range and for amplifying the receiver signals, said wideband amplifier comprising a field effect transistor (11) which has a gate, a drain and a source and is arranged to amplify the received signals, said transistor (11) having a negative feedback circuit (12, 15) for feeding back a signal at said drain to said gate, and a gate bias circuit (13) between said gate and said source for setting a gate bias of the field effect transistor (11), characterised in that said feedback circuit includes a feedback capacitor (15) arranged to cut off a direct component of current between the gate and the drain and a feedback resistance (12), said gate bias circuit includes a gate bias resistance (13), and wherein said field effect transistor (11), formed by ion implantation, has a substantially flat characteristic of mutual conductance (g m ) versus voltage (V GS ) between the gate and the source with respect to the center of an operation region to thereby suppress a second order intermodulation distortion (IM₂) of an output of said wideband amplifier. A wideband amplifier according to claim 1, wherein said predetermined frequency range is a UHF and/or a VHF frequency range and said field effect transistor (11) is formed from GaAs substrate. A wideband amplifier according to claim 1 or claim 2, wherein said field effect transistor (11) has an active layer (1) having a carrier doping profile in which the carrier concentration is lower near a surface of the gate electrode (2) and is higher in a deeper portion of the active layer compared with a conventional field effect transistor, said field effect transistor having formed by controlling the condition of ion implantation to a semiconductor substrate. A method of inspecting a field effect transistor for the wideband amplifier according to claims 1 to 3 in order to ensure a sufficient low level of second order distortion in a high frequency range, said method comprising the steps of: applying two different voltages on an input terminal of a field effect transistor; measuring a difference between mutual conductances relating to each of said predetermined voltages; comparing said difference with a predetermined value; and determining whether said field effect transistor is acceptable for the wideband amplifier according to whether said difference is lower than said predetermined value.

Inventors: GODA KAZUHIDE, NAMBU SHUTARO, YAMASHITA YASUHISA, GODA, KAZUHIDE, NAMBU, SHUTARO, YAMASHITA, YASUHISA
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1990
Main classification: H03F3
IPC codes: H03F1, H03F3