Method for gain control of field-effect transistor
United States Patent Hayashi et a1.  
 [5 METHOD FOR GAIN CONTROL OF FlELD-EFFECT TRANSISTOR [76] Inventors: Yutaka Hayashi, 9-14. 4-chome.  
 Hon-machl. Hoya; Yasuo Tarui. 6-4. 5-chrome. Minamisawa. Kurume-machi. Kitatama. Tokyo. both of Japan [22] Filed: Nov. 13. 1973 [21] Appl. No.: 415,345  
 Related US. Application Data [60] Division of Scr. No. 264.965. June 21. 1972. and a continuation-in-part of Scr. No. 24.166. March 21. 1970.  
 [52] US. Cl 332/31 T; 307/264; 307/279; 330/29; 330/35 [51] Int. Cl. ..1-103c 1/36 [58] Field of Search........... 332/31 T: 330/29. 38 R. 330/38 M. 35; 307/264. 279  
 [56] References Cited UNITED STATES PATENTS 3.311.756 3/1967 Nagata ct a1 330/35 1 1 Apr. 22, 1975 3.391.354 7/1968 Ohashi ct al. 332/31 T 3.443.240 5/1969 Santilli 330/29 3.480.873 11/1969 Caner 330/29 3.525.050 8/1970 Wolf ct a1. 330/29 Primary Eruminvr-Alfred L. Brody Attorney. Agent. or FirmR0bert E. Burns; Emmanuel .l. Lobato; Bruce L. Adams [57] ABSTRACT The gain of a field-effect transistor having a single insulated gate is controlled by varying the forward current between the base and the source. Amplitude modulation is obtained by applying an AF signal together with a forward bias to a terminal connected to the base through a resistor. An RF signal is applied to the gate. for example through a transformer. and the amplified signal appears at a load coupled to a resonant circuit connected to the drain and one terminal of a resistor which is decoupled by a capacitor. The other terminal of the resistor is connected to a power source.  
 9 Claims. 11 Drawing Figures SIGNAL GAlN SOURCE CONTROL PATENIEDAPRZZIQYS 3.879.688  
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  SIGNAL SOURCE I GATE- BIAS FIG.3  
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 K Reduced Gain Output Signal- (No Distortion) Input Signal q i i i I I e V e L 8 C n e r e f e R METHOD FOR GAIN CONTROL OF FIELD-EFFECT TRANSISTOR This is a division, of application Ser. No. 264,965, filed June 21, 1972, which is a continuation-in-part application of our earlier US. patent application Ser. No. 24,166, filed Mar. 31, I970.  
 BACKGROUND OF THE INVENTION This invention relates to a method for gain control of a field-effect transistor, more particularly, to a method for gain control which provided for a minimum of distortion due to cross modulation in a field-effect transistor which does not have a tetrode configuration (i.e., does not have two insulated gates).  
  Methods employed heretofore for the gain control, amplitude modulation, etc., of field-effect transistors include one wherein the transfer conductance gm is changed by varying the DC gate bias voltage, and another wherein the value of gm is also changed by reverse-biasing a base region where a channel is formed of the field-effect transistor with respect to the source thereof.  
  According to the methods described above, however, an input signal drives the gate from the turn-on region to the cut-off region if the signal voltage is high when the gain of the field-effect transistor is decreased. This means that the device operates under a time variable transfer transconductance. The resultant increased distortion due to cross modulation renders the methods virtually impracticable. An attempt to circumvent this undesirable effect has been made by the use of tetrode fieldeffect transistors having two insulated gate electrodes, which, however, have also given rise to various difficulties in their operation at high frequencies.  
  On the other hand, conventional efforts to make use of field-effect transistors in the high frequency band by shortening their channel length have materialized, to give one example, in the development of a method wherein the channel length is determined in accordance with a difference in the lengths over which two impurities are diffused. Yet, at the present, this method is also not free from drawbacks in that the production of such tetrode field-effect transistors according to the method requires complex processes.  
 SUMMARY OF THE INVENTION Therefore, it is a principal object of the invention to provide an improved method for gain control of a fieldeffect transistor wherein all deficiencies attendant to the prior methods mentioned above are overcome.  
  It is another object of the invention to provide an improved method for gain control of an ordinary fieldeffect transistor having only one insulated gate with a minimum of distortion due to cross modulation.  
  It is a further object of the invention to provide an improved method for gain control of an ordinary fieldeffect transistor having only one insulated gate, which does not need an extra bias of reverse polarity, that is, with a gain control voltage of the same polarity as that of the drain bias voltage.  
  It is another object of the invention to provide an improved and excellent method for gain control of a fieldeffect transistor using very simple circuits.  
  Characteristic features and functions of the invention will be further described in connection with the accompanying drawings, in which the same or equivalent members are indicated by the same numerals and characters.  
 BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows an equivalent circuit of a short channel field-effect transistor with means for effecting gain control in accordance with the invention.  
  FIG. 2 graphically represents the output characteristics of the field-effect transistor due to base current;  
  FIG. 3 graphically represents a characteristic example of variation in gm due to a forward bias applied between a current limiting resistor of the base and the source;  
  FIG. 4 is a schematic diagram of a circuit for providing a controlled gain by the method of this invention;  
  FIG. 5 is a diagram illustrating control of amplification of a field-effect transistor in accordance with the invention, and  
  FIGS. 6 to 11 illustrate schematically various gain control circuits that can be used in the field-effect transistor circuit of the present invention.  
 DETAILED DESCRIPTION OF THE INVENTION In a field-effect transistor with a sufficiently short channel, as illustrated in FIG. 1, a current between the drain D, and source S, electrodes increases as the de- I vice acts as a bipolar transistor when the base B, is forward-biased with respect to the source by means of a gain control 10 and when a base current flows due to such forward bias. Actual examples of the above phenomenon are schematically shown in FIG. 2, wherein the curve (a) represents an output characteristic between drain and source when the base is kept forwardbiased by the gain control 10 up to the instant at which current begins to flow, whereas the curves (b), (c) and (d) respectively denote the output characteristics when the forward current is increased.  
  When utilized as a tuned amplifier, the field-effect transistor has its drain connected with an LC resonance circuit 12; in this case, the point of operation undergoes a series of changes as indicated by A, B, C and D in FIG. 2 along with increases in the base current after further connecting a resistance 14 with a bypass capacitor 16 in series with the LC resonance circuit 12. Accordingly, the drain voltage gradually decreases. On the other hand, since the value gm relative to the fieldeffect transistor itself decreases substantially in proportion to a decrease in the drain voltage when the drain voltage is less than a pinch-off voltage, the signal amplification factor measured across a load coupled to the resonent circuit decreases along with decrease in gm and, further increase in the output conductance.  
  In the above instance, the gate bias voltage applied by a gate bias circuit 18 is not changed, and the bias voltage between base and source electrodes due to the gain control 10 is also kept almost unvaried, so that the change in the threshold voltage of the field-effect transistor is only negligible and the effective gate voltage, obtained by subtracting the threshold voltage from the gate voltage, is kept nearly at a constant level. It is accordingly possible to decrease only the gain without any corresponding increase in distortion due to cross modulation even though the amplitude of the signal applied to the gate G, by an input signal source device 20 may be large.  
  Shown in FIG. 3 is an example of a characteristic curve indicated by the value gm with regard to the forward voltage present between base and source when a high resistance is connected in series with the base.  
  Further, at frequencies in excess of the upper limit of frequencies acceptable in a bipolar transistor, without varying the drain voltage, the output conductance increases due to a charge injected to the base, so that the desired gain control is made possible by the base forward voltage or current. For the purpose of amplitude modulation, a carrier wave may be applied to the insulated gate (indicated by G in FIG. 1), and a signal of a frequency lower than that of the carrier wave may be applied to the base terminal.  
  In the circuit shown in FIG. 4, in which the method of this invention is applied, an n-channel MOS transistor capable of operating in bipolar fashion is indicated at 40. The gate, drain, base and source electrodes are indicated at G,D,B, and S respectively. An input RF signal from a signal source 22 is passed to the gate electrode G by means of a transformer 23, the secondary of which forms part of a resonant circuit with a variable capacitor 24. The amplified signal appears at the resonant circuit 25 connected to the drain, and is passed through a transformer 26, the primary of which forms part of the resonant circuit 25, to a load represented by the resistor 27. A bias voltage V decoupled by a capacitor 28 is applied to the gate electrode as shown. A positive supply voltage V,,,,, decoupled by a resistor 29 having resistance R and capacitor 30 is applied through resonant circuit 25 to the drain electrode. The gain control signal represented by a controlled positive voltage supplied by the gain control means 31, is applied through a resistor 32 to the base electrode.  
  The gain control obtained with the circuit of FIG. 4 is illustrated by FIG. 5 in which curves M and N represent I vs. V with different gains. It will be seen that the curve is merely changed in inclination so that variation in gain is obtained without distortion.  
  Amplitude modulation can be obtained with the circuit by applying an AF signal together with a positive bias to the base B by means of the circuit 31.  
  The gain control circuit which is indicated by the block in FIG. 1 is a circuit which can supply variable and controllable forward current between the base and source of the field-effect transistor (FET). Thus, it can be either a variable voltage source with a resistor in series as illustrated by way of example in FIG. 6 or a variable current source as illustrated&#39;in FIG. 7. The value of the voltage or current is controlled by hand in the case of manual control and by a control signal in the case of automatic gain control (AGC) or by an AF signal in the case of amplitude modulation. Simple and practical ways of providing controlled base forward current are illustrated in FIGS. 8 and 9 where the gain of the amplifier is manually controlled by adjusting a potentiometer R A circuit for control by amplitude modulation is shown in FIG. 10 where DC bias current can be adjusted by a potentiometer R and modulation efficiency can be determined by a resistance R,,. Automatic gain control (AGC) can be achieved by a circuit as illustrated in FIG. 11 where an amplified and rectified rf signal is compared with a reference level and the difference between them is amplified by a differential amplifier A. The output voltage from the amplifier A is the control signal. If the amplifier a has a low output resistance the output is supplied to the base&#39;terminal through a series resistor R,,. If the amplifier A is of high output resistance or current source type, the output can be directly supplied to the base terminal of the F ET.  
  The control circuits illustrated in FIGS. 6 to 11 are also applicable to the field-effect transistor circuit illustrated in FIG. 4 where the gain control is represented by the block 31. In this event the series resistor R, is the resistor 32 shown in FIG. 4.  
  As herein used the term forward bias means a positive bias for the p-type base of an n channel IGFET with reference to the source and a negative bias for the n-type base of a p channel IGF ET with reference to the source. The polarity shown in all figures of the drawings is for an n channel FET. The polarity must be reversed for a p channel F ET.  
  While preferred embodiments of the invention have been shown by way of example in the drawings, it will be understood that the invention is in no way limited to these embodiments.  
 What we claim and desire to secure by Letters Patent l. A method for controlling the gain of a field-effect transistor of the type having a base, a source, a drain, and a gate which is receptive of an input signal during use and capable of operating as a bipolar transistor when said source, base and drain are respectively used as an emitter, base and collector, said method comprising the steps of: applying a constant voltage to said gate to bias same for an operable range of input signals; applying a voltage to said drain having an operable range within the pinch-off voltage characteristics of said field-effect transistor to bias said drain; applying a forward bias voltage to said base with respect to said source; and varying the base-to-source current to effect control of the gain of said field-effect transistor and t effect a constant gate threshold voltage.  
  2. A field-effect transistor circuit for controlling the gain thereof to reduce output distortion of an input signal applied thereto and amplified thereby, said circuit comprising: a field effect transistor having a base, a source, a drain, and a gate receptive of an input signal applied thereto during use and capable of operating as a bipolar transistor when said source, base and drain are respectively used as an emitter, base and collector; means for applying a substantially constant gate bias voltage to said gate for an operable range of said input signal means for applying a bias voltage to said drain having an operable range within the pinch-off voltage characteristics of said field-effect transistor; and means for applying a forward bias voltage to said base with respect to said source including means for varying the forward current between said base and said source thereby effecting variations in the gain of said fieldeffect transistor, whereby the variations in the base-tosource forward bias current to effect control of the gain of said field-effect transistor and said constant gate bias voltage effect a constant gate threshold voltage whereby the input signal is amplified within the pinchoff voltage characteristics of said field-effect transistor thereby reducing the distortion in the amplified signal.  
  3. A circuit according to claim 2, wherein said means for applying a bias voltage to said drain comprises a resistor, a resonant circuit, a load coupled to said resonant circuit, a power source connected in series with said resistor and&#39;capacitor means decoupling said resistor; wherein said means for applying a gate bias voltage comprises a bias voltage source connected to said gate and a first signal source coupled to said gate; and  
 wherein said means for applying a forward base current to said base comprises a second signal source.  
  4. A circuit according to claim 3, in which said resonant circuit comprises an LC resonant circuit connected to said drain electrode in series with said resistor.  
  5. A circuit according to claim 4, in which said LC resonant circuit includes the primary of a transformer, the secondary of which is connected to a load resistance.  
  6. A circuit according to claim 5, in which said capacitor means comprises a capacitor connected to said source-drain circuit between said LC resonant circuit and said resistor.  
  7. A circuit according to claim 3, in which said first signal source is connected with the primary of a transformer, the secondary of which is connected between said bias voltage source and said gate.  
  8. A circuit according to claim 7, in which said source is connected to ground and in which a variable capacitor has one terminal connected to ground and another terminal connected to said gate.  
  9. A circuit according to claim 7, in which said source is connected to ground and a capacitor has one terminal connected to ground and another terminal connected between said bias voltage source and the secondary of said transformer.  
 * l l =l= =l UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PATENTNO: 3,879,688 0 DATED April 22, 1975 lNVENTORtS) Yutaka HAYASHI and Yasuo TARUI It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown beiow:  
 On the Title Page in column 1, after the Inventor is listed and before Filed, please insert:  
 Q Assignee: Kogyo Gijutsuin, Tokyo, Japan Signed and Sealed this fifteenth D3) 0f June 1976 Q [SEAL] Arrest:  
  RUTH C. MASON C. MARSHALL DANN a Arresting Officer (&#39;ommiser&#39;mier oj&#39;Parenrs and Trademarks