| 摘要 |
1,043,900. Field effect transistor circuits; frequency modulation. RADIO CORPORATION OF AMERICA. Nov. 20, 1963 [Dec. 3, 1962], No. 45855/63. Headings H3R and H3T. An insulated - gate fieldeffect transistor comprises a biased gate electrode 61, Fig. 3, separated by a highresistance silicon dioxide layer from a lightly doped P- type silicon substrate 64 having source and drain electrodes 62, 63 forming with the substrate reversed biased rectifying junctions 72, 71 which act as variable capacitors when a variable control voltage across terminals 73, 74 is applied to the substrate electrode 64. With the stray capacitances between gate electrode 61 and source 62, and between source 62 and earth (67 being an R.F. choke), the transistor circuit forms a Colpitts oscillator with the frequency determined by inductor 69 and the capacitances. The oscillator may be frequency controlled by an A.F.C. voltage or frequency modulated by a signal voltage applied at 73, 74 over choke 75 to the capacitors 71, 72. When the signal voltage becomes positive, rectifier junction 72 forms a resistor, the net effect decreasing the oscillator frequency. Alternatively, a switching voltage forwardly biasing both rectifier junctions 71, 72 may key the oscillator on and off. Capacitor 76 which is optional also influences the circuit frequency characteristics, and an additional capacitor between the gate and source electrodes 61, 62 may be provided to reduce the increase in oscillation amplitude arising with positively increasing substrate voltage. In a modification, Fig. 6 (not shown), the source electrode 62 is positively biased by a resistor in series with the choke 67. Also a variable resistor or a signal source is connected between the positively biased drain and the substrate electrode 64 so that diode 72 is always forwardly biased and varies the effective value of capacitor 71. In Fig. 7 the transistor circuit is an R.F. or I.F. tuned amplifier with input applied over capacitor 92 and A.G.C. also applied to the gate electrode 91 to vary the current and therefore the voltage across a tapped resistor 100 in series with a tuned output circuit 99. The D.C. voltage at the resistor tapping 102 is applied to the substrate electrode to vary the transistor capacitance shunting the tuned circuit and lower its frequency response with increased A.G.C. voltage. This variation of the frequency response of circuit 99 with the A.G.C. voltage results in an essentially flat frequency response characteristic over a predetermined signal pass-band. This arrangement may be used as a television receiver inter-carrier I.F. amplifier to provide enhanced response to the video carrier when the carrier signal level is weak. |
