GB844107A - Improvements in or relating to oscillators for high-frequency heating - Google Patents

Abstract

844,107. Valve oscillating circuits. PHILIPS ELECTRICAL INDUSTRIES Ltd. May 1, 1958 [May 1, 1957], No. 13931/58. Class 40(6). An oscillator for high frequency heating comprises a back coupled grid controlled oscillator valve with a pentode included in the D.C. circuit of the control grid to stabilize the grid current with varying load. A resistor is connected in parallel with the pentode and between its cathode and the control grid of the oscillator valve there is included a further resistor the end of which remote from the cathode of the pentode is connected to the control grid of the pentode via a path which includes a constant voltage means. In the embodiment shown a Colpitts oscillator has a valve 1, with a control grid D.C. circuit 13 which comprises a resistor 14 in parallel with a pentode 15, which has, in its cathode path, a variable resistor 16. The end of this resistor 16 is connected via a constant voltage device 18 and a resistor 28 to the control grid of the pentode. If, as a result of an increase in the oscillator load, the grid current of valve 1 falls the grid potential of the pentode will rise and hence its resistance will fall with the result that changes in grid current are counteracted. The resistance of circuit 13 is thus maximum at maximum grid current and decreases with increase in load on the oscillator. If the resistor 16 is adjusted so that the pentode is substantially cut off in the unloaded condition, the total grid current will then be substantially absorbed by the resistor 14. Fig. 2 (not shown) gives curves of the grid current absorbed by the pentode 15 and the resistor 14 individually and also the total grid current as functions of the load. As described, the screen grid potential of the pentode 15 is derived from a separate source 25 and in order to avoid undue increase in screen current when the anode voltage of pentode 15 ceases e.g. when the oscillator valve is switched off, the negative terminal of the source 25 is connected to the cathode of pentode 15 via resistor 26 and to its grid by resistor 27.