Patent GB-743585-A (GB)
Title: Improvements in or relating to circuits for amplifying electrical oscillations
IPC: H03F3/66
Filed: 1952-10-17
Abstract: 743,585. Valve circuits. PHILIPS ELECTRICAL INDUSTRIES, Ltd. Oct. 17, 1952 [Oct. 22, 1951], No. 26088/52. Classes 40 (5) and 40 (6). A circuit for amplifying, detecting, frequency changing or division or for removing amplitude modulation comprises a valve having two control grids to which the signal is applied and an anode circuit including at least one circuit tuned to a frequency other than that of the input signal and coupled to the control grids of the valve by feed-back paths of opposite polarities whereby oscillations are produced having a frequency different from that of the input signal and amplification takes place as a result of the conversion conductance of the tube. In Fig. 1 the input frequency f is applied through a circuit 4 tuned to the input frequency to two control grids of the valve in opposite polarity. The anode circuit 5 is tuned to f/2 and provides positive and negative feed-back respectively to the control grids. It is explained that the self-oscillation is caused by the conversion conductance of the valve as well as by the direct gain and that the oscillation frequency is relatively insensitive to the resonant frequency of the anode circuit. The circuit is even less sensitive in this respect if phase correcting circuits (Figs. 3, 4 and 5, not shown) are associated with the feed-back paths. The output will follow the amplitude and frequency changes of the input and rectifier circuit 18, 19 will provide a voltage corresponding to the amplitude modulation envelope of the input wave. The output of the rectifier may be fed back to the input and according to the time constant used the mean amplitude will be stabilized or envelope feed-back will be applied. In the latter case the circuit may be used as an amplitude limiter for a frequency modulated signal. An automatic gain control voltage for earlier amplifying stages may also be obtained from the rectifier. The rectifier may alternatively be fed from a further anode circuit tuned to the signal frequency (Fig. 7, not shown), both,short and long time constant circuits being provided, the voltage from long time constant circuit being also applied to a clamping diode connected across the input circuit to provide further amplitude limitation. In Fig. 8 the anode tuned circuit 35 is tuned to a frequency fm not integrally related to the input frequency f and the second tuned circuit 34 is tuned to the sum or difference between these frequencies (fŒfm). Oscillation at both frequencies occurs. If fm is small compared with f and the input signal is frequency modulated the frequency swing of the frequency (f+fm) is compressed. When the circuit is used merely as a frequency changer the networks 34, 6, 7 are preferably arranged such that as the frequency increases the phase of the feed-back first increases and then decreases. Further circuits may be added in series with 34 and 35, each tuned to a frequency different by an amount equal to f from the next and all frequencies may be stabilized by applying a stabilized frequency to one or both control grids and having a value equal to the sum of two tuning frequencies, preferably the central frequencies. For the reception of suppressed carrier waves one of anode tuned circuits of Fig. 8 may be tuned to the carrier and one to a modulation frequency. If a limiter is provided to limit the carrier oscillation amplitude the oscillation in the other circuit corresponds to the modulation on the input signal. A non- linear impedance such as a lamp, or a rectifier associated with a long time constant circuit may be associated with one of the anode tuned circuits to control the amplification.