GB521999A - Improvements in and relating to modulating and amplifying systems - Google Patents

Abstract

521,999. Valve modulating and amplifying circuits. BRITISH THOMSON-HOUSTON CO., Ltd. Oct. 27, 1938, No. 31114. Convention date, Oct. 29, 1937. [Class 40 (v)] In order to increase the efficiency of the modulating and amplifying system of a radio transmitter the load of the power amplifier is made variable under the control of the modulation, a portion of the output power being rectified and fed back to replenish the direct current power supply to the amplifier. The power amplifier 1 is driven from an exciter stage 44 which is plate modulated by means of the audio frequency transformer 46. The anode of valve 1 is shunt fed through a choke 2 from the source of anode potential 3 and the tuned output circuit 7 is connected in series with the input end of a quarter wavelength section of network 14. The output end of the network 14 is connected to the anode 22 of the modulatorrectifier valve 23 whose cathode is connected to the positive terminal of source 3, and whose grid is fed with modulating potential. The bias and phase of the modulating voltage on the grid of valve 23 is such that when the modulation is 100 per cent. positive the valve 23 is non-conducting and the end 13 of the network 14 simulates a short circuit and the full power output of valve 1 is delivered to the aerial. At carrier level, due to the conductivity of the modulator-rectifier a resistance equal to the aerial load resistance is inserted in series with the tank circuit 7. As a result valve 1 delivers only one half its maximum power and only one half of this is fed to the aerial, the other half (one quarter of the maximum) being rectified in 23 and fed back to the anode supply. The grid drive on 1 is such that at carrier level the anode voltage swing is a maximum and the negative half cycles of modulation in the output are produced by the downward swings of the driving voltage. Further action of the modulator rectifier valve is prevented during the negative half cycles of modulation by holding the bias on that valve fixed. This may be done by making carrier conditions correspond to zero bias and by providing the audio system connected to transformer 30 with such poor regulation that the grid of valve 23 cannot be driven far positive. In a modification, Fig. 2 (not shown), the quarter wave section is not employed and the power amplifier is connected to the load over a link coupling circuit with the modulator-rectifier valve in shunt in the coupling circuit. In order to reduce overall distortion, a portion of the aerial output may be rectified and the audio frequency fed back in a negative feed back sense.