Patent GB-478552-A (GB)
Title: Improvements in and relating to the operation of modulated high frequency transmitters and class b low frequency amplifiers
IPC: H03F3/32
Filed: 1937-04-03
Abstract: 478,552. Valve modulating and amplifying circuits. POHJANPALO, J. J. April 3, 1937, No. 9526. [Class 40 (v)] The efficiency of a radio-modulating system is increased by using two or more valves or valve groups, one group commencing to operate when the aerial current is low and reaching maximum efficiency when the aerial current reaches its mean or unmodulated value and then going gradually out of action. The second group comes into action as the aerial current rises beyond its mean value, and operates alone and at maximum efficiency when the aerial current reaches its peak value. The system is also applicable to Class B low-frequency amplifiers. Fig. 1 shows a grid modulating system in which the carrier is applied at L3 to the parallel-connected grids of the respective valves P1, P2, the modulation being applied through transformers T1, T2. When there is no modulation, the valve P2 is inoperative owing to its high negative grid bias, but the valve P1 is operating at maximum efficiency, its output voltage Ea1 being equal to the anode supply voltage ea. During negative half-cycles of modulating voltage, the output Ea1 of the valve P1 is reduced. During positive half - cycles, the modulation does not affect the grid of the valve P1 owing to the provision of a rectifier valve P3. The valve P2, however, comes into action and contributes to the output into the aerial. The anode of this valve is connected to a centre tapping on the output coil L1, and as its output voltage Ea2 rises towards its full value ea giving maximum efficiency for the valve P2, the valve P1 goes gradually out of action owing to its anode alternating voltage Ea1 approaching a value equal to twice the supply voltage ea. Instead of using a centre tap connection on the coil L1, the anodes of the valves P1, P2 can be connected in parallel to the load circuit L1, C1, but the anode supply voltage of the valve P2 may be double that of the valve P1, Fig. 2 (not shown). A similar circuit may be used for the final amplification stage of a transmitter, the grid circuits of the valves P1, P2 being fed with carrier from separate modulators, Fig. 3 (not shown). The circuit of Fig. 1 may be modified by providing more than two valve groups, a corresponding number of tappings being provided on the coil L1, Fig. 4 (not shown). When series modulation is employed, Fig. 5, a modulating-valve M1, M2 is connected in series with each carrier amplifier P1, P2 and the supply voltage for the pair P2, M2 is twice that for the pair P1, M1. The valve M2 is so biassed that no current flows through P2, M2 except during a positive half-cycle of modulating voltage in its grid circuit. The system shown in Fig. 6 employs parallel-feed anode modulation for each carrier amplifier P1, P2. The anode supply voltage for the valve P1 is ea, and for the valve P2 is zero. During negative half-cycles of modulation, the valve M1 operates through the transformer T3 to reduce the voltage applied to the valve P1. During positive half-cycles, the valve M2 operates to apply positive voltage through the transformer T4 to the anode of the valve P2, this voltage rising to the value 2ea at full output. Fig. 7 shows a Class B low-frequency amplifier in which the input is applied through two push-pull transformers T1, T2 to the grids of two pairs of valves P1, P2, the outputs being tapped to the mid-points and ends of the output transformer T3. The valves P1 are biassed to bottom bend, whilst valves P2 are more heavily biassed. At high inputs the valves P2 supply the load circuit, and the valves P1 go gradually out of action. The input to valves P1 must be so arranged that it cannot rise above the value at which the valves P2 begin to operate. The input circuit of the modulator M2 of Fig. 5 may be arranged as shown in Fig. 8, in which the grid-cathode circuit is shunted bv a resistance R2 in series with a diode valve P3. At large amplitudes of modulating voltage, the diode passes current and the voltage swing on the grid is reduced.