Patent GB-709036-A (GB)
Title: Improvements relating to electronic amplifiers
Inventor: HINTON MAITLAND JOSEPH GRESLEY
IPC: H03F3/40
Filed: 1952-02-08
Abstract: 709,036. Valve amplifying circuits; modulating systems. NAGARD, Ltd. May 8, 1953 [Feb. 8, 1952], No. 3432/52. Class 40 (5) Relates to the amplification of direct or alternating current signals by modulating the input on a carrier, amplifying the modulated carrier and detecting the amplified signals ; and consists mainly in employing as modulator a rectifier system which short-circuits the input signal at carrier frequency, and in providing means whereby the carrier frequency is substantially prevented from appearing at the signal input terminals. In Fig. 1, the input signal is fed at terminals a, b across one diagonal of a bridge of four rectifiers D1 ... D4, and the carrier is fed at c, d across the other diagonal. Alternate half-cycles of the carrier render all the rectifiers conductive and short-circuit the input signal at carrier frequency. The resistance R tends to shield the input terminal a from the carrier. The modulated carrier is amplified at CA and passes to a detector D to yield an amplified signal at the terminals y, z. In Fig. 2 the shielding of the signal input from the carrier voltage is effected by connecting the signal input to the grid of a cathode-follower V1, the upper rectifier terminal being connected to the cathode. H.T. supply for the valve V1 is taken from the terminals of a pair of neon tubes N+ and N-, earthed at their junction and connected to the main supply through resistors R+, R-. A second cathode-follower V2 takes its grid voltage from a variable tap T1 on a potential-divider PI connected across the neon tubes. The resulting cathode voltage is supplied to a lead I" connected to the lower rectifier terminal. The cathodes of the valves V1, V2 are joined by a resistor P2 to which the HT negative lead is connected by a variable tapping T2. The tapping T1 permits variation of the carrier amplitude corresponding to zero signal, and it may be calibrated in volts to indicate the voltage applied at the terminal a, the tapping being adjusted until a zero signal is obtained at the output terminals y, z which may be connected to an oscilloscope. The tapping T2 may be adjusted so that the carrier level is zero when the grids of the valves V1, V2 are at the same potential, e.g. at earth potential. A variable condenser c' may be provided to adjust the relative phases of carrier supplied to the lines l<SP>1</SP>, l<SP>11</SP>. In a further modification, Fig. 4 (not shown), the resistor R, Fig. 1, is replaced by a rectifier system similar to the system D1 ... D4 but with polarities reversed. The carrier is applied to the horizontal diagonal and the input line is in series with the vertical diagonal. Its action is to block the line l<SP>1</SP> whenever the system D1 ... D4 short-circuits l<SP>1</SP> and l<SP>11</SP>. Fig. 5 shows an arrangement employing only two rectifiers D9, D10 in the modulator and two other rectifiers D11, D12 for blocking the input line. Both pairs are fed with carrier from a valve oscillator Vo, and they are so arranged that during one series of halfcycles of the carriers, D11, D12 are blocked and D9, D10 are conducting, and during the other series, the reverse obtains. A duplicate arrangement of the Fig. 2 circuit is described, Fig. 3 (not shown), in which two independent signals, or two phase-opposed versions of the same signal, are transmitted. Each signal is applied across switch-controlled attenuators to its own cathode-follower valve V1 which may be in the same envelope with the corresponding level control valve V2. The two valves V2 take their grid voltage from a double potentiometer connected across the H.T. supply, so arranged that if the contact arm is rotated to give a rising voltage on one grid, a falling voltage is imparted to the other. The two signals jointly modulate the carrier in a four-rectifier bridge whose output is taken to pentode amplifiers in cascade. Detection is effected by two separate oppositelypoled rectifiers, and the resulting signals are amplified in separate amplifiers.