Patent GB-657690-A (GB)
Title: Improvements in and relating to electric amplifiers
IPC: H03F3/40
Filed: 1948-04-22
Abstract: 657,690. Valve amplifying circuits. BLACKMAN, P. F., and SUN-VIC CONTROLS, Ltd. April 22, 1948, No. 11123. [Class 40 (vi)] [Also in Group XXXVIII] In a D.C. amplifier, the input converted by commutating means having contacts to a square waveform, is fed to an A.C. amplifier connected to a full-wave rectifier to give a D.C. output.. This output is applied to the input to give negative feed-back. A D.C. signal applied at terminals 10, 12 and filtered by inductor 23 and capacitor C7 is applied over contact banks T5, T6 of a switch to a vibrator V energized from the A.C. supply. The switch is shown in the "Test" position but, in normal operation, it is placed in one of the other positions. The substantially square wave is fed to valves V1, V2 in push-pull to produce a triangular wave in their anode circuits due to capacitors C5, C6. After further amplification in valves V3, V4 the triangular wave is reconverted to a square waveform by the transformer feeding the phase-sensitive network P.S.R. The A.C. supply is fed through a limiting resistor R13 to a transformer having a non-linear resistor 19 known . under the Registered Trade Mark " Metrosil " across the secondary to produce a squarer output. This output is applied as a polarizing voltage to the network PSR. A D.C. potential appears across leads 17, 18 and is a function of amplitude of the A.C. passing through the amplifier. This potential is smoothed over resistor R19 and capacitor C6 and applied to cathode follower V5. With the switch in the test-position shown, resistor R17 is adjusted until the current in meter M2 is zero. When the switch is turned to any other position, a resistor R21 ... R24 is included in both the input and output circuits to provide negative feed-back. If the output voltage across terminals 20, 21 is required to be proportional to the input, a resistor RX is connected across terminals 20, 22 so that a fraction of the output voltage is fed back over resistors RX, R20 and one of resistors R21 ... R24 to the input. When the output current is to be proportional to the input voltage, the load is connected directly across terminals 20, 22. The performance of the amplifier may be checked by depressing the push-button PB when the switch is in the "test" position, a potential being applied over resistor.25 to give a definite deflection of the meters M1, M2. The valve V5 is protected from a large positive bias by rectifier SR, and a further, rectifier may be inserted in the cathode lead, which it is stated reduces undesirable negative feed-back. A. delay switch RL1 energized from the heater supply, and shown in the operated position, maintains the junction of the meters M1, M2 at the potential of lead 16 until valve V5 is conducting. In a modification, Fig. 2 (not shown), the input potential is derived from a standard cell, the output being substantially equal to that of the cell but capable of supplying considerable current. By a slight modification of the feedback circuit, Fig. 3 (not shown), potentials in excess of the standard cell may be obtained. By employing input transformers with single primaries each connected to one contact of the vibrator V, Fig. 4 (not shown), and using two amplifiers, two inputs may be amplified. One amplifier may be employed to maintain a standard voltage related to that of a standard cell, (see Group XXXVIII). Specification 622,491 is referred to.