Patent GB-849348-A (GB)
Title: A multistage direct current amplifier
Inventor: JIRMUS JAROSLAV
IPC: H03F3/36
Filed: 1957-08-01
Abstract: 849,348. Valve amplifying circuits. TESLA NARODNI PODNIK. Aug. 1, 1957, No. 24379/57. Class 40(6). [Also in Group XXXV] A multistage direct-current amplifier is provided with overall negative feedback and, in addition, with positive feedback across the last two stages only. In one embodiment, Fig. 2, a pentode first stage E1 is direct coupled to a triode second stage E2 which is coupled in turn to a third stage E3 by means of a common cathode resistor R8. Positive feedback across the stages E2, 3 is obtained by returning the anode resistor R4 of the first stage to a tap on the anode resistor R1 of the last stage, and overall negative feedback by connecting one, 1<SP>1</SP>, of the input terminals 1, 1<SP>1</SP> to a tap on a resistance network R2 across the output terminals. Alternative methods for varying the amount of positive feedback are described in the Specification (Figs. 3-5, not shown). In a modification, Fig. 6 (not shown) the output is taken from the anode circuit of the valve E2 via an additional cathode follower stage, and the negative feedback is injected into the cathode circuit of the first stage E1 which comprises a triode. In another modification, Fig. 7, the first stage comprises a pair of cathode-coupled triodes E1 2, each of which drives one of the succeeding pair of cathode-coupled valves E3, 4. Positive feedback is obtained by returning the anode loads R27, 28 respectively, of the valves E1 2 to the anode circuits of the valves E3, 4 respectively, the amount of feedback being controlled by a variable shunting resistor R13. Negative feedback is obtained by returning the grid of the valve E2 to a potentiometer R11, 12 across the output terminals. In a modification of this embodiment, Fig. 8 (not shown) the valves E1, 3 are not provided with anode loads, and compensation for input capacities and of the frequency characteristic is obtained by connection of condensers (1) between anode of valve E2 and grid of valve El, (2) similarly with E4 3, and (3) across the output potentiometer from which negative feedback is derived: the grid of the final stage E4 is also connected to a point in this potentiometer. The Specification also describes two applications of the arrangement to voltage or current measurement. The first, Fig. 9 (not shown) is derived from that of Fig. 2, the valve E1 being replaced by a hexode or heptode with the signal applied to the outer grid, and the inner grid and cathode being connected in known manner so as to avoid drift due to heater voltage variation. The amplifier is terminated in a cathode-follower from which negative feedback is derived, the measuring instrument being connected between the cathode of this valve and that of an undriven balancing valve. The second application, Fig. 10 (not shown) is based on the arrangements of Figs. 7, 8, and in this the amplifier ends with a stage comprising a pair of series-connected valves, the measuring instrument being in the cathode circuit of the lower, driven valve. The Specification also describes an application of the arrangement to a voltage regulating circuit (see Group XXXV).