Patent GB-859986-A (GB)
Title: Improvements in or relating to voltage amplifying devices
IPC: H03F1/50
Filed: 1957-04-16
Abstract: 859,986. Valve amplifying circuits. PHILIPS ELECTRICAL INDUSTRIES Ltd. April 16, 1957 [April 19, 1956], No. 12423/57. Class 40(6). In a direct-coupled amplifier in which drift is reduced by feeding back to said amplifier an amplified version of the drift component derived by alternately and periodically applying, by means of a switch, the input voltage of the amplifier and a suitably attenuated voltage derived from its output to an A.C. amplifier, a rectified output voltage of the A.C. amplifier being applied to the direct-coupled amplifier to stabilize the output thereof with respect to the input, the effect of variations in the input voltage at the input terminals of the A.C. amplifier are compensated for by applying a voltage derived from the output of the direct-coupled amplifier to a suitable input electrode of the A.C. amplifier in an opposing direction. The output from the directcoupled amplifier 1, Fig. I, is applied to a potential divider 14, 15 to provide at 6 a signal identical with the input signal apart from drift. This is applied to the movable contact of a continuously driven switch 3 whereby it is applied intermittently to the input of an A.C. amplifier 2 together with the input signal which is applied continuously from terminal 4. It may be shown that this results in the application to the amplifier 2 of an alternating current signal having a frequency equal to the switching frequency and an amplitude proportional to the drift component introduced by the amplifier 1. This signal is amplified by amplifier 2 and is then rectified synchronously by the switch 3 to derive an amplified direct-current signal corresponding to the drift component. This is smoothed by network 18, 19 and applied to the main amplifier 1 with such polarity as to reduce the drift introduced thereby by a factor depending upon the gain of the amplifier 2. To compensate for the input voltage appearing at the terminals of the amplifier 2 a further voltage from a network 11<SP>1</SP> 12<SP>1</SP>, 13<SP>1</SP> and corresponding in form with said voltage at the terminals of amplifier 2 is applied to at least one electrode of the input stage of the amplifier 2 to prevent amplification of said input signal by the amplifier 2. Fig. 3 shows the amplifier circuit in greater detail. The direct-coupled amplifier 1 comprises two cathode-coupled double-triode-stages 21, 27 followed by an output cathode follower 32. Negative feedback is provided over resistors 25, 26 to the stage 21, the resistor 25 being variable to vary the overall gain of the amplifier. The potential divider 14, 15 at the output of the amplifier also comprises a variable resistor whereby the division ratio is maintained equal to the overall gain. The divided voltage, of equal amplitude with the input voltage apart from drift, is applied through a network, which is identical with one at the input to A.C. amplifier 2, to a cathode follower 36 supplying the movable contact of the switch 3. The amplifier 2 comprises two R.C. coupled pentode stages 42, 54 followed by a triode output stage 60, the output of which is rectified by the switch 3 filtered by long-time-constant network 18, 19, further amplified by triode 65 and then applied by cathode follower 68 to one triode of the double triode stage 27 as a correction voltage. The networks 37, 38, 11, 12, 13 at the input to the amplifier 2 prevents the application thereto of signal components having frequencies close to the frequency of operation of the switch 3. The network at the output of the potential divider 14, 15 is made identical with the network 37, 38, 11, 12, 13 so that the forms of the signals passed by the two networks shall be precisely the same apart from the drift voltage present on one. In addition, alternating-current components of the input signal appearing at the control grid of pentode 42 are compensated for by an identical signal developed by cathode-follower 46 and applied to the cathode of pentode 42. The voltage between screen-grid and cathode in this valve is stabilized by a neon tube 50. Figs. 4 and 5 (not shown) illustrate alternative arrangements for the input stage of the amplifier 2 employing a transistor and a pair of cathodeconnected triodes respectively.