GB380377A - Improvements in or relating to amplifier circuit arrangements - Google Patents

Abstract

380,377. Valve amplifying circuits. MARCONI'S WIRELESS TELEGRAPH CO., Ltd., Marconi House, Strand, London. -(Assignees of Farnham, P. O. ; 104, Elcock Avenue, Boonton, New Jersey, U.S.A.) March 21, 1932, No. 8401. Convention date, March 19, 1931. [Class 40 (v).] In a tuned high-frequency amplifier employing one or more dynatron or negative-resistance valve stages, means are provided (1) for maintaining stability and substantial uniformity of gain over the tuning range, and (2) for maintaining stability when volume is controlled by regulating the bias on an input grid. Fig. 4 shows a two-stage amplifier employing screengrid valves V1, V2 as dynatrons, and with a tuned input circuit L, C and tuned intervalve and output circuits L2, C2. When the tuned circuits are adjusted over the tuning range, their impedance as viewed from the platecathode terminals of the valve is altered, and the gain (which depends on the ratio of the external impedance to the internal impedance) is also altered. To overcome this, the tuned circuit L2, C2 is connected to the plate through a small condenser Cm, and is shunted by a series combination of a coil L1, resistance R1 and large capacity C1. This has the effect of maintaining the external impedance substantially constant during tuning, and so effects the object (1). Self-oscillation is also prevented, as the external impedance cannot become greater than the internal impedance. The shunt circuit L1, R1, C1 may be replaced by a parallel combination of inductance and capacity in series with a resistance, Fig. 8 (not shown). In Fig. 7, the external impedance is kept constant, by linking the tuning condenser C2 to a variable resistance R in the tuned plate circuit ; whilst in another modification; Fig. 10 (not shown), the tuning condenser is linked to a variable self-biasing resistance in the control grid circuit. Referring to the second object of the invention, this is attained in the circuit of Fig. 4 by interposing resistances R in the plate-supply circuits. As volume is increased by reducing control grid bias Ec, the resistance R (owing to the negative characteristic of the plate circuit) adds to the plate voltage and so maintains stability. Where two stages are employed, a single resistance R may be employed, with a variable tap 10 connected to the supply source. Instead of employing the resistance R, the grid-bias control Ec may be linked mechanically to the plate-voltage control EB in such a way that EB increases as Ec decreases, Fig. 11 (not shown). Stability under volume control may also be effected by interposing a resistance in the high-voltage screengrid circuit, Fig. 9 (not shown). The Specification includes sets of characteristic curves of the valves, and other curves showing (a) substantial uniformity of gain over the tuning range, (b) that the circuit is more selective than the normal tuned amplifier, and (c) that selectivity increases at high gain ratios.