GB575639A - Improvements in or relating to thermionic valve amplifiers - Google Patents

Abstract

575,639. Valve amplifying circuits. HADFIELD, B. M. Dec. 14, 1943, No. 20985. [Class 40 (v)] In a thermionic valve amplifier circuit, the phase displacement between the input and output is maintained substantially zero over the working frequency range by means of negative feedback. An impedance is provided in the cathode circuit having a ratio of susceptance to admittance equal to the product of the ratio of susceptance to admittance of the anode circuit impedance multiplied by the ratio of the sum of the grid-cathode mutual conductance and cathode circuit conductance to the grid-cathode mutual conductance at low frequencies. In the basic valve circuit (Fig. 1), the frequency range over which a given amplification may be maintained without exceeding a given phase-shaft is limited by the anodecathode capacity of the valve. It is shown that the phase-shift may be greatly decreased without reduction of either the frequency-range or the amplification, by the addition to the cathode circuit of admittance Ac comprising elements similar in form to those comprising the anode load admittance Aa and bearing a numerical relationship to them such that the ratio Ac/Aa is equal to Gc/g/g, where Gc is the conductive term of the admittance of the cathode circuit and g the mutual conductance of the valve from the grid to the cathode. In Fig. 4, the anode circuit admittance may comprise inductance La shunted with capacity Ka, both in series with resistance Ra and the whole shunted with capacity Ca. Corresponding elements Lc, Kc, Rc, Cc comprise the cathode circuit admittance, corresponding elements 'being so proportioned that the required condition is attained. The arrangement of Fig. 3 (not shown) is similar, but omitting the shunt - condensers Ka, Kc, while in Fig. 2 (not shown) the arrangement is that obtained by omitting also the inductances La, Lc. The improvement in phase and amplitude characteristics which may be obtained with this last arrangement is illustrated by the curves of Fig. 6 (not shown). The conductive term of the cathode circuit admittance (Gc) may be made equal to the mutual conductance g from grid to cathode, or at least their ratio should lie within the range 3 to 0.33. In Fig. 5 (not shown), a three-stage amplifier incorporates the invention in each stage, overall negative feedback being applied from the last anode through a suitable impedance on to a resistance in the first cathode circuit.