GB1234656A - - Google Patents

Abstract

1,234,656. Transistor amplifier. PHILIPS' GLOEILAMPENFABRIEKEN N.V. 8 Oct., 1968, No. 47661/68. Heading H3T. An IGFET having a first gate 6 receiving a signal to be amplified and a second gate 10 receiving a gain-controlling D.C. signal Vg 2 has a source resistor 18 whose value is sufficiently high (load line AB, Fig. 2, not shown) as to ensure that when the voltage (Vg 2s ) between the second gate 10 and the source 8 is changed ( + 4 v. through -1.5 v.) from maximum gain to second gate cut-off (A to B), the voltage between the first gate 6 and the source 8 (Vg 1s ) changes, in an opposite sense, by an amount (- 2 v. to + 3 v. ) which is at least as great as the difference between its value at maximum gain (- 2 v.) and its value at the first gate controlled cut-off point (C, - 5v.). The arrangement thus provides high amplfication (at A) for small input signals, but as the input signal magnitude increases the gain can be reduced and a linear operating range maintained, by moving the working point (along line AB) further from the non- linear part of the Vg 1s -I DS characteristic in the region of cut-off (C) so that the increasing input signal has an increased length of linear characteristic to work on. Modulation distortion and cross-modulation are reduced. Enhancement or deplation mode is possible, using P or N channel transistors. The input 1 is coupled to the gate 6 through a capacitor 2, and the gate is connected to a bias level provided by resistors 20, 21 through a coil 3 tuned by a capacitor 4. The drain is also tuned 11, 12 and the output is taken therefrom through a capacitor 14. It is said to be known to have a small resistor for R18.