ES349407A1 - Field Effect Transistor Amplifier Circuits - Google Patents

Abstract

In an amplifier having a plurality of stages 10 1 , 10 2 ... 10 n connected in cascade, each stage consists of a common source connected insulated gate transistor 21, 14 which has a drain load impedance 20, 16 and the output 15 1 , 15 2 from each stage is connected to the gate of the succeeding transistor, the load impedance of at least one of the transistors is formed by the source-drain path of a further insulated gate field effect transistor 20 connected as a source follower and having the gate disposed to receive input signals and a feedback circuit 40 is connected between the output 15 n of the last stage 10 n and the gate 21g of the transistor 21 in the first stage 10 1 so as to provide a degenerativefeedback voltage. The feedback circuit may include an insulated gate field effect transistor 40 or this may be replaced by a resistor. Changes in the D.C. level at the output due to thermal effects, threshold voltage changes, supply voltage drift and component aging is fed back to the input 21g to provide degenerative feedback to compensate for these changes. The variations due to the A.C. signal, which may be at radio frequency, are filtered out by a capacitor 41. The load impedances of the 10 2 ... 10 n stages are shown as insulated gate field effect transistors 16 connected as diodes or alternatively these can each be replaced by a resistor. Several inputs can be connected to separate gate electrodes 20g, 16g 2 -(the earth connection to gates 16g 2 being removed), so that the output represents the combination of these inputs. The input signal can be disconnected from the first stage 10 1 in which case the gate 20g is connected to earth 12. Automatic gain control may be included in the circuit by connecting the source to drain path of an additional insulated gate field effect transistor between the gate and drain electrodes of one of the transistors 14. The AGC voltage signal is then supplied to the gate of the additional transistor. The input 17 1 to the amplifier may be fed from a source (70, Fig. 2, not shown) of slowly varying signal or D.C. signal via the conduction path of an enhancement type insulated gate field effect transistor (80) which conducts alternately with a further similar transistor (90). The input enhancement type transistors (80, 90) conduct alternately when their inputs increase above a certain threshold level and by connecting a detector circuit to the output of the amplifier, operating synchronously with the input transistors (80, 90) the output is converted back to D.C. The circuits may be constructed of individual components or may be formed as an integrated circuit. P or N type transistors may be used of the enhancement or depletion types. The input 19 may be a parallel tuned circuit of a transducer such as a microphone.