GB1444765A - Contrallable current source - Google Patents

Abstract

1444765 Function generators PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd 6 Sept 1973 [9 Sept 1972] 41923/73 Heading G4G [Also in Divisions G3 and H3- H5] In an arrangement constituting a logarithmically controllable constant current source, a pair of identical transistors 8, 11, Fig. 1, have like bias networks 9, 10 and 12, 13 respectively as emitter circuits and their bases are connected together and to the output of a differential amplifier 4. The collector of the transistor 11 is connected to the current output terminal 3 and via a resistor 14 to a source of potential + U S2 . The collector of transistor 8 is connected via a non-linear network 6 to the in-phase input of the amplifier 4, which is also connected via a resistor 5 to a source of potential +U S1 . The out-of-phase input of amplifier 4 is connected to a source of potential U C the value of which determines the output current I C . In alternative arrangements, P.N.P. transistors may be used (Fig. 2, not shown) or the bases of the transistors may be earthed, their emitters being connected via equal resistors to the amplifier output ; in this case the feedback network is connected to the out-of-phase input of the amplifier and the control potential is applied to the inphase input. The arrangement may be used in a gain-control circuit (see Divisions H3-H5) in which the feedback network comprises diode 41, Fig. 5, and a number of diodes 42-45 with progressive biases so as to obtain a logarithmic voltage/current characteristic.