High G temperature compensated current source

#The present invention relates to a current source which has a given ratio of an output current I to an input current i and comprises a first series combination which includes a first resistor connected in series with the main current path of a first transistor, so as to be passed through by the input current, and a second series combination which includes a second resistor connected in series with the main current path of a second transistor for producing the output current, the first and second transistors being arranged so as to form a first current mirror circuit.

#Current sources of this type are generally used having small ratios G (up to about 10) of output current to input current. For these uses the second transistor has an emitter surface G times greater than that of the first transistor (or it is constituted by G individual transistors which are identical to the first transistor and arranged in parallel) so as to obtain the same base/emitter voltage drop in the first and second transistors, and avoid variations of the ratio G as a function of temperature.

#For higher ratios G, for example, ranging to 100, such a solution leads to prohibitive dimensions for the second transistor, and in that case arrangements with an operational amplifier will be used. Such solutions are used, for example, by MATRA COMMUNICATION (French patent application 88 01645, dated Feb. 11, 1988, more particularly, FIG. 5), SGS-THOMSON (report of the TEA 7063 circuit--Telephone Speech and Peripherals Line Control) and MOTOROLA (Product preview of the TCA 3385 circuit--Telephone Ring Signal Converter).

#The invention will be better understood by reading the following description, given by way of non-limiting example, with reference to the appended drawings, in which:

#FIG. 1 shows a current source having a high ratio of the output current to the input current and using an operational amplifier,

#FIG. 2 shows a current source according to a preferred embodiment of the invention,

#As shown in FIG. 1, an operational amplifier AP includes a resistor R at its non-inverting input and a resistor R' at its output B (the emitter of a transistor T arranged as an emitter follower). The resistor R' is connected to the inverting input of the amplifier AP. An input current i is injected into the input A and passes through the resistor R. The amplifier AP maintains the voltages at A and B, equal so one has:

#G=I/I=R/R'

#The ratio G is defined with good accuracy, but on the other hand, an operational amplifier requires many components and poses problems of stability and frequency response.