A constant-current source comprising:
- a constant-current output circuit for supplying a constant current provided with one or more transistors having bases interconnected through a base line,
- a first circuit which provides a first current signal for setting a strength of the constant current to be delivered from the constant-current output circuit,
- a second circuit which generates a second current signal in response to a potential of the base line,
- a third circuit which operates to minimize any deviation of the second current signal from the first current signal, causing the potential of the base line to change, and
- a DC power supply for energizing at least the first, second and third circuits, wherein
- the first circuit comprises:
- a first resistance connected to a first electrode of the DC power supply at one end thereof,
- a first transistor of a first conductivity type having an emitter connected to the other end of the first resistance and a base connected to the base line,
- a constant voltage source having a second electrode connected to a second electrode of the DC power supply,
- a second transistor of a second conductivity type having an emitter connected to a first electrode of the constant voltage source and a collector connected to a collector of the first transistor through a branch point where a difference current corresponding to a deviation of the collector current of the second transistor from the collector current of the first transistor is branched,
- a regulation circuit which supplies a base current to the second transistor so as to minimize the deviation,
- a second resistance connected to the second electrode of the constant voltage source at one end thereof, and
- a third transistor of the second conductivity type having an emitter connected to the other end of the second resistance, a base connected to the base of the second transistor and a collector connected to the second circuit, wherein the second circuit comprises:
- a third resistance connected to the first electrode of the DC power supply at one end thereof, and
- a fourth transistor of the first conductivity type having an emitter connected to the other end of the third resistance, a base connected to the base of each transistor in the constant-current output circuit through the base line forming a current-mirror circuit and a collector connected to the collector of the third transistor through a branch point where a difference current corresponding to a deviation of the collector current of the fourth transistor, which is the second current signal, from the collector current of the third transistor, which is the first current signal, is branched to be supplied to the third circuit, the first resistance being determined so that a ratio of the first resistance to the third resistance equals a reciprocal of a ratio of the collector current of the first transistor to the collector current of the third transistor, and the first, second, third and fourth transistors having such transconductances that the ratio of the transconductance of the fourth transistor to that of the first transistor is equal to the ratio of the transconductance of the third transistor to that of the second transistor.