1. Field of the Invention
The present invention relates to a constant current circuit.
2. Description of the Related Art
As a voltage source employed in a semiconductor integrated circuit or the like, there is known in general a voltage source including a bandgap circuit using a bandgap voltage of pn junction in a diode or transistor. For example, in FIGS. 1 to 4 in Japanese Patent Laid-Open Publication No. Hei8-339232, there is disclosed a reference voltage generation circuit (referred to as “a reference voltage circuit” in the Japanese Patent Laid-Open Publication No. Hei8-339232), in which a reference voltage is generated utilizing a difference between base-emitter voltages of a pair of transistors, a voltage between both ends of a resistor (hereinafter, referred to as “a voltage across a resistor”) having a positive temperature coefficient is offset by a forward drop voltage of the pn junction having a negative temperature coefficient, and a reference voltage without a temperature coefficient is output.
Here, there is shown in FIG. 6 a reference voltage generation circuit having a configuration similar to that shown in FIG. 3 of the Japanese Patent Laid-Open Publication No. Hei8-339232. In a reference voltage generation circuit 21a of FIG. 6, assuming that a voltage across a resistor R9 is VR9 and a forward drop voltage of a diode D1 is VD, an output voltage Vout is:
                    Vout        =                              VR            ⁢                                                  ⁢            9                    +          VD                                        =                                            (                              R                ⁢                                                                  ⁢                                  9                  /                  R                                ⁢                                                                  ⁢                5                            )                        ·                          (                              k                ·                                  T                  /                  q                                            )                        ·                          ln              ⁡                              (                N                )                                              +          VD                    and thus, the temperature coefficient can be made 0 by making a positive temperature coefficient (R9/R5)·(k·T/q)·ln(N) of VR9 equal to an absolute value of the negative temperature coefficient of VD.
In this way, a resistance value, a ratio between their respective emitter areas of transistors, and the like are set to offset the temperature coefficient in the bandgap circuit, so that a temperature compensated reference voltage can be output.
However, when the current source is required as a power supply of a semiconductor integrated circuit or the like, even if a current I5 flowing through the resistor R9 of the reference voltage generation circuit 21a in FIG. 6 is output as an output current, the temperature coefficient cannot be made 0. For example, as shown in FIG. 7, in a current supply circuit 2a with such a configuration that the current I5 in FIG. 6 is supplied to an outside load (not shown), an output current Iout is:Iout=(1/R5)·(k·T/q)·ln(N)and it has a positive temperature coefficient.
Thus, a constant current independent of temperature cannot be output.