Reference voltage generating circuit and constant voltage circuit

A constant voltage circuit for generating a predetermined constant voltage from an input voltage by using a predetermined reference voltage generated by a reference voltage generating circuit as a reference, wherein the reference voltage generating circuit includes:

• a depletion-type n-channel field-effect transistor serving as a first field-effect transistor having one node thereof coupled to a power supply voltage;
• a second field-effect transistor having one node thereof coupled to another node of the first field-effect transistor and having a highly-doped n-type gate;
• and
• a third field-effect transistor having one node thereof coupled to another node of the second field-effect transistor, another node thereof coupled to a ground voltage, and a highly-doped p-type gate,
• wherein a gate of the first field-effect transistor is coupled to a joint point between the first field-effect transistor and the second field-effect transistor, substrate gates of the first through third field-effect transistors coupled to the ground voltage, and the gate of each of the second and third field-effect transistors coupled to a joint point serving to provide the reference voltage between the second field-effect transistor and the third field-effect transistor, and wherein each of the second and third field-effect transistors is configured to have such a ratio of a channel width to a channel length that a characteristic indicating a relationship between a gate-source voltage and a drain current exhibit a positive temperature dependency for both of the second and third field-effect transistors or a negative temperature dependency for both of the second and third field-effect transistors,
• and wherein a channel of the second field-effect transistor having the highly-doped n-type gate has impurity density equal to an impurity density of a channel of the third field-effect transistor having the highly-doped p-type gate.