| 摘要 |
PROBLEM TO BE SOLVED: To provide an AC amplifier circuit of relatively simple constitution without the need of high resistance. SOLUTION: An emitter-follower circuit 10 is constituted of a constant current source 3-1 and a transistor Q1. A comparator circuit 30 is constituted of a constant current source 3-2 and two transistors Q2 and Q3, compares the base voltage of the transistor Q1 with a reference voltage V<SB>ref</SB>and changes the base current of the transistor Q2 corresponding to the size. A base current compensation circuit 20 is provided with a constant current source 3-3 having the current value of the sum of the current value of the constant current source 3-1 and the current value which is the half of the current value of the constant current source, a transistor Q4, and a current mirror circuit 40 making the same current as the base current of the transistor Q4 flow into the connection point of the bases of the transistors Q1 and Q2. An operation is performed so as to make the base voltage of the transistors Q1 and Q2 be equal to the reference voltage V<SB>ref</SB>, and the input impedance of the base of the transistors Q1 and Q2 is increased by appropriately setting the emitter ground current amplification factor of the transistor Q2 and the current value of the constant current source 3-2. COPYRIGHT: (C)2003,JPO
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