| 摘要 |
PROBLEM TO BE SOLVED: To provide a gain variable circuit in differential form which is strong against saturation. SOLUTION: The gain variable circuit is constituted of: a differential amplifier circuit section A; a bias circuit section B; and a control circuit section C. The differential amplifier circuit section A is a two-stage amplifier circuit constituted of a differential amplifier circuit of the first stage using transistors Tr1, Tr2 and a differential amplifier circuit of the next stage using transistors Tr3, Tr4. When control voltage is not applied to an input terminal Vcont of the control circuit section C, the transistors Tr1, Tr2 are biased by the bias circuit section B to operate in an active area, and become states for performing usual amplification operations. When the control voltage is applied to the input terminal Vcont, a base current is increased compared with a normal amplification operation state, but since a collector current is fixed, a voltage between a collector and an emitter is lowered. As a result, the transistors Tr1, Tr2 operate in a saturation area, and gains are lowered. COPYRIGHT: (C)2011,JPO&INPIT
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