| 摘要 |
PURPOSE:To obtain a stable gain variable amplifier with a simple linear circuit by using the 3rd differential amplifier to control the currents flowing to the 1st and 2nd differential amplifiers to which the input signal is supplied and at the same time feeding back the outputs of the 1st and 2nd differential amplifiers. CONSTITUTION:The current flowing to transistors TR44 and 46 is decided for the 3rd differential amplifier 42 by the output of a gain setting circuit 54. The 1st and 2nd differential amplifiers 2 and 4 are connected in parallel in terms of alternating current and amplify the signal supplied from an input terminal 18. This amplified signal is delivered to an output amplifier 68. In this case, the current flowing to both amplifiers 2 and 4 is decided by the amplifier 42 and accordingly the gain is decided. A feedback circuit 80 is connected to an output terminal 78, and the output is fed back to the terminal opposite to the input terminal of the amplifiers 2 and 4 respectively. Thus the operation is stabilized with a gain variable amplifier. Then the remaining sounds can be completely erased in a close-tight mode with an electronic volume. |
