| 摘要 |
An improved variable gain circuit is described having an input for an audio signal and an output for an audio signal. An operational amplifier having inverting and non-inverting inputs and an output has its input connected to the audio input. The first differential cell having inverting and non-inverting inputs, output, and a first control current terminal, has one input connected to the output of the operational amplifier and the other input connected to a reference potential. The output of the first differential cell is connected to one of the inputs of the operational amplifier in negative feedback relation and a source of controlling current is connected to the control current terminal. The second differential cell is also provided having inverting and non-inverting inputs, an output, and a second control current terminal. One input of the second differential cell is connected to the output of the operational amplifier and the other of the inputs is connected to the source of reference potential. The second control current terminal is connected to a second source of controlling current and the output of the second differential cell provides a current having a gain which is substantially the ratio of the second to the first controlling current.
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