| 摘要 |
A voltage controlled low pass filter comprising first and second circuits connected in series between a filter input and a filter output and a third circuit for adding a DC bias voltage to the filter input with AC signals to be filtered. The first circuit has a gain inversely related to the DC bias voltage and is operative to convert any signal applied to the filter input into a first circuit output signal which is a logarithmic function of the applied signal. The second circuit has a gain directly related to the DC bias voltage such that the overall gain of the first and second circuits is unity. The second circuit also has a bandwidth which is inversely related to the gain of the second circuit, and is operative to convert the first circuit output signal into a signal at the filter output which is an exponential function of the first circuit output signal. The filter has a cut off frequency which is dependent upon the DC bias voltage and the filter output signal is a replica of the signal applied to the filter input for frequencies below the cut-off frequency.
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