A narrow-band RC active bandpass filter, in which the center frequency may be changed by changing the resistance values of certain resistive elements, has the certain resistive elements switchably connectable into and out of the RC network so that the ratio of time during which the resistors are connected to the network to the time during which resistors are not connected to the network determines the effective resistance thereof. A programmable timing circuit has a plurality of outputs with each output supplying a different number of equal width pulses during a predetermined like time period whereby each output corresponds to a different ratio. The timing circuit actuates and deactuates the switchable connection from a selected output thus deriving the required effective resistance for a predetermined center frequency. The bandwidth of the passband is set by a different resistive element and remains substantially unchanged as the center frequency is changed by changing the effective resistance of the certain resistive elements.

A circuit for multiplying two variables wherein one variable is represented by the pulse width of the pulses of a variable pulse width, constant frequency rectangular waveform signal and the second variable is represented by the level of a variable amplitude signal. When the value of the first variable is zero, pulses of the rectangular waveform signal have a definite non-zero pulse width. An oscillator cicuit produces a reference rectangular waveform signal having a frequency equal to that of the first rectangular waveform signal. From these two rectangular waveform signals there is developed a third rectangular waveform signal having the same frequency but having the width of its pulses directly proportional to the value of the first variable so that the pulses are of zero width when the first variable is zero. A fourth rectangular waveform signal is developed from the third rectangular waveform signal and from said variable amplitude signal such that the pulse amplitude of said fourth rectangular waveform signal is proportional to the value of the second variable and the pulse width is directly proportional to the value of the first variable. The fourth rectangular waveform signal is passed through a low pass filter to produce a DC signal whose magnitude is directly proportional to the area of the output pulses of the fourth rectangular waveform signal and hence to the product of the two variables.

A variable transconductance method and circuit is disclosed along with a filter and an amplifier using the same. An output transconductance is set by selectively adding at least two transconductances which are set according to a select signal for variably selecting an output transconductance.

A bandpass filter suitable for use in modem circuitry and other applications which require the filter's center frequency to be switched is disclosed. The filter circuit has the characteristic that although the center frequency is changed the overall gain of the filter remains constant. This characteristic results from using a plurality of filter sections which each have low individual gain and providing compensating gain at the input of the filter. In addition, compensating gain elements are used in each filter section so that when the center frequency of the filter is changed, the gain of each filter stage remains constant.

A high-impedance programmable attenuator utilizing field-effect transistor (FET) switching in both the high-impedance and low-impedance sections. In the high-impedance section, hybrid passive attenuator elements are employed to provide attenuation of electrical signals, and both series and shunt forms of active switching are employed to align the appropriate signal paths. The low-impedance amplifier section includes a FET-switched resistive network.