There is provided an AM receiver in which the bandwidth of an intermediate frequency amplifier is continuously varied in accordance with the received field strength. This AM receiver includes a combining circuit for combining a first AM detection output A from a narrow band intermediate frequency amplifier and a second AM detection output B from a wide band intermediate frequency amplifier to produce an AM signal output of A(1-a.sub.M)+a.sub.M B (where, 0.ltoreq.a.sub.M .ltoreq.1). By varying the coefficient a.sub.M, the bandwidth of the intermediate frequency amplifier can be equivalently continuously varied between the wide band and narrow band.

A radio receiver is disclosed having a wide filter and a narrow filter, each of which is connected with the front end and outputs an intermediate frequency signal. To the output of the filters are connected first and second detectors, respectively. A subtracter receives the outputs of the detectors and subtracts one from the other. A switch receives the output of the subtracter and directs the outputs of detectors in such a way that the switch is switched to a first position for directing the demodulated output from the first detector to a following circuit when the difference is below a reference level, and to a second position for directing the demodulated output from the second detector to the following circuit when the difference is above the reference level.

The filter has a fixed Q and is coupled to a source of AC signals. A source of pulses is applied to a variable-voltage circuit, the circuit being responsive to each pulse to produce a threshold voltage which varies from a first value toward a second value. A comparator circuit has one input coupled to the filter and a second input coupled to the variable voltage circuit. The comparator produces a signal for a duration related to the time the amplitude of an AC signal from the filter is at least equal to the threshold voltage.

A receptive condition automatic selection device for an FM receiver comprises a band width changing circuit for changing a band width of an intermediate-frequency amplifier of the receiver, a first detector for detecting a noise level contained in a demodulation signal from a demodulator, a second detector for detecting a tuning condition of the receiver by the demodulation signal and a bistable circuit connected to said first and second detectors for producing a control signal for the band width changing circuit. The band width of the intermediate-frequency amplifier can be automatically changed to either a narrow band or a wide band in accordance with receptive conditions of the FM receiver.

A plurality of parallel branch paths are connected in an intermediate frequency or high frequency amplifying circuit of a radio receiver for the purpose of switching the bandwidth. Between an intermediate point of each branch path and the ground is connected output electrodes of a transistor, and the base electrode of the transistor is selectively connected to a source of control voltage via a transfer switch to render the selected transistor conductive.