a controllable attenuator has an input and an output , and comprises a first resistive element , a first capacitor connected in series between the input and the output , a first controllable shunting transistor connected between the output and a supply terminal via a second resistive element , and a controllable bypass transistor connected between the input and the output . the controllable attenuator may form part of a radio receiver circuit , the attenuator being positioned between a matching circuit and a low - noise amplifier .