Wideband, variable-bandwidth distributed amplifier

A distributed integrated amplifier circuit, suitable for use at gigahertz frequencies, comprising:

• an input transmission line;
• an output transmission line;
• and
• a plurality of amplifier cells having a respective plurality of cell inputs distributed along said input transmission line and cell outputs distributed along said output transmission line, each amplifier cell comprising:
• a first amplifier stage, having an input coupled to a respective one of said cell inputs and producing a first amplified signal at a first stage output;
• a second amplifier stage arranged to receive said first amplified signal at a second stage input and having an input impedance which is variable in response to an bandwidth control signal input, said second amplifier stage also having an output coupled to a respective one of said cell outputs;
• and
• a reactive feedback pathway coupling a negative feedback signal from said first stage output to said input of said first amplifier stage;
• wherein the amplitude of said negative feedback signal varies in direct relation to said input impedance of said second amplifier stage thereby imparting to said cell a frequency response characteristic which varies in response to said bandwidth control signal input.