Reduction of Noise in Wideband Distributed Amplifiers

A general solution is presented to a classical problem in wideband distributed amplifiers, a noise figure characteristic that increases at low frequencies to values much higher than the underlying transistor noise capabilities. Early studies showed that this is caused by the input line termination resistor, but this understanding has not been used to implement circuits with improved noise figure. We report that an effective solution is the recasting of the input artificial transmission line into a transformer filter that matches the more favorable high resistor values to the external impedance. Improvements of about 1 dB in noise figure at the lowest frequency of operation for a 2 to 18 GHz distributed amplifier are demonstrated.