 By using a noise diode, the output noise of the amplifier is measured using two input noise levels (the Excess Noise Ratio) and by measuring the output noise factor (referred to as Y) the noise figure of the amplifier can be determined without having to measure the amplifier gain.Any amplifier generates noise.In a radio receiver the first stage dominates the overall noise of the receiver and in most cases thermal, or Johnson noise, determines the overall noise performance of a receiver.