Induced Grid Noise and Noise Factor

IT is easily shown that for any type of internal structure the power-frequency spectrum sg(ƒ) of the current fluctuations appearing at the control grid of a triode valve is related to the spectrum sa(ƒ) of the anode current fluctuations by where Yg(ƒ) is the increase, from the ‘cut off’ to the working conditions, of input admittance at the grid with the anode effectively earthed; gmis the high-frequency mutual conductance of the triode. In the case of the ‘induced’ (transit-time) grid noise, and for moderate angles, the relevant component of grid admittance is largely susceptive, being where Ctis the value of the ‘space-charge capacitance’ measured on the valve. Thus, setting The modifications to (2) introduced by distribution of electron velocities and trajectories are discussed elsewhere1, and direct experimental verification is also described.