Noise In Tubes and Transistors

Author: N.H. Crowhurst

Both tubes and transistors generate noise components. For a tube this can be measured and calculated as an effective resistance value connected between grid and cathode. Changing the amplified bandwidth will modify the noise1 voltage in exactly the same way as it does with an ordinary resistor. The value of resistance calculated from measurements shown serves to give the effective noise the tube will produce, however the circuit happens to be arranged.

In transistors, the calculations for noise source are not as simple as in tubes. Connection between emitter and base and between collector and base each are associated with a resistance value inside the transistor. Each of these effective resistances causes a noise component, in the way just discussed, although neither of them is a constant resistance value. The input and output resistance are dependent on each other and dependent on the temperature of the transistor at its critical junction point. This temperature, in turn, is dependent on the combined currents, which produce a small amount of heat. These fluctuations also cause noise, which is predominant at low frequencies.

Measuring a tube's equivalent noise resistance