Noise of the JFET amplifier

By analyzing the different noise sources of the silicon JFET amplifier, engineering formulae for equivalent noise voltage referred to the input of the JFET amplifier are derived. Formulae are obtained for common signal source impedance and particularly for capacitance-type signal source impedance. If the parameters of the JFET, the signal source, and the load are known, these formulae can he used to estimate the overall noise of the JFET amplifier over a wide frequency band. The theoretical curve of the overall equivalent noise voltage spectral density at frequencies from 0.5 Hz to 50 kHz has good correlation to a measured experimental curve. The contribution of the different noise sources to the overall noise is shown. For modern low-noise JFET's, 1/f noise prevails over the channel thermal noise at frequencies f/spl les/100 Hz. When using high-capacitance-type signal source impedance, contribution of the biasing resistor thermal noise to the overall noise predominates at low frequencies f/spl ges/1 kHz and room temperature. At upper frequencies f/spl ges/1 kHz, overall noise is due mainly to the JFET's thermal channel noise.