A second letter is added to the subscript of the symbol for an FET y parameter to indicate the configuration for which it applies: s for common source, II for common drain, and g {or common gate. The common-source y parameters warp those most frequently given in specifications. These are summarized in Table 9-4. The values of the y parameters for a field-effect transistor are antecedence a certain small capacitance that exists between each pair of FET t terminals. This capacitance is inherent in the structure of an get, because every such device contains conducting surfaces separated from each other by dielectric material, the fundamental ingredients of capacitance. For example, the gate and channel of a JFET are separated from each other by the depletion region of a reverse-biased PN junction, and the gate and channel of a MOSFET are separated by a layer of silicon dioxide. Thus, there is a small gate-to-source capacitance, C,everything fielded transistor. Similarly, there are gate-to-drain capacitance, Cg and drain-to source reap stance, Figure 9-30 shows a JFET circuit in which these capacitance s are identified by dashed lines, to emphasize his a capacitance not a capacitor. While the internal capacitance s are small in .value. on the order of a few picadors, they are the dominant component of FET impedance at high frequencies, where heir capacities repentance is small compared to large restrictive components. Therefore, the value of an FET y parameter is quoted at a specific frequency, often in terms of a real and an imaginary part that rec .resent the conductance and acceptance components. The most important FET 1’parameter is the forward transfer admittance, which can be expressed as
where g” is the real (conductance) component of Ylt and bit is the imaginary (sustenance) component is precisely the trans-conductance parameter, gm. that we have employed W tre,fl,?fitly:’t. previous discussions and examples, Thus. (Yft) = gft = gift Siemens -10) Stated another way, gm is the value of Yrs. The common-source short-circuit output admittance soberness expressed in terms of its real and imaginary components: yo. = gill + Siemens (9-71) The real part of yo., designated go” is the reciprocal of that we have previously referred to as the ac drain resistance. Thus, rJ = ohms (9-72) By neglecting the very large gate-to-source resistance of an FET, as we have done in the past, it can be shown that
where p is the real (conductance) component of and is the imaginary (sustenance) component. gels is precisely the trans conductance parameter, gm. that we have employed ‘f,Q gently’ preview.s discussions and examples, Thus = Siemens ~-7(j) Stated another way, is the value of Yrs. The common-source short-circuit output admittance also be expressed in terms of its real and imaginary components: Yos = gos Siemens (9-71) The real part of Dos. designated the reciprocal parliamentarian that we have previously referred to as the ac drain resistance. Thus = \Ig”,ohms (9-72) By neglecting the “cry large gate-to-source resistance of an FET, as we have done in the past, it can be shown that Figure 9…:j shows a typical set of manufacturer’s specifications lor the y parameters of an N-channel JPET. These show the variation. we would expect ‘n the imaginary part of each y parameter versus frequency: As frequency increases, the capacities acceptance increases, corresponding to the decrease ill capacities reluctance. Notice that all graphs are given for the conditions V’IS = 15 V and
VIi.~= 0 V. Recall that the value of gIll in an Nchannel JFET Increases with increasing
