AN2450 datasheet(12/35 Pages) STMICROELECTRONICS

12 / 35 page

Voltage gain and input impedance

AN2450

12/35

DocID12784 Rev 6

Figure 4. Shrinking effect of

 value increase

Starting from Equation 16 on page 7 we can obtain the expression of the normalized input

impedance Zn of the resonant tank:

Equation 27

whose magnitude is plotted in Figure 5, at different Q values, with

 = 0.2.

The red and blue curves in the above mentioned figure represent the no-load and short-

circuit cases respectively, and are characterized by asymptotes at the two normalized

resonance frequencies fno and fnr (= 1). All the curves at different values of Q intercept at

normalized frequency fn.cross:

Equation 28

At frequencies higher than the crossing frequency fn.cross, the input impedance behaves

such that at increasing output current Iout (that is at increasing Pout and Q) it decreases

(coherently to the load resistance); the opposite happens at frequencies lower than fn.cross,

where the input impedance increases, while the output load resistance decreases.

Zn fn  Q





Zin fn  Q





-------------------------------

jfn

 jf

--------------------

1fn

–

jfn

----------------

fncross



----------------

AN2450 Datasheet(PDF) 12 Page - STMicroelectronics