Measurement Results for the Frequency-Tunable CMOS RF Power Amplifie - Linear CMOS RF Power Amplifiers for Wireless Applications - page 118

Fig. 7.26 Suggested frequency-tunable RF PA with adjustable control circuit

Layout Parasitics Although we knew that layout is of fundamental impor-
tance in RF IC design, its consequences were devastating in our case because
slight variations on parasitic capacitances at the output matching network can
play havoc with the desired transformed impedance.
Coupled Inductors The design of coupled inductors is a difficult subject and
if the achieved coupling factor is different from the predicted value, the trans-
formed impedance can be very different from the desired value.

These two issues directly affect the transformed impedance and, hence, the per-
formance of the amplifier. Therefore, it is a hard task to design a frequency-tunable
PA based on coupled inductors and achieve a good performance in the first design.
Moreover, the only parameter that we had control to check the circuit per-
formance was the control circuit bias voltage. Although this voltage affects the
transconductance g m2 of the control circuit according to (6.27)onp.92, it would
have been safer to foresee another possibility of adjustment in case of malfunction-
ing. Our suggestion is to add to the control circuit parallel branches that can be
switched on and off, as indicated in Fig. 7.26 , thereby changing the current I ctrl .
Furthermore, the capacitances associated to these extra branches change the equiv-
alent impedance of node v y in Fig. 6.12 on p. 90. Changing the impedance of this
node alters the current ratio responsible for the final value of the tunable inductance.
With these considerations, a possible strategy is to make a first design in which
the integrated transformer is carefully simulated with an electromagnetic software—