Design of the Frequency-Tunable CMOS RF Power Amplifie - Linear CMOS RF Power Amplifiers for Wireless Applications - page 83

Fig. 6.5 Integrated CMOS
RF power amplifier schematic

to provide more flexibility for adjustment. The RF choke is also an off-chip com-
ponent. The RF PA was designed to operate in class A with a quiescent current of
87 mA and a supply voltage (VDD) of 2.5 V. Resistor R stab = 500 and capacitor

C stab = 1 pF implement a shunt feedback that guarantees unconditional stability at

all frequencies at the expense of reduced power gain. R stab was implemented with
a non-silicided polysilicon resistor and C stab with a MIM capacitor. Although the
minimum channel length for the CMOS technology used is 0.11 µm, 2.5 V devices
are limited to 0 . 24 µm. This was the value chosen for M 1 . It is obvious that choos-
ing a smaller length would result in a better RF performance, but it would require a
supply voltage of 1.2 V, which would limit the output power that could be attained
with the same transistor size.

6.4.2 Coupled Inductors

In the previous chapter, the coupled-inductor technique was introduced as our choice
to implement the tunable inductor for the output impedance matching network of the
frequency-tunable PA. This choice was motivated by the property of the π -matching
network for which keeping the capacitors fixed and varying only the inductor, the
desired transformation can be achieved for different frequencies. Moreover, the pos-
sibility of Q enhancement makes the technique attractive due to the low quality
factor of integrated inductors in CMOS technology that limits the attainable output
power.
Some considerations about the ratio between the RF and control currents must be
done before designing the coupled inductors. First, we recall some equations from
last chapter:

I ctrl
I RF =

α + i β = r( cos φ + isin φ), (6.13)

L eq = L( 1 + αk) = L( 1 + kr cos φ), (6.14)

R eq =− ωkβL + R Ls =− ωkLr sin φ + R Ls . (6.15)

The domains of α and β values can be determined by checking the corresponding
equivalent inductance and resistance. Figure 6.6 depicts these domains for different