Design of the Dynamic Supply CMOS RF Power Amplifie - Linear CMOS RF Power Amplifiers for Wireless Applications - page 29

Fig. 3.13 CMOS RF power amplifier schematic (integrated parts)

the evaluation board. Following the procedure explained above, the final value of
the transistor size was calculated to be W = 672 µm and L = 0 . 24 µm (minimum

length for 2.5 V devices), with I DC = 87 mA and for a maximum output power of

16 dBm (40 mW). The optimum resistance was calculated to be R opt = 18 .

3.3.2 Stability

The components R stab1 , R stab2 , and C stab in Fig. 3.13 are used to guarantee uncon-
ditional stability at all frequencies. For a circuit with only one active device, the
Rollett criteria [ 26 ] can be used to determine whether the power amplifier is stable
or not [ 8 , 18 ]. For this purpose, the PA is considered a 2-port network.
Rollett's fundamental condition is

2 2 2

1 −| S 11 | −| S 22 | +| |

K = > 1 , (3.20)

2 | S 12 S 21 |

where K is known as the Rollett stability factor and

= S 11 S 22 − S 12 S 21 . (3.21)

An auxiliary condition 2 must still be respected:

2 2 2 > 0 .

B 1 = 1 +| S 11 | −| S 22 | −| | (3.22)

In Agilent's simulator, Advanced Design System (ADS) [ 4 ], the conditions
( 3.20 ) and ( 3.22 ) can be verified through the functions called stab_fact() and
stab_meas() [ 1 ].
These conditions stems from the fact that for unconditional stability, the real part
of the impedance of port 1 and port 2 must be positive for any value of passive source
or load impedances. Mathematically, this means that | Γ IN | < 1 and | Γ OUT | < 1for

| Γ S | < 1 for a given frequency range. 3 all | Γ L | < 1 and If | Γ IN | > 1, the power of

2 Alternative auxiliary conditions can be found in [ 21 , 25 ].
3 Γ IN and Γ OUT are the input and output reflection coefficients.