Measurement Results for the Dynamic Supply CMOS RF Power Amplifie - Linear CMOS RF Power Amplifiers for Wireless Applications - page 43

Fig. 4.4 Spectrum analyzer screenshot showing the maximum switching frequency

Fig. 4.5 Envelope
processing function

An example of a function, shown graphically in Fig. 4.5 , that complies with these
requirements is

⎧
⎨

V s1 if G env V env + V knee <V s1 ,

V s (P in ) = G env V env + V knee if V s1 ≤ G env V env + V knee ≤ V s4 , (4.2)

⎩

V s4 if G env V env + V knee >V s4 ,

where G env is the envelope amplification gain, V env the input envelope amplitude,
and V s the dynamic supply voltage which is a function of the input power ( P in )—
with V s1 and V s4 being the inferior and superior limits as suggested in Fig. 3.1
on p. 18.
The circuit that implements the function described in ( 4.2 ) and illustrated in
Fig. 4.5 is shown in Fig. 4.6 . The main component values are given in Table 4.2 .
The commercial power detector LTC5508 [ 17 ] from Linear Technology is used at
the input for envelope detection. The detected envelope ( v env ) is then applied to the
input of the envelope amplifier. The gain, and the minimum and maximum dynamic
supply voltages can be adjusted. The core of this amplifier is the wideband opera-
tional amplifier OPA690 [ 5 ] from Burr-Brown. This circuit was implemented in a