Design of the Frequency-Tunable CMOS RF Power Amplifie - Linear CMOS RF Power Amplifiers for Wireless Applications - page 87

Fig. 6.8 Layout of the
interleaved coupled inductors
with patterned ground shield
and dummy structures
6.4.3 Control Circuit
As shown in the block diagram of Fig. 6.4 , the control circuit forces a current I ctrl in
the secondary winding (control winding) of the coupled inductors which depends on
the input RF signal. I ctrl is related to the current I RF flowing in the primary winding
(RF winding) of the coupled inductors because I RF itself depends on the input RF
signal. Figure 6.9 shows a possible implementation of the control circuit.
The circuit of Fig. 6.9 is designed using the Signal-Flow Graph (SFG) tech-
nique [ 13 ]. The SFG proves useful for representing circuit equations and deriving
its transfer functions, especially for circuits with multiple loops. Examples of SFGs
in circuits with transformers can be found in [ 5 , 10 ].
We also use the Driving-Point Impedance (DPI) concept [ 20 , 21 , 26 ]. Together
with the SFG technique, it provides a systematic procedure (known as the DPI/SFG
analysis [ 21 , Chap. 4]) for deriving the transfer functions of a circuit.
6.4.3.1 Analysis of the Control Circuit
The small-signal model of the control circuit of Fig. 6.9 is depicted in Fig. 6.10 .Two
transconductances are used to establish a relationship between I ctrl and I RF . We can
further simplify the analysis by replacing the coupled inductors by their two-port
network representation shown in Fig. 6.11 .