Frequency-Tunable Capability - Linear CMOS RF Power Amplifiers for Wireless Applications - page 65

Fig. 5.2 Balanced amplifier
block diagram

over a 600 MHz bandwidth. GaAs and CMOS implementations have been recently
reported in [ 11 ] and [ 26 ], respectively. The main problem of balanced amplifiers is
the need of quarter wavelength couplers that are usually not practical in integrated
circuits at low gigahertz frequencies [ 61 ].

5.3.1.4 Drawbacks

The broadband techniques described in this section are not suitable for the design of
the frequency-tunable CMOS RF PA. The lossy-matching technique decreases the
power gain of the amplifier which is already low in CMOS designs. The distributed
amplifier requires high- Q inductors difficult to realize with on-chip CMOS coils.
Furthermore, the number of inductors required can be limiting in terms of silicon
area. The balanced amplifier requires quadrature couplers which, besides having
bandwidth limitations, require long transmission lines when operating at low giga-
hertz frequencies, thereby occupying a large Si area.

5.3.2 Narrowband Techniques

In narrowband amplifiers, impedance matching is normally achieved with impedance
matching networks composed of passive reactive elements. They achieve their best
performance at the operating frequency for which they are designed. Outside the
bandwidth for which the performance is optimum, the matching is poor and the
amplifier is out of specifications.
In all the narrowband techniques covered in this chapter, at least one element
of the matching network is tunable. The tunable element can be a capacitor or an
inductor. In this section, we classify the narrowband techniques according to the
way the value of this component is tuned:

• Varactors

- Barium-Strontium-Titanate (BST) Capacitors [ 1 , 38 , 49 , 55 ]
- MOS Capacitors [ 43 , 50 ]
- Varactor Diodes [ 23 , 36 ]