Page 105 - Complete Wireless Design

Amplifier Design

104 Chapter Three

The Q of the L network, which must be low for broadband circuits to obtain
a wide bandwidth, and for power amplifiers to minimize high circulating cur-
rents and thus high losses, is not selectable, so its usage is limited in such
devices.
The T network (Fig. 3.8) is another quite popular impedance-matching
network; it can be designed to furnish almost any impedance-matching lev-
el between stages and a selectable and low Q for wideband and power ampli-
fier use.
Pi networks are chosen for the same reason as the T network, and are found
extensively in matching applications of all types. The pi network and its equiv-
alent circuit are displayed in Fig. 3.9. By altering the ratio between capacitors
C and C the output impedance of the load can be matched to the source, as
1 2
well as decreasing the harmonic output. And while the pi network is a low-
pass filter, it can have a small resonant peak and a high return loss at this
point, at a certain frequency where the loss of the circuit decreases below that
of its bandpass value (if the Q is high enough; Fig. 3.10).

Distortion and noise. Two unavoidable, but distinctly undesirable, parts of any
circuit are distortion and noise. Distortion can deform the carrier and its side-
bands at the transmitter or receiver, causing spectral regrowth and adjacent

Figure 3.8 The impedance-matching T network.

Figure 3.9 (a) The pi impedance-matching network and (b) its equivalent circuit.

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