Page 113 - Complete Wireless Design

Amplifier Design

112 Chapter Three

3.1.2 Amplifier design with S parameters
S parameters characterize any RF device’s behavior at different frequencies
and bias points. With the information that S parameters supply, the designer
can calculate a device’s gain, return loss, stability, reverse isolation, and its
input and output impedances. Knowing the transistor’s port impedances is
required so that the necessary matching network can easily be designed for a
proper impedance match from stage to stage. This matching is vital so that
maximum power is delivered to the load at a high return loss (minimum power
reflected back toward the source from the load).
Impedance matching of active devices is essential because not only will the
typical transistor not have a 50-ohm resistive Z and Z , but its reactances
IN OUT
will also vary over frequency. This means that for maximum power transfer
into the system’s impedance, which is normally 50 ohms, a matching network
must be used to match the active device to the system’s impedance, and some-
times over a wide band of frequencies. However, utilizing LC components is
the dominant matching technique, so the match will be perfect only over a
very narrow band of frequencies. There are, nevertheless, techniques for
impedance matching that work quite well over a very wide band of frequen-
cies, and these will be discussed. For further information, consult the influen-
tial work on practical amplifier design RF Circuit Design by Chris Bowick.
As mentioned above, a device that must be matched will normally not be at 50
ohms resistive and furthermore will be either inductive or capacitive. This com-
bination resistive and reactive elements in the active device’s Z and Z is
IN OUT
referred to as a complex impedance. So the matching network’s job is not only to
match the active device to the system’s impedance, but also to cancel the reactive
element to allow for a 50 j0 match (or 50 ohms resistive, with no capacitive or
inductive reactances). This is called conjugate matching, and supplies a perfect
impedance match. Nevertheless, in order to decrease the gain of a transistor at
various desired frequencies for gain flattening, or to purposely design an ampli-
fier with less gain within its bandpass, as well as for optimal noise figure (NF),
a perfect match may not be desired for certain amplifier applications.
It is usually advisable that all circuits, even discrete circuits in the middle
of an IF chain, have a Z and Z of 50 ohms. Although we could match at
IN OUT
any sensible impedance from discrete stage to discrete stage, it would make it
quite difficult to perform accurate interstage tests with the 50-ohm test gear
commonly available. Thus, after each stage is tested, a 50-ohm circuit can then
be confidently placed within the system for reliable cascaded operation.
When designing matching networks, we will take the S-parameter 2-port
representation of the transistor, ignoring any effect the DC biasing network
may have on these parameters in the final design. This is quite valid if only
small amounts of RF feedback are produced by high values of R (Fig. 3.15; the
f
feedback resistor) in an amplifier’s bias network. In this way, the S parame-
ters will be satisfactory for computing not only the matching networks, but
also for the software simulations of the circuit’s responses. However, if an
amplifier utilizes a low value resistor for R in order to employ heavy feedback,
f

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