Page 208 - Complete Wireless Design

Amplifier Design

Amplifier Design 207

The actual type of coupling in a discrete circuit, sometimes combined with
impedance matching, will depend on the sort of signal being amplified. DC,
low-frequency AC, high-frequency AC, and wideband amplification, will all
have specific requirements.

3.9.2 Design of decoupling/coupling circuits
Decoupling. RF must not enter an amplifier’s power supply, and the power
supply’s voltage to the amplifier must not influence any circuits that are before
or after the amplifier. Both occurrences would have a negative effect on sys-
tem operation, as any alternating current into the amplifier’s DC bias supply
can cause circuit instabilities and noise throughout a system; while passing
DC beyond the immediate amplifier stage area would affect the bias of any fol-
lowing amplifier—or be sent to ground as a short. In performing as coupling
and decoupling elements, capacitors and inductors prevent any of the above
from occurring.
However, in order to function as desired, coupling and decoupling capacitors
must not be near their parallel (high-impedance) resonant mode, while decou-
pling inductors must not be close to any series (low-impedance) resonant
modes—nor should the inductors be above their maximum frequency of oper-
ation, in which case they would start to become capacitive.
Inductors are far from perfect components, and possess parasitic capaci-
tances. So when an amplifier must be able to function properly across a wide
band of frequencies, two RFCs (Fig. 3.97) will normally be required: a low-
inductance coil that works at very high frequencies without encountering any
series resonances, and a high-inductance coil used to block the lower frequen-
cies. This is necessary because the low-frequency, high-impedance inductor will
begin to pass the higher frequencies of the passband through the natural turn-
to-turn capacitance of any coil. An additional bypass capacitor to ground may
be placed between the larger coil and ground to further decouple any RF into
the amplifier’s supply. This type of decoupling will permit the amplifier’s wide
passband to sustain a nearly flat gain response over its entire frequency range.

Coupling. There are various coupling techniques that can be used between
stages, depending on frequency, cost, performance, and impedance-match-
ing needs.
Capacitor coupling (Fig. 3.111), also referred to as RC coupling, is found in AC
and RF amplifiers only, and is capable of amplifying over a very wide bandwidth
(the amplifier’s required impedance matching circuit will limit this bandwidth,
however). As shown in the figure, the series coupling capacitor C blocks the DC
C
bias to the next stage, but allows the RF signal to pass unattenuated. The cou-
pling capacitor and R form a voltage divider, allowing most of the RF signal to
6
be dropped across the high resistance of R located at the input to the next stage.
6
The voltage divider functions as described because the capacitor has a much low-
er impedance to the RF than does the resistor. This signal across R will then add
6
to or subtract from the second stage’s emitter-base junction, forcing its collector
current to vary through R , producing an amplified output voltage.
7
Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com)
Copyright © 2004 The McGraw-Hill Companies. All rights reserved.
Any use is subject to the Terms of Use as given at the website.