Page 124 - Complete Wireless Design

Amplifier Design

Amplifier Design 123

Figure 3.20 Terminating low frequencies into 50 ohms to prevent instability.

cies, and with a high enough feedback path somewhere on the board, oscillation
can become a problem if the layout is poor. In addition, the higher the gain of an
amplifier stage, the more likely oscillations will begin to break out; 25 dB of gain
is considered the maximum for stability from a single stage.
Neutralization (degenerative feedback), as mentioned above, is sometimes
used to stabilize a potentially unstable amplifier. Nonetheless, the amplifier
neutralization procedure will be successful only if the positive feedback path
that created instability and oscillations is internal to the transistor, and not if
poor layout and/or lack of input/output shielding creates the return path.
Neutralization is also problematic with wideband transistor amplifiers
because of the variations in input and output capacitance of a bipolar transis-
tor with changes in frequency and bias currents, as well as the neutralization
retuning requirements for transistors in different production lots.
Another viable technique for creating a stable amplifier is to simply reduce
the gain of the stage. This works because an amplifier, as mentioned above,
must reach the Barkhausen criterion to oscillate (just as an oscillator must).
This means that reducing the feedback and/or the gain will stabilize an ampli-
fier. Unfortunately, reducing stage gain appreciably is often an unacceptable
solution, both from an economic and an efficiency standpoint.

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