Page 112 - Complete Wireless Design

Amplifier Design

Amplifier Design 111

10 VSWR 1 2
ML 10 log 1
VSWR 1

where ML mismatch loss, dB, and VSWR voltage standing wave ratio,
dimensionless units.
Amplifier efficiency is another meaningful specification in many applica-
tions. The efficiency of an amplifier is the percentage of the RF output power
compared to the RF and DC input power, and can easily be calculated by:

P
out
Eff 100
P P
IN DC
where Eff efficiency of the amplifier, %
P RF output power, W
OUT
P RF input power, W
IN
P power supplied to the amplifier by the DC bias, W
DC
As a useful aside: In amplifier design, the desirable specifications, such as a
high P1dB, low noise, high efficiency, good gain flatness, proper wideband
operation, high gain, and high return loss can frequently be in opposition with
each other because of real-life internal transistor design limitations.

3.1 Small-Signal Amplifiers
3.1.1 Introduction
Small-signal amplifiers are needed to increase the tiny signal levels found at
the input of a receiver into usable levels for the receiver’s detector, or into the
proper levels required of the final power amplifier of a transmitter. These
amplifiers are Class A or AB for linear operation, high sensitivity, and low dis-
tortion in digital, AM, and SSB systems.
A receiver’s first RF amplifier will be of the small-signal, high-gain type and
must not produce excessive noise, since any noise generated within this first
stage will be highly amplified by later stages, decreasing the SNR.
Because of the high operating frequencies, RF amplifiers may sometimes be
neutralized in order to counteract any possible positive feedback and its resul-
tant self-oscillations. However, designing with a transistor that has uncondi-
tional stability at the frequency and impedance of operation has now become
much more prevalent.
The voltage gain of the small signal amplifier can be calculated as V /V ,
OUT IN
and when two or more are cascaded, their voltage gain is multiplied. However,
the decibel is more frequently used, with these values simply added, or dB
dB, when stages are cascaded.
There are four vital considerations in any discrete RF amplifier design: the
choice of the active device, the input and output impedance-matching network,
the bias circuit, and the physical layout. Each of these will be discussed in detail.

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