GUIDELINES ON SELECTING HIGH POWER RF AMPLIFIERS (PAGE 1)
COPING WITH REFLECTED POWER
by Thomas Mullineaux
OPHIR RF, Inc.
Los Angeles, CA
ABSTRACT
System engineers face trade-off judgments when selecting high power amplifiers for
applications that involve high levels of reflected power. Making well-informed decisions on
this all-important topic is essential and this article explores the main issues and factors to be
considered.
INTRODUCTION
When selecting an amplifier for a particular application, one of the factors taken into
consideration is the ability to work into a poor match. Applications where amplifiers run into
perfect loads with no mismatch are rare. The vast majority of applications see reflected power
levels at about 10-30% of the forward power level. There will be occasions when 30-60%
reflected power occurs, and there will be rare occasions when 60-100% reflected power
occurs. For a variety of reasons, many amplifier manufacturers opt to design their RF
amplifiers to cope with about 60% reflected power.
CLASS A AMPLIFIER DESIGN OPTION
From the start of the design process, the option to design Class A amplifiers that can
withstand 100% reflected power is always open, but this invariably involves higher cost and
significant trade-offs in efficiency, weight and size. Also, the availability of older RF devices
capable of Class A operation is a factor, since no designer wishes to create a new design
using soon to be obsolete parts. More importantly though, there is the often unacceptable
consequence of forward signal distortion. High amounts of reflected power can change the
operating characteristics of an amplifier to the extent that significant forward signal distortion
occurs. So the benefit of continuing to operate into a poor match becomes very dubious. For
instance, when using an amplifier to test a component or subsystem for compliance to a
particular specification, what real purpose is served if the forward signal is so distorted that
the test results are invalid?
MICROWAVE AMPLIFIERS VERSUS RF AMPLIFIERS
Happily, this phenomenon does not apply to broadband microwave amplifiers. The
topology most commonly employed in microwave amplifier designs has the advantage that
reflected power is diverted away from the output transistors.
©2002 OPHIRRF, Inc.
5300 Beethoven Street, Los Angeles, CA 90066 TEL: (310)306-5556 FAX: (310)577-9887 WEB: www.ophirrf.com E-MAIL: [email protected]
GUIDELINES ON SELECTING HIGH POWER RF AMPLIFIERS (PAGE 2)
This makes microwave amplifiers inherently more immune to reflected power problems.
Unfortunately, the same design technique cannot be implemented with RF broadband
amplifiers. No reflected power diversion takes place and the output transistors ‘see’ all of the
reflected power. Distortion of the forward signal results when reflected power levels are high.
The plot below helps explain the problem. Look at the top trace first. This shows the gain
of the RF amplifier operating into a good match (little or no reflected power). Note that at low
input powers the gain is fairly constant and then, like all amplifiers, gain compression occurs
(the gain trace drops) as the power is increased.
The plot shows
actual swept
power curves for
a RF amplifier
operating into a
good match and
the same amplifier operating into
a poor match.
During a power
sweep test, the
frequency is fixed
and the input
power is swept
from a low value
to a higher value.
The output curve
shows the ratio of
the output power
of the amplifier to
the input power,
i.e. the gain of the
amplifier.
Now look at the lower trace. This shows the gain of the RF amplifier operating into the poor
match (63% of forward power reflected back). Note that although the basic shape is the
same as the upper trace, for identical power sweep conditions, the gain has dropped by 2dB.
It follows that for any input power level, the gain is lower than when the amplifier is operating
into a good match, and that the amplifier goes into compression at a lower power level. The
2 dB reduction means the power output has dropped by 37%. The consequence here is that
the RF amplifier is behaving as though it is a power amplifier of much lower power. So a
200W amplifier will behave like a 126W amplifier. Hence the high probability of forward signal
distortion. Clearly, higher reflected power levels such as 90% of the forward power level will
increase the probability of forward signal distortion considerably.
©2002 OPHIRRF, Inc.
5300 Beethoven Street, Los Angeles, CA 90066 TEL: (310)306-5556 FAX: (310)577-9887 WEB: www.ophirrf.com E-MAIL: [email protected]
GUIDELINES ON SELECTING HIGH POWER RF AMPLIFIERS (PAGE 3)
It is important to remember the distinction between microwave (e.g. 1-2GHz, 0.8-4.2GHz, etc.)
and RF amplifiers (80-1000MHz, 2-500MHz, etc.). With microwave amplifiers, the reflected power
is diverted, and deterioration in gain is not an issue.
CLASS AB RF AMPLIFIER OPTION
Owing to this deterioration in performance at high reflected power levels, most broadband RF
amplifier manufacturers opt for Class AB designs that cope with about 60% reflected power. This
makes for an amplifier that is less expensive, and gives the added benefits of smaller size, lower
weight and higher efficiency. To protect the amplifier output stages from damage from
exceptionally high reflected power (usually caused by accidental disconnection of the amplifier
load), the amplifier is designed to switch off when a certain reflected power threshold is exceeded. The switch-off threshold can be made user adjustable up to the maximum threshold limit.
For those special cases where switching off at high reflected power levels is unacceptable (such
as mission critical applications, or component tuning where bad match spots may be hit), more
elaborate techniques such as foldback can be employed. Generally foldback is avoided in
preference to simple threshold protection.
CONCLUSION
Common sense says ‘something has to give’ when large amounts of reflected power hit a RF amplifier’s output transistors. This is true even if the transistors are able to withstand 100% reflected
power.
So beware marketing statements like
…..(the) amplifier will continue to operate into high VSWR …….
These type of statements imply there is absolutely no consequence to operating into a poor
match, when in reality all that is actually being stated is
.... (the) amplifier will not be damaged when operated into high VSWR..…
a very big difference.
©2002 OPHIRRF, Inc.
5300 Beethoven Street, Los Angeles, CA 90066 TEL: (310)306-5556 FAX: (310)577-9887 WEB: www.ophirrf.com E-MAIL: [email protected]