|With so many choices from QRF, this question is asked quite
often. The answer is hinted at in the "Sweet Spot
Article", but is not evident to all who read that article. I will try to
expand on the choices in these pages.
|Since all amplifiers introduce distortions, choosing the right
amplifier for the job at hand is important to minimize the build-up of those
distortions. The first rule to remember is NEVER buy MORE gain than is necessary! And the
second rule is to choose the
correct output hybrid technology for the output level desired
in YOUR application.
|First rule: When you buy too
much gain, the input hybrid is usually where the extra gain exists. The input hybrid
is usually a push-pull device and therefore is limited in CTB performance. If you
wanted to buy a 45 dB gain amplifier, the input chip would probably be a 27 dB
push-pull hybrid. Even if the output hybrid were a 20dB GaAs power-doubled hybrid,
the distortions of the input chip could be the limiting factor of the total amplifier
performance. When you consider the rules from the sweet spot
article, overcoming the input hybrid noise figure by 3 to 6 dB would mean input levels of
+10 to +13 dBmV for the 27 dB gain push-pull hybrid. This would yield an output level from the INPUT hybrid of +37 to +40 dBmV!
Looking at the chart below, you can see the input device is already outside the desired CTB performance range for push-pull devices.
|Second rule: Match required output levels to the correct amplifier hybrid technology.
A "rule of thumb" was published in our full page ad in the April and June 1998
issues of Communications Technology. Here it is in simple terms:
|RF Output Level
|25 to 32 dBmV
||Silicon Push-Pull (PP)
|32 to 36 dBmV
||Silicon Power-Doubled (PD)/GaAs PP
|36 to 40 dBmV
|40 to 45 dBmV
||Feedforward/GaAs PD or QUAD GaAs PD
|This chart is a suggested general rule of thumb for broadband
signal amplification solutions. There are exceptions to the rule based on the number
of RF carriers to be amplified which will be addressed later in this text. The chart
and paragraph above are lifted from the "Sweet Spot" article with a modification
to the second and third category to include GaAs PD hybrids in the 32 to 40 dBmV
range. The reason these are added here is that the popular 20 dB gallium arsenide
(GaAs) PD hybrids that we use in our newer products have lower noise figures in the 4 to 5
dB range. With such low noise figures, these new chips have opened up the dynamic
range of amplifiers we have sold for years to new performance levels!
|There is a limit to the low-end range CTB
performance of GaAs PD hybrids. Since the chip designers have traded some CSO
performance for improved CTB performance, there is no "free lunch" in these new
Gallium Arsenide chips. That is why our line of QRBA forward isolation amplifiers
are still popular with push-pull silicon hybrids where the
output levels meet the chart limits shown above. So what is
the answer to which amplifier I should buy from QRF? Click HERE for the next page to find out!