Amplifier
Operational "Sweet Spot" Page 1 |
| Almost every day, someone asks me: "What are the best
input and output levels for this amplifier?" That is a good question which can
be answered easily or with complex analysis. As always, the simple answer is the
best. So here goes: Always choose the correct amplifier
for the job at hand. That may not sound like the answer you expected, but it
is the right one. Now the details... |
| Rule 1. Input signal levels should
ALWAYS exceed the noise figure of the amplifier by 3 to 6 dB. This may sound
easy, but some folks are fooled. Here's why. For practical purposes, the input
gain device of a two-stage amplifier establishes the noise figure for the entire
amplifier. This rule can be broken by too much interstage loss
due to equalizers, pads, attenuators and response networks. And of course the
input noise figure of the amplifier is equal to the noise figure of the input gain stage
(hybrid, transistor, etc.) plus the loss of any input passive devices (equalizer, diplex
filter, test point, power chokes, etc.) that precede that gain stage. Don't forget
that the noise figure of this input gain stage may have a different dB rating at 50 MHz
from the value at 450/550/750 MHz. Be sure to check these specifications. If
the published data does not specifically say that there is a different noise figure at a
given frequency, it "should" be safe to "assume" the published value
is at the highest amplifier operating frequency. Typically the
noise figure for a 450 or 550 MHz hybrid amplifier chip is 1 dB better at 50 MHz than it
is at the top frequency. It could be as much as 1.5 to 2 dB for a 750 MHz
chip. |
| "Well now, with a noise figure like that, why can't I hit
the input hybrid harder and get a better carrier to noise ratio?" That brings
us to the other battle cable TV has waged since cable began: unwanted mixer
products, lines in the pictures, composite-triple-beats (CTB), etc. This is of
course, the result of over-driving any amplifier to the point where intermodulation
(unwanted mixing of signals) produces other undesired signals (CTB, etc.). It can
happen in the input stage of an amplifier if the input levels are too high, but is usually
most prevalent with the higher signal levels produced at the output stage of the RF
amplifier. |
| Rule 2. 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 |
Hybrid Technology |
| 25 to 32 dBmV |
Push-Pull (PP) |
| 32 to 36 dBmV |
Power-Doubled (PD) |
| 36 to 40 dBmV |
Quadra-Powered |
| 40 to 45 dBmV |
Feedforward/GaAs PD |
|
| This chart is a suggested general rule 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. < CLICK HERE FOR PAGE 2 > |