| Amplifier
Operational "Sweet Spot" Page 4 |
| The newest and most advanced RF hybrid technology of today is the
Gallium Arsenide (GaAs) push-pull and power-doubled hybrids. The
power-doubled designs can operate at output levels with low distortions similar to
feedforward hybrids. They accomplish this level of performance and yet
consume less power than the silicon power-doubled hybrids that preceded them. |
| The noise figures of this new family of hybrids are lower than
their silicon brothers, on the order of 3 to 4 dB noise figure across their entire
frequency range. The second-order and third-order distortion levels are also
improved! The bottom line: Amplifiers
built with GaAs hybrid technology have a wider dynamic range
of input and output levels than any amplifier design that has
come before them. |
| Given this new level of performance, one should not lose sight of
the original premise of this article. Always exceed the noise figure of any
amplifier gain stage by 3 to 6 dB to minimize the noise contribution from that gain
device. Choose an output level from any amplifier gain stage that does not violate
that noise figure rule, yet still provides a suitable RF output level with acceptable
second-order and third-order distortion levels. |
| I contributed to the design and testing of a new
CATV trunk amplifier in 2003 / 2004. The amplifier had GaAs power-doubled RF
amplifier chips at the input gain stage and the output gain stages. It was designed
to operate at 32 dB trunk gain with 12 dB of RF output slope With GaAs RF
amplifiers, as the RF signal slope change is adjusted by 1 dB, there is a 1 dB change in
the CTB. Remember that older silicon amplifiers with a change of 1 dB of RF slope
only provided a 0.7 dB change in CTB. In basic terms, an RF amplifier with a FLAT RF
output signal signature will get better CTB performance as the RF slope is changed from
flat to 6, 8, 10, or even 12 dB difference between channel 2 (54-60 MHz) and channel 78
(near 550 MHz. With the 10 dB slope change from flat, the CTB for a silicon
amplifier would improve by only 7 dB. The GaAs amplifier would improve by 10 dB CTB
performance. Don't forget, if you put in TOO MUCH SLOPE, the lower channels can
violate rule number 1 at the input to the final RF output amplifier stage. |
| So, there you have it -
the secret to operating wideband RF amplifiers in "The Sweet Spot". |
| Click here for the Technical
Articles menu |