Amplifier Operational "Sweet Spot"       Page 3
Summing this all up, the amplifier on the previous page works fine in the range of +36 dBmV to +40 dBmV output signal levels with 110 NTSC analog television channels.  Below that output range, the C/N would suffer.  Above that range, the CTB would suffer.  To get more output, feedforward (FF) hybrid technology comes into the picture.  (Actually, feedforward saves your picture quality.)  The internal losses of the delay lines and directional couplers inside the feedforward hybrid provide the cancellation of unwanted distortions (CTB, etc.).  These same internal losses degrade the noise figure of the feedforward hybrid.  A 750 MHz feedforward chip has typical noise figure ratings of 9 dB at 50 MHz and 13 dB at 750 MHz.  The noise figure of the output gain stage determines the MINIMUM RF output level.
For the 750 MHz feedforward hybrid, the numbers add up like this:   Noise figure = 13 dB.  Add 3 dB minimum signal level above noise figure = +16 dBmV input at 750 MHz.  Now add the 24 dB gain of the hybrid to reach a minimum output level of +40 dBmV.  With the input signal level at 5 dB above the noise figure, the output would be +42 dBmV for the "sweet spot" operational levels.   It's that easy!
The last thing to consider (in case you did not notice above) is the noise figure difference at 50 MHz versus the top frequency of the feedforward hybrid.   Let's say that the QRAM750-30F headend amplifier is installed in a master headend feeding broadcast signals to a large array of laser transmitters in the next room from the modulators and satellite receivers.   This layout would have a length of cable from 50 to 100 feet in length followed by splitters and couplers to feed the lasers.  Here is the way to improve the CTB performance: set the output slope of the QRAM driver amplifier with some slope (not flat) to reach the majority of lasers with flat signal levels.  The signal advantage here is for every 1 dB of slope change out of the QRAM, there is approximately 0.7 dB improvement in the CTB.  The thing to remember is that you should not exceed 4 dB of slope on the amplifier.  Why?  What was the noise figure of the feedforward hybrid at 50 MHz?  It was 4 dB better than the noise figure at 750 MHz!  If you exceed 4 dB slope output, the C/N will suffer for the low band channels.  The result is another amplifier "sweet spot" has been found.  Use it to your advantage!
Before we leave QRAM, there is one more thing that recently came up.   With ONE channel, the QRAM750-17Q can really put out a strong signal.  A single MHW7205C power doubled hybrid has its 1 dB compression point at +82 dBmV.  Do the math if you need to amplify just ONE channel:  output around +84 dBmV is possible.   Feedforward amplifiers cannot do this due to higher internal hybrid passive losses.
Now, the special case when the input of trunk module is not push-pull:   SA 550 MHz feedforward "FT" series amplifiers used a power-doubled input hybrid to drive a feedforward output hybrid.  The output chip has a noise figure of 9 and 11 dB at 50 and 550 MHz respectively.  Considering the 3 to 5 dB signal level over the noise figure rule:  The minimum output level for the low band from the amplifier would want to be 9 + 3 + 24 = 36 dBmV.  Channel 78 must be at least 2 dB higher.
And last of all, for those of you using a single hybrid line extender, it is probably a 34 dB gain chip.  The noise figure rule still applies.  Typical noise figures for 450 and 550 MHz 34 dB hybrids are about 5 to 6 dB.  Add 3 dB plus the gain of the hybrid and the minimum output level for these LEs is around 43 dBmV for the low band unless you are willing to sacrifice the C/N since this is one of the last two amplifiers in the system.  It's that simple.
When I first wrote this article, the Gallium Arsenide (GaAs) hybrid from Motorola was not in production.  Now that QRF is building amplifiers with this new hybrid technology, a page four of amplifier sweet spot is needed to complete the story!  Click here for page 4.