• Understanding Amplifiers, At Least a Little.



    If you browse through this site, you'll find a number of articles that tell you that amplifiers cannot amplify that which is not there but that they can help you get your signal to your TV. I have been asked on occasion why a certain TV channel was lost after adding an amplifier when the station was intermittent before. Shouldn't the amplifier have amplified the poor signal into a good usable signal?

    When I went through school, I didn't love to push numbers at first, but somewhere around learning algebra, I started seeing connections and really started to like math, especially in physics class when these numbers actually told me something about how the physical world worked.

    So in electronics when I first learned about amplifiers, it was like a magic box that could make something weak into something strong. However, there were a few Gotchas associated with amplifiers.

    Gotcha 1: An amplifier can only amplify what is there. This is sort of obvious, but we get caught at times because we assume we have enough weak stuff of what we want so we assume we'll have more of it to use after we amplify it.

    Gotcha 2: An amplifier amplifies everything the same. (For our purposes we'll take this as true, for now.)

    Gotcha 3: An amplifier introduces noise. Yup, it does.

    Let me put Gotcha 1 in a different way: an amplifier cannot amplify what it does not receive. If the amplifier cannot receive the TV signal, it cannot amplify it.

    In one of my classes I learned the term GIGO, for "garbage in garbage out," meaning that if we put garbage into a device or an equation we'd get garbage out of the device or equation.

    Let's go back to our weak intermittent signal and think about this: if I have an intermittent weak signal at the bottom of my antenna and I amplify it in a "perfect amplifier" (A perfect theoretical amplifier, introduces no noise, amplifies everything, and amplifies everything equally.) Then after the amplifier there will be a much stronger intermittent signal.

    In this case, since the amplifier amplifies everything, it will amplify the noise just as well as it amplifies the signal; thus, if the signal is too weak to hear or view, the amplifier will amplify this and so we have a bigger stronger bunch of noise. If it gets only the noise, it will amplify only the noise (noise in noise out, garbage in garbage out). Let's say we can see the signal just barely above the noise. Since the amplifier amplifies everything equally, it amplifies the noise just as much as the signal and so we have the same signal-to-noise ratio. If that ratio wasn't enough to allow the radio or TV to use the signal before it was amplified, it won't be any better after amplification, just stronger.

    This is similar to being in a crowded lunch room where everyone is talking and you can't quite hear your friend across the table from you. So, you shout to ask her to talk louder and she does. But this just causes everyone around you to also talk louder for the same reason and so you still can't hear what she's saying. This is the same thing that happens to our TV, first without the amplifier and then with it.

    That covers Gotchas 1 and 2, now for Gotcha 3. This is the really BIG Gotcha of all amplifier Gotchas. It's one that most beginners don't want to consider or may not even know about.

    Any time we build an amplifier with electronic devices, we introduce some noise along with the signal we're amplifying. If the signal is not far enough above the noise without the amplifier, it will be even worse with the amplifier. Yes, that's right: an amplifier makes the signal-to-noise ratio worse. Not just sometimes but every time.

    I decided to write about this because I recently added an amplifier to our attic antennas and I now have a few examples with real data that show all of these gGotchas. I took measurements before I added the amplifier and again after I added it. The time it took was minimal, around 10 - 15 minutes, so I'm assuming that conditions did not change enough to make the comparison invalid.

    Let's look at some of the data to see which Gotcha it demonstrates.

    Gotcha 1:
    Here we can see a clear example of Gotcha 1: if there is a weak signal and the S/N is 0, then after amplifications the weak signal is stronger, by about 15 dB but the S/N is still 0, so we still have nothing to watch.
    Ch 8: 30.3dB, S/N: (No Data, Flashing), AMP: 39.3dB Flashing, No data


    Gotcha 2:
    Here we have a nice strong signal and after amplification we see an even stronger signal, but the S/N hasn't improved which shows that the amplifier clearly amplifies both the signal and the noise.
    Ch 9: 36.1db, S/N: 18.1, & BER: 0.00, AMP: 51.3db, S/N: 18.5, & BER:0.00


    Gotcha 3:
    Here's a signal that was intermittent to begin with, and then after the amplifier is put in place, we see that while the signal has improved, the S/N ratio is much worse. In fact, it's so bad the analyzer can't even calculate it. In other words, it can't see the signal for the amplified and added noise.
    Ch 41: 37.9dB, S/N: 18.1, & BER: 0.16, AMP: 51.5dB, searching (No S/N or BER)


    Gotcha 2 revisited:
    Remember I said that we would take Gotcha 2 for being true for now? Here's where we have to pay attention to amplifier specifications. There is a good example of this at solidsignal.com with the new low noise amplifiers.

    Here's the first lower noise amp at:
    Antennas Direct PA-18 Ulta Low Noise Antenna VHF UHF TV Antenna Pre-Amplifier
    This page shows an amplifier which "provides a gain of 15db and a 1.8 dB maximum noise figure." They did not tell us how the noise or gain varies due to frequency so we have to assume these things. In cases like these, it is always better to assume the worst, which would be a gain of about 12dB and a noise figure of 1.8dB for the channels of interest. We hope that this way we won't be disappointed when this amp is used.

    For the new lower noise amp, we can see frequency dependent variations for both gain and noise:
    Winegard LNA200 Boost XT 20dB Outdoor Digital TV Antenna Pre-Amplifier with Clear Circuit Technology (LNA-200)
    This page shows that both the gain and the noise introduced are different for the different sections of the TV band. The gain is "20 dB L-VHF, 15 dB H-VHF, 16.5 dB UHF" and the noise introduced is, "3 dB VHF, 1 dB UHF" so clearly this is a very low noise figure amp for UHF but it is what I'd call a more normal noise amp for the VHF channels.

    The simple rule is that it is always better to use a lower noise amp. On the other, hand it is not always better to have the highest gain possible. There are times when there can be too much gain. This would be the case when one is much closer to some stations than to others. Here one would have some good strong signals and then some much weaker stations. In this case, it might be better to pick a low noise, medium gain amp. Or, one might need the highest gain amp and then have to add an attenuator after the amplifier right at the input to the TV if it needs to be adjusted, depending on the station.


    All my blog articles are listed at: Karras' Corner
    or
    Karras' Corner Article Links on my KE3FL web site.

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