Video: Termination Insensitive Ampifiers - Page 1
Forum section: electronics / rf-microwave

author: kk9jef
author_rank: Newbie
timestamp: August 13, 2017, 09:11:16 pm
content: My first ever YouTube video, covering the basic description and properties of termination insensitive RF amplifiers, of the type used in, for example, N2CQR'sDIGI-TIA Transceiveror N6QW'sZIA.Style is very much cribbed from W2AEW, W6KWF, and others. Hope this is helpful to some, but either way, I had a great time making it.

author: Howardlong
author_rank: Super Contributor
timestamp: August 13, 2017, 09:42:00 pm
content: Very good (for me at any rate). I wish I had your quick presentation style, I'm rubbish when it comes to putting over a concept quickly, my brian has trouble finding the right words and I innevitably end up with lots of umms and ahhs.Yours seem slightly higher brow than W2AEW's videos, whose are more of an intermediate level.As a matter of interest, how long did you spend putting that together?

author: kk9jef
author_rank: Newbie
timestamp: August 13, 2017, 10:10:28 pm
content: Thanks Howard! It was a lot of fun to put together.It took me about 5 hours from first sitting down with a a pencil and paper to having it edited together - that includes assembling the common-emitter board, but the actual TIA board itself I had previously made as part of another project.I've be noodling about this video for a couple weeks, though, just thinking it through in spare moments and on my commute. So I had a decent idea of the scope and flow of the video before I started in earnest.

author: rfeecs
author_rank: Frequent Contributor
timestamp: August 13, 2017, 10:34:24 pm
content: Very nice smooth presentation.Big thumbs up for listing references to the circuits you mention.Some nitpicks:At around 11:55 the impedance is partially dependent on the 15 ohm resistor that you have covered up, not the 150 ohm.I don't think I've actually ever heard the term "Termination Insensitive Amplifier", especially in this context. Termination insensitive mixer yes, termination insensitive filter yes, amplifier no. I would probably have expected this to be called "high reverse isolation amplifier".

author: retrolefty
author_rank: Super Contributor
timestamp: August 13, 2017, 10:37:57 pm
content: Very nice.Are there any modern IC amplifiers available that exhibit input and output termination insensitive properties?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 14, 2017, 12:22:36 am
content: Looks good, just one point:Buffersdoprovide useful gain. They do not provide voltage gain, but when the source and load impedances are unequal, voltage or current gain, alone, is not a useful metric; power gain is. After all, at RF, matching networks and transformers are fairly practical, so that power flow and impedance matching are the norm.Cheers!Tim

author: w2aew
author_rank: Super Contributor
timestamp: August 14, 2017, 02:09:18 am
content: Nice job! Comments made on the YouTube video page...

author: ConKbot
author_rank: Super Contributor
timestamp: August 14, 2017, 05:24:49 am
content: Quote from: rfeecs on August 13, 2017, 10:34:24 pm"Reverse isolation" measurements will be at the nominal expected impedance though. A S12 measurement for gain and S21 for reverse isolation. I.e. A high reverse isolation amplifier What you use in a receiver on a mixer input, if you were trying to minimize LO leakage to the antenna. It definitely is providing isolation, but I don't think "Reverse isolation" is the right term, especially since is providing a constant impedance forward and reverse.

author: KE5FX
author_rank: Super Contributor
timestamp: August 14, 2017, 06:29:17 am
content: Nice work on the vid, looking forward to more.That said, the normal nomenclature for this sort of thing really is "isolation amplifier." Yes, it's two ways of saying the same thing, but most of the literature will use the isolation amplifier terminology. An amplifier whose input Z is influenced by its load is by definition termination-sensitive, and also by definition has poor isolation.This is particularly likely to cause confusion when combined with your use of the "TIA" acronym. A TIA is a transimpedance amplifier, used to convert the output of a current source like a phototube directly to a voltage for further processing. Think of an opamp with negative feedback and something driving its virtual ground directly.

author: rfeecs
author_rank: Frequent Contributor
timestamp: August 14, 2017, 04:55:48 pm
content: Quote from: ConKbot on August 14, 2017, 05:24:49 amFirst, you have S12 and S21 mixed up. S21 is the gain, S12 is the reverse isolation.This amplifier would ideally be a "Unilateral amplifer:"https://en.wikipedia.org/wiki/Amplifier#Unilateral_or_bilateralQuoteIn terms of S-parameters, a unilateral amplifier is one with S12=0. There is no path from the output back to the input. So it has infinite reverse isolation.For the input to be sensitive to the output, changing the load will change the load reflection coefficient. If there is no path back to the input, this change cannot be seen on the input and the input impedance does not change.Similarly, if the source reflection coefficient changes. S22 is measured by injecting power into the output port and measuring the reflection. If there is no path in the reverse direction back to the input port, then the a change of source impedance cannot be seen on the output.It's all explained in detail in the classic app note from HP,"S-Parameter Techniques for Faster, More Accurate Network Design":http://www.hparchive.com/Application_Notes/HP-AN-95-1.pdfThe first equations on page 11 show that if S12 = 0, S11 and S22 do not change with load or source impedance.Here's another good old HP app note "S-parameter design":http://www.sss-mag.com/pdf/AN154.pdfSo a high reverse isolation amplifier is a "termination insensitive amplifier". (Of course the gain, output power, noise figure will all still be sensitive to termination. That's why I don't much like the name "termination insensitive".)

author: G0HZU
author_rank: Super Contributor
timestamp: August 14, 2017, 10:53:59 pm
content: Nicely presented....I have one main observation... having watched the whole thing and seen the W7ZOI amplifier I was a bit surprised at your (earlier) choice of common emitter amplifier at 7:50 in the video. This amplifier didn't seem to be designed for 50R input and output and it looked a bit out of place in this respect.I think it would have made more sense to have a common emitter stage that looked like the first stage of the W7ZOI amplifier but redesigned for 50R in and out. This would have fitted in nicer with your diode ring mixer and receiver example.I drew a quick and dirty example circuit of what I kind of expected to see at 7:50 in the video. My circuit below isn't a great design or great performer but it is a 50R in and out amplifier with about 15dB gain that I tried to base on the front end of the W7ZOI amplifier. I tried to make the minimal number of changes to components values so my changes haven't been optimised for device bias point vs device beta spread. But it is going to be OK for a demo. It has a similar spec for gain, but this amplifier has poor reverse isolation compared to the W7ZOI version that follows after it in your video. See the image below. Sorry for the quick and crappy drawing. I'm a bit poorly at the moment so can't do much on the PC at the minute.