Push-Pull Output Transformers - Part III, The Final Countdown:

Discussion in 'Amateur Radio Amplifiers' started by KD2NCU, Sep 28, 2017.

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  1. WW1WW

    WW1WW Ham Member QRZ Page

    Of course the current into an inductor is equal to the current out of an inductor, it's just a piece of wire.
    So you were trying to understand the pros and cons of different output stages, where are we with that discussion?
    I know this much:
    1) you can use a structure with T2 feeding the DC and it works fine regardless of the output final transformation ratio 4:1, 9:1, 16:1, I built them all and tested them.

    2) You can replace T2 with 2 separate inductors and the performance is essentially the same the major difference is mechanical layout.

    3) You can feed the DC through the center tap of a conventionally wound output transformer without any T2 or individual inductors and the performance is the fine as long as the cross sectional area of the transformer cores are adequate to handle the total flux without excessive nonlinearity. I built this structure and the problem was by the time the cores were big enough to provide low distortion the physical windings had excessive leakage inductance which limited the high frequency performance.

    4) You can feed the DC through the center tap of a transmission line transformer, again without T2 or any inductors and get excellent performance as long as the proper core material with adequate cross section is used. This last structure gave me the best performance when both distortion and bandwidth are considered.

    There is no closed form solution for these output stages because, a) the high frequency performance of ferrite material is not fully specified and, b) the physical limitations of the output power transformer makes it's design optimization an iterative process.

  2. KD2NCU

    KD2NCU Ham Member QRZ Page

    Hello KG7. I see you came out from hiding under your rock to make another highly intelligent comment. KG7 would you like to pick up where K7 bailed out because he can't support junk science of electrons stored in magnetic fields? Please do. There's lot's more to enjoy. In addition to continuing that discussion with or without K7, I'm going to post some more analysis in a bit that you can try to refute technically if you have the knowledge and ability to do so, or you can just shriek "wrong" and then run and hide like you did a while ago. You gonna stick around for a while this time? Done any good reading since you ran away? Still believe a core can't saturate unless it's grounded? Can you cite any reference or use any analysis to prove that? Still believe all Ruthroff transformers are grounded? Still believe the bifilar coil is NOT AC grounded? Still believe it's not an autotransformer? Still believe it doesn't provide a 1:4 impedance transformation? C'mon, shriek "you're wrong" or insult me and then run away again. Or, grow a pair of ferrites and stick around. What's it gonna' be KG7? Hurl insults and run and hide again, or try to prove you actually know anything?
  3. WA4SIX

    WA4SIX Ham Member QRZ Page

    Here's the deal. I have started to do quite a bit of studying about this, but am a slow learner.
    I have gained some insight from this thread. I will look into it further.
    I am not a transformer guru. I do not know how to model circuits.
    What I would like to see is a full circuit modeled by you, with values. Do it with & without the "T2". Maybe we can all benefit from it.
    I remember being told something about honey, vinegar & flies.

    KD2NCU likes this.
  4. KD2NCU

    KD2NCU Ham Member QRZ Page

    I see WW1 is back now too. So WW1, K7JEM is saying that the current going into the center tap of the bifilar feed coil does not have to equal the sum of the two currents leaving the other ends of the two coils because electrons can be stored in magnetic fields. He also says the current going into the center tap of the coil will be flat DC, minimal or negligible ripple at 10 amps while the single on transistor current has only risen to a few milliamps. Do you subscribe to that? Because up above you seem to be saying no. But the other day, you sounded confused. Do you remember what you said? I do. Here's what you said in response to me asking where all those trons and amps are going when the current going into the circuit is supposedly 10 amps but the current leaving the circuit is only a few milliamps. Here's what you said, "You forgot V=L dI/dT. The collector voltage is a half sine but the supply voltage is pure DC. The energy is stored in the magnetic field of the inductor. Also in a real amplifier Q2 is not out of the picture. In the off state it presents between 200 and 5000pf of capacitance depending on the device." What were you really trying to say here? Do you think the current entering the center tap of the bifilar feed coil can be 10 amps of "Pure DC" as measured after all the bypass capacitors just as it enters the center tap while the single on transistor current has only risen to a few milliamps? If so, where did all those trons and amps go? There are no capacitors in the circuit. It's really a ridiculously simple question. Either you think the current into the center tap is "Pure DC" with minimal ripple while the collector current is very small and you can explain where the trons and amps are going or you cannot. Really think the off transistor leakage capacitance explains this 10 amp discrepancy? No, I don't think you believe this either. Can you explain this? What do you think Is below looks like?
    KG7, you still around. Care to try to explain this?
    As a reminder, here's the circuit.
  5. KD2NCU

    KD2NCU Ham Member QRZ Page

    I have provided several such analysis using accepted circuit analysis techniques and you and others just kept saying wrong, wrong on so many levels, etc. I don't think that anyone actually looked at any of the materials I provided.
    If you would like to tone this down and have a mature technical discussion I will resubmit the analysis and some new analysis as well and I'd be very happy to review it like mature adults using actual physics and circuit analysis. The guy you see being an A.H. on this thread, I'm not really that guy. I'm that guy in response to the wonderful reception I've received. I taught electrical engineering and electronics in the US Navy in addition to training operators of nuclear submarines, I've been an engineering manager for many years hiring and training young engineers and that's what I've enjoyed the most. I prefer to be THAT guy.
    I like your post above very much and would enjoy a rational mature discussion with you. I'm THAT guy. How would you like to proceed?
    PS, I had not seen this post yet when I posted my last snotty post to you. Sorry.
  6. KD2NCU

    KD2NCU Ham Member QRZ Page

    OK, KG7SWP, let’s try this. Since it’s the Bifilar Coil that seems to get us trying to rip each other’s lungs out, let’s start there and work our way up to the whole circuit a piece at a time.
    There are several ways to analyze it. I’ll start with the way that makes the most sense to me in terms of just explaining what is going on instead of getting all wrapped up in math and equations right away.
    Since I don’t know everyone’s background, and they probably vary over a large range, I’m going to be very thorough, not leave any steps out, not make any assumptions, etc. I’m sure a lot of people already know a lot of this analysis but since I don’t everyone’s background, well you get the picture. If it sounds like I’m talking down to anyone, I assure you that’s not the intent. If you catch me making unsubstantiated assumptions, call me on it and make me justify and validate anything that doesn’t make sense.
    Let’s start small and take a little bit at a time, then stop, ask each other questions to clarify, etc., and maybe we can resolve small disagreements and questions with tiny arguments and discussion instead of hatchet throwing contests.

    KG7SWP, since you’re the only one of us that’s recently demonstrated even a shred of sanity and human decency, (including me), you be the hall monitor and pull us back in line and smack us if we start throwing hatchets again. Or maybe serve us all a big dish of beef chow mein from Lee Ho Fook's or pina coladas from Trader Vic's?

    Start by Making a Simple Transmission Line Transformer-Type Common Mode Choke:
    Wind a closely spaced pair of wires around a core keeping the pair close together as shown in the diagram at left below. The pair of wires forms a parallel transmission line just like a flat 300 ohm TV line only its impedance is a lot lower than 300 ohms typically closer to 25 ohms. More on this later.
    Winding it around the core turns it into a common mode choke. As a common mode choke, it tends to block current components that are not equal and opposite and not block currents that are equal and opposite in the pair of wires.
    Use the right hand rule to show that as long as the currents in the two wires are equal and opposite, the flux produced by the two individual wires tends to cancel. There are other ways to make a common mode choke but the transmission line approach usually more closely approximates the ideal device at high frequencies and is generally broader band than some other design approaches.
    Circuit Symbols:
    The circuit symbol for all of these would be the symbols below.
    Since we can’t see the three dimensional view of the core and coil we can no longer use the right hand rule to tell whether flux from two currents is adding or cancelling. So we use phasing dots on the schematic representation in order to tell whether two currents are producing flux that adds or cancels.
    In the case of transformer action where the current or voltage in one winding is producing current or voltage in another winding, the phasing dots also allow us to determine the actual polarity of the current and voltage in the second winding.
    In the first diagram, the two currents produce flux that adds so these currents experience inductive reactance. These are common mode currents.
    In the second diagram, the two currents produce flux that tends to cancel. They do not experience any significant inductive reactance. These are differential mode currents.
    What happens to inductance and inductive reactance when two coils are coupled like this?:
    In a coil of wire, Faraday’s law says that the voltage across the coil will be proportional to the number of turns and the time rate of change of the flux through the coil. There should actually be a negative sign accounting for Lenz’s law which says that the induced voltage always opposes the change that caused the induced voltage. (No perpetual motion machines allowed.) We can ignore the negative sign for now, we just want to know how much inductive reactance will be present.
    Conclusion: When two coils are interacting like they are in a transformer, common mode choke, bifilar coil, baluns, etc., we can’t assume that there is necessarily ANY inductive reactance present at all. We have to analyze the circuit to determine whether the currents are experiencing any inductive reactance.
    I think, in part, we keep ending up at each other’s throats because some people are essentially assuming that since the bifilar coils look like inductors, they must be acting like inductors and presenting inductive reactance to the currents flowing. We can’t make this assumption. We have to analyze the currents through the coils to determine if there is any inductive reactance being presented.

    Add a Center Tap for DC Supply Connection to Complete the Bifilar Coil:
    •Add a jumper to create the center tap where the DC source will be connected. The common mode choke inside the dashed line will continue to attempt to force the AC currents in the close pair of wires to be equal and opposite and will try to snuff out any AC components that are not equal and opposite.
    •So if there are any significant AC current components in the pair of wires of the coil, they will be overwhelmingly equal and opposite. Being equal and opposite, their flux tends to cancel and the common mode choke lets them flow relatively unimpeded by inductive reactance. Equal and opposite AC currents in the two wires are not seeing any significant inductance or inductive reactance.
    Examine Common Mode and Differential Mode Currents:
    •We still have a common mode choke inside.
    •Common mode currents tend to get blocked.
    •Differential currents (equal and opposite) can flow with close to zero inductive reactance.
    How good is this model, how well do real coils match this description of the perfect coil?
    This depends entirely on how well the coil is designed and assembled.
    Larger numbers of turns increase the degree to which the device chokes components that are not equal and opposite.
    Improper and inconsistent spacing of the pair of wires degrades the performance of the device because they're making a lousy transmission line with inconsistent intrinsic impedance.
    The right choice of core material for the frequency range and power impacts the performance of the device.
    The gauge and spacing of the wires determine whether the transmission line impedance is 25 ohms or whatever and there will be an optimum impedance depending on the rest of the circuit.
    In short, the less perfectly the device is designed and assembled, the less perfectly it will match the theoretical performance.
    How much attenuation does a typical coil present to common mode currents and differential mode currents?
    I’ve seen numbers like 20 to 40 db attenuation of common mode currents, ie reductions of the common mode current down to 1% to 0.01% of what the common mode current would be if there were no common mode choke and insertion losses of tenths of a db. So they can be reasonably close to ideal performance. But I haven't dug very deeply. If anyone can add more information regarding this please so reference.

    So at this point, we have a common mode choke that we’ve added a center tap to. We’re going to connect our DC supply to the center tap and we’re going to connect the two remaining wires to the collectors/drains of the transistors.
    The basic common mode choke at the center of the device will continue to try to prevent current components that are not equal and opposite from getting into the coil, not doing much of anything to currents that are equal and opposite, so if any AC currents are present in the wires, they will be overwhelmingly equal and opposite.

    Let's take a breather, ask each other questions, and discuss.
    KG7SWP, please tightly control access to the hatchets and meat axes.
  7. KD2NCU

    KD2NCU Ham Member QRZ Page

    Hey WW1. Why are you throwing snotty little barbs like this at me? What did I ever do to you? And why do you need to fight K7's battles for him? And where ya been? You jumped into the fray the other day, made more meaningless comments, I asked you some technical questions, you disappeared. Explaining how this "Real" circuit works is actually quite trivial. Be glad to. So why would you think this crappy little circuit is difficult or complex to explain? Does this pathetic little circuit actually seem difficult or complex to YOU? It's not to me. Why are you posting crap like, "Gee, I wonder if KD2 can explain ..... " What are you, 12 years old? A more pertinent question is, do YOU actually know anything technical or do you just know how to run your mouth? Here's a challenge. First, you convince me that you actually know anything and would even understand my explanation of this pathetic little Class C amp by answering the questions I posed to you earlier when you jumped into the fray, about whether the center tap of a push pull can have 10 amps of "Pure DC" going into it while the collector or drain current is only a few milliamps, and if so, how you might account for the discrepancy in current, or maybe the bifilar coil center tap current really does mimic very closely the transistors' collector/drain currents and whether you really think leakage current through the off transistor even comes close to explaining the 10 amp current discrepancy in K7's theory, and if not, why you even brought it up, and what you meant by "You forgot V = L di/dt" and what you really intended to try to prove with that pathetic statement. Come on and lay it all out and explain what that circuit is doing with real analysis if you can. Can you? Or can you just throw out crap like, "Dude, you forgot V = L di/dt, dude." Don't be cute, clever, or coy like K7 was trying to be once it became clear he was in over his head and right before he ran away. Either you believe the current into the center tap is "Pure DC" or you don't. If you do, then either you can explain the 10 amp discrepancy using "real" physics and legitimate circuit analysis or you can't and instead you have to use junk science and junk physics and hand waving garbage to explain it. Either you're going to show me you know something or you're not. Either you're going to run and hide, or you're not. Now by the way, the circuit I provided was from a "Real" circuit, just as real as yours and K7's trivial little class C amp.
    You first convince ME that YOU know anything other than how to run your mouth and can analyze the push-pull circuit with legitimate physics and circuit analysis.
    If you can, then I'll be happy to explain this little class C amp for you. If you can't because you don't know physics and "Real" circuit analysis and have to use junk science like K7 to explain it, I'm not going to waste my time explaining a simple class C circuit to a junk science loud-mouth that wouldn't know the right answer anyway.
    Or maybe you'd be more comfortable making another snide comment and running away again. Does anyone actually get intimidated by or let you win an argument with that limp drivel of yours?
    Are you in or out?
    Last edited: Oct 12, 2017
  8. KD2NCU

    KD2NCU Ham Member QRZ Page

    Welcome back everyone. I see everyone looks eager and ready to learn. Awesome!
    Let’s do a little review.
    So what have we learned so far?

    Jimmy: We learned that the physics of the bifilar coil is such that it lets equal and opposite (differential mode) AC currents flow freely in the pair of transmission line wires as long as the currents in the two wires are equal and opposite and that if they are equal and opposite, those currents experience essentially zero inductive reactance and currents that are not equal and opposite are severely attenuated by inductive reactance.
    So what can we conclude in general about AC currents in the bifilar coil?
    Suzie: We can conclude that if there are any significant AC currents in the bifilar coil they will be equal and opposite and they will experience no significant inductive reactance and that any AC currents flowing that are not equal and opposite are probably so small as to be insignificant.
    Timmy: But wait, the windings of the coil look all squiggly like inductors, so they must be causing inductive reactance and making “Pure DC”. I read it on the internet!
    Suzie: Shut up Timmy.
    Timmy: You mean just because I see an inductor-looking thingie, I can’t assume it’s causing inductive reactance and turning stuff into Pure DC. I have to analyze the currents to determine what the device is doing? Oh yeah, now I remember, and especially when two coils are interacting I can’t assume anything. I have to analyze what they are doing, I can’t just assume. Sounds like work. Sounds like I have to think a little.
    Suzie: Grow up Timmy.
    What else have we learned?
    Bobby: We learned that there are a bunch of internet trolls lurking about that like to hurl 6th grade insults and use junk science to argue defenseless positions supporting junk science assumptions like electrons jumping out of coils and then jumping back in, and electrons “bunching up” and stuff and they get all mad and stuff when you ask them to support it with actual physics and established circuit analysis techniques and they usually hurl one more insult and run and hide at that point. Why do they do that?
    I don’t know, probably several reasons: Sometimes they don’t have enough training and education to argue some of this stuff. Once they argue a bad position they don’t have big enough ferrites to admit they were wrong so they act all huffy, say, “This is stupid”, and run and hide.
    Timmy: What are ferrites?
    Suzie: Something you’ll never have.
    Alex: Are they bad people because they don’t have the training and knowledge to argue some of this stuff?
    No, not at all. They are only bad people if they choose to argue and insult and shout “you’re wrong on so many levels” and refuse to accept “Real” physics and real circuit analysis from someone with more training and experience on a particular topic and continue to argue and shout junk science.
    Johnny: Why do they continue to do this?
    Because people apparently let them get away with it. Apparently some people actually let them win arguments using shouting, insults, and junk science.
    Amy: But how can we tell when they are starting to use junk science?
    When they start responses with stuff like “You’re wrong on so many levels”, “This guy probably can’t …” ie; refer to you in the third person with an insult, scream “you forgot V = L di/dt” as though that means or proves something, or bring up a series of red herrings and diversions, that’s when you can be pretty sure they don’t really know what they are talking about and can’t construct a logical argument so they have to resort to 6th grade antics.
    Bobby: But what should people do if they really don’t know a topic?
    Suzie: They should admit it, ask questions, look at and analyze the alternative viewpoint being put forth, at least consider the possibility that THEY might be wrong and be open minded to the possibility that there’s more going on than they understand rather than shouting, insulting, and digging in their heels.
    Well put Suzie.
    Timmy: But weren’t we taught to listen with our mouths and hear only what we want to hear?
    Apparently some do think that that’s a good way to learn.
    Suzie: Timmy, you are so lame. We were taught to listen with our ears and only open our mouths when we actually know what we’re talking about or have a good question on the topic, or have something constructive to say or add to the conversation. Geez, are you lame, Timmy.
    Timmy: Everyone was taught that?
    Suzie: Apparently not.
    Johnny: So are the trolls gone now?
    Oh no, they’re still out there lurking about, looking for opportunities to critique and shoot snotty little darts at the work of others without ever putting their own analysis out for others to critique. As a matter of fact, they are watching us closely right now hoping for an opportunity to scream, “See, you’re wrong!”. Maybe I’ll deliberately introduce an error somewhere to see if they’re smart enough to catch it.
    Timmy: Sounds cowardly of them.
    Suzie: Wow Timmy, nothing gets by you, does it!
    Jimmy: Ok, Ok, so we know there are trolls out there, shouldn’t we be scared and intimidated?
    No, confront them with real physics, real knowledge, and real logic and analysis and if you don’t fully understand a topic, research it. They hate that stuff.
    Suzie: Ok we get the big picture about the trolls, can we get back to a technical discussion about the bifilar coil now?
    You bet.

    Put the Lime in the Coconut:
    Let’s put the feed coil in our Push-Pull amp and analyze.

    •Let’s assume class AB with crossover effects eliminated by perfect base biasing and perfectly matched transistors and a perfectly designed and constructed coil.
    •For now, let’s assume the antenna and antenna feed are all pretty well matched which would indicate that the voltage and current at the primary are essentially in phase, ie; load seen at the primary of T3 is resistive.
    •Let’s assume Q1 is conducting and Q2 is off.

    •The collector/drain current will be essentially a half wave sine pulse, the positive 180 degrees of a sinewave, given that it is a class AB linear amp.
    •The load current will also be a half wave sine pulse because it is a linear amp looking into a resistive load.
    What else can we say at this point?

    •Suzie: If there are any large AC components of I1 and I2, they will be equal and opposite. Any components are not equal and opposite will be negligible because the action of the coil blocks them.
    What can we say about Is, the current coming from the DC source?
    •Suzie: Not much yet. The only thing we know is that it has to equal I1 + I2 at all times.
    •Timmy: How do we know that?
    •Suzie: Because Kirchhoff’s current law says the sum of the currents going into a node has to equal the sum of the currents leaving the node (conservation of charge).

    Let’s start to look at the currents in the circuit in some more detail.

    •What can we say about the load current IL and I2?
    We can say they are essentially equal because Q2 is off and the collector leakage current is probably negligible compared to the load current that is going to flow in the output transistor of a high power amp.
    •What else? We can say that I2 is a half wave sine pulse because I2 and IL are physically the same current and we know that IL is sinusoidal in a linear amp.
    •Timmy: Hold on now, how can we say that I2 is a sine pulse? I thought it was “Pure DC”?
    •Bobby: Why would you think that Timmy?
    •Timmy: (Shrieking now) Because it’s an inductor!
    •Suzie: Now remember what we said Timmy, just because it has round squiggly piggy-tail thingies doesn’t mean it’s actually an inductor and especially when it’s interacting with another coil.
    •Timmy: But the … (Suzie: Shut up Timmy!).
    •What can we say about the collector current of Q1? We can say that it has to equal I1 + I2 because of Kirchhoff’s current law and because I2 and IL are the same current.
    Anything else?
    •Timmy: Well, can’t we say that I1 is also a half sine pulse because the coil forces AC currents in the two wires to be equal and opposite and when they are, they experience essentially no inductive reactance and because the coil suppresses any components that are not equal and opposite? What do you guys think?
    •(Class sits in blank stunned silence. So quiet you can hear eyes blink. Suzie recovers first.)
    •Suzie: I think you’re a frickin’ genius, Timmy, that’s what I think!
    •Timmy: And I also just noticed that if I1 and I2 are both sine pulses that means that the currents in the coil are equal and opposite and will experience no inductive reactance!”
    •Suzie: Is it getting hot in here or is it just me?

    OK, class, good progress, that’s enough for today. Let’s summarize what we learned:
    •Timmy: Well, we analyzed the currents in the circuit and figured out that during Q1’s conduction cycle, the collector current is a half sine pulse, the load current in the final transformer has to be a half sine pulse, and by using Kirchhoff’s current law, and by understanding the properties of the bifilar feed coil and Kirchhoff’s current law, we proved that the currents in the bifilar feed coil also have to be half sine pulses and that they are equal and opposite so they experience no inductance.
    •Suzie: Oh god, I’m falling in love!
    •Billy: So are the trolls gone for good?
    •Oh no, they’ll be back. We know they are watching and there’s no way they can remain silent through all of this good stuff!
    •More next time class.
    Last edited: Oct 13, 2017
  9. KM1H

    KM1H Ham Member QRZ Page

    And here I initially thought this thread title would help me with a pair of BIG triodes:eek: So far 8 pages with no end in sight:rolleyes:
  10. WA1GFZ

    WA1GFZ Ham Member QRZ Page

    KD2NCU I follow you and have one comment. I agree a common mode choke wants to have balanced current in both windings. But, In the case of a push pull amplifier one device is conducting current through 1 winding while the other winding is sitting there with the load of the output transformer and output C of the device turned off. So one winding is acting like a primary and the other winding is acting like a secondary (1:1 Transmission line transformer). As we induce power to the second winding the loss monster is stealing some of the energy in the ferrite core and resistance of the windings. The load impedance of of the secondary winding does not match the source impedance of the primary so a field is generated in the core. This can become a source of RF feedback in an attempt to force better balance.
    Woody, I found feeding DC into the center tap of a transformer in a typical flux transformer requires a big core and long conductors because a big imbalance is created in the cores. It is more like two single ended amplifiers driven out of phase with a common secondary combining the two phases. This big size limits the high frequency performance. I found that the core can be a lot smaller if the transformer is balanced fed with DC so there is a smaller offset like in KD2NCU's T2. So I wind my transformers with a pair of 25 ohm coaxes (2 X 2 turns) 1:2 step up turns ratio and send the DC into the shields at the common point CT. The RF comes out of the center conductors AC coupled to block DC. It does need an output Balun. I run 300 watts of RF and feed DC using a 1 inch by 1 1/8 inch type 43 sleeve that runs stone cold 160 through 6 meters. Again there is some offset in the core just like KD2NCU's T2 that can be a source of RF feedback. A flux transformer center fed would require 4 of these cores and I bet won't work very well above 15 meters because I have tried it. Frank WA1GFZ

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