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. KD2NCU

    KD2NCU Ham Member QRZ Page

    I had set out to learn more about the pros and cons of different push-pull output configurations. Some of the “conventional wisdom” I was finding regarding the purpose and operation of the so called “Bifilar Balanced Feed Choke” in Push-Pull amplifiers just wasn’t making sense to me and was not standing up to technical analysis. I’ve continued to search for a well-documented technical analysis of this configuration and what it does and does not do. It turns out that the bifilar DC feed coil T1 below used in push pull output circuits while often called a “Balanced DC Feed Choke” or “Bifilar DC Feed Choke” or “Common Mode Choke” is very well known and documented as a Ruthroff 4:1 Voltage Transformer. Most of you probably know the Ruthroff 4:1 voltage transformer better as a Ruthroff 4:1 Voltage Balun often used to interface unbalanced 50 ohm lines to balanced 200 ohm lines, or 75 ohm coax to a 300 ohm antenna. If you read up on the construction and connection of the bifilar coil, the Ruthroff 4:1 Voltage Transformer, and the Ruthroff 4:1 Balun, you will find that they are all the same device.

    While it is called various kinds of chokes in various publications and there are offhand references in various publications that say it keeps the RF out of the DC supply I’ve been unable to find any technical reference that actually analyzes it using electromagnetic fundamentals to show how it provides any choking action or keeps RF out of the supply as typically configured in push-pull outputs. For example, I can find scads of rigorous analysis of transistors, transformers, baluns, tubes, Gunn diodes, etc., that explain in detail how they work and derive equations describing their operation. I haven’t been able to find any such rigorous derivation showing this device to provide any such choking action as used in the push pull configuration. However, I do find extensive documentation that analyzes this identical configuration as a Ruthroff 4:1 voltage transformer. Has anyone else come across an actual thorough and meaningful technical analysis or derivation of how this device functions as a choke in this configuration? Knowing what the device actually does and does not do seems essential to the correct design of the push-pull output impedance matching and DC supply low pass filtering.

    At this point it’s evident that calling it a choke is a misnomer and very misleading. It’s also clear that the word choke is overused and has become very imprecise. It’s now a meaningless garbage term because it has been applied at one time or another to just about anything with one or more turns of wire on it. In addition to the device being called a “choke” of one type or another, in identical configurations in the literature and patent citations, it's also called a bifilar transformer, an impedance transformer, a balun transformer, an output matching transformer, a reversing transformer, a 4:1 transformer, a transmission line transformer, etc. So we need to focus on understanding how it works in the circuit rather than shutting off our ability to think and presuming a certain behavior once we see the overused word “choke”. Be more curious and skeptical.

    Below are a collection of independent references, citations, excerpts, and information that demonstrate that the bifilar feed coil used in the push pull output and the Ruthroff 4:1 Voltage Transformer/Balun are constructed identically, are connected identically, and are one and the same thing and which very clearly analyze what the device does and does not do.
    This blog limits me to 10 attached or embedded objects so I have removed some diagrams to stay under the limit. If you go to the attached PDF file, you will see this article in its entirety including all images and diagrams.

    In some past threads I started on this topic, some have responded with deeply insightful statements like, “this has no basis in fact” [the previous thread cited plenty of facts and analysis that those responders were either unwilling, unable, or afraid to review and understand] and powerfully constructed arguments like “it’s called a choke in bunches of places so it has to be a choke and stop RF cuz that’s what chokes do” and penetrating logic like “Is not!” and “Does so!” but they were either unable or afraid to challenge the analysis I presented on its technical merits. There’s a lot of independent information below. Have some courage to review and digest it and then let’s have a meaningful technical discussion about it if you have or are aware of an alternate technical analysis that derives and supports a different conclusion but hopefully those with a different perspective can bring me something a little more analytical than “Does so!” and “Dude, it’s a choke, that’s what chokes do, bro.”

    The connections of the bifilar feed coil, T1 and the Ruthroff 4:1 Voltage Transformer/Balun are identical.

    The bifilar 4:1 Balun/Transformer is alternately interfacing the unbalanced or single ended outputs of the individual transistors to the balanced primary winding of the final transformer T2.
    [image removed]

    What is a Ruthroff 4:1 Voltage Transformer/Balun?
    It’s frequently configured as a transmission line type transformer that provides a 2:1 turns ratio and a 4:1 impedance transformation ratio. Among other applications, it is used to convert from balanced to unbalanced lines and to provide a 4:1 impedance transformation. It is constructed and connected EXACTLY like the “balanced feed choke” used in push-pull output circuits. Note that it can be depicted and analyzed as an autotransformer.
    Reference: A Simplified Analysis of the Broadband Transmission Line Transformer by Jerry Sevick. https://www.highfrequencyelectronics.com/Feb04/HFE0204_Sevick.pdf
    [image removed]

    Reference: Some Broad-Band Transformers, C. L. RUTHROFF, MEMBER, IRE, Bell Telephone Labs, Holmdel, N.J., 1959
    This is the original paper that created the Ruthroff 4:1 Voltage balun and is the basis for many types of broadband transformers and baluns including the “bifilar feed coil” used in push-pull amplifier outputs.


    How is the Bifilar DC Feed Coil Made?


    Reference: CRYSTAL SETS TO SIDEBAND, By Frank W. Harris, KØIYE
    This excerpt clearly indicates that the bifilar coil is a transformer, a tapped coil, and can either step up or step down the voltage. Note that it is made exactly like the 4:1 voltage balun above.
    “The power to the final is delivered by a small bifilar wound choke. Bifilar wound transformers were discussed in chapter 6. Wind about a dozen turns of a pair of #26 wires onto a small ferrite core. The two wires are wound on the core simultaneously as if they were one wire.
    Although the transformer is wound with two parallel wires, the two wires are soldered together to make one winding that orbits the toroid twice. Bifilar wound transformers are a kind of tapped coil. As a result, the impedance (voltage) can be stepped up or down by connecting the output to either all of the coil for high voltage, or just half of the coil to step down.”
    [image removed]

    The 4:1 Ruthroff Voltage Balun and the Balanced Feed Coil used in push-pull outputs are the same thing.

    Their construction, connection, circuit symbols, operation, and function are all identical.
    [image removed]

    The same coil can function as a balun, a push pull feed coil, a common mode choke, or an ordinary transformer depending on the external connections.
    [image of common mode choke and simple transformer removed]

    As connected in push-pull output circuits, the device functions as a 1 to 4 impedance transformer.
    In the diagram below, each transistor will see 1/4th of the impedance presented by T2’s primary winding as confirmed by numerous technical references.


    Reference: Balun Project, Tom Berger K1TRB, pg 2, http://personal.colby.edu/personal/t/trberger/pages/pdfs/ProjectBalun.pdf
    In this case the author literally connected the same physical coil first as a choke balun and then as a Ruthroff 4:1 transformer to prove a point. In one case the device functions as a current balun or common mode choke, in the other connection the same device functions as a Ruthroff 4:1 impedance transformer.
    [image removed]

    Reference: The Balun and The UNUN, pg 1, 6

    Shows 4:1 balun/transformer circuit, as bifilar transformer, and as autotransformer.

    Reference: RF Transformer Design
    http://www.foxcomputer.se/RF Transformer Design.pdf
    Analyzes 4:1 Ruthroff balun/transformer. Excerpt shown below.

    [image removed]

    Reference: Low Cost 1000 Watt 300 Volt RF Power Amplifier for 27.12MHz
    Advanced Power Technology Application Note APT9701
    This article shows a push-pull circuit with the typical bifilar DC feed coil and refers to it as a “Balanced Feed Choke”, (BFC1).
    “It [Balanced Feed Choke] also acts as a 4:1 impedance transformer which greatly simplifies matching the drain impedance to 50 ohms.”
    The device is an autotransformer when connected as the 4:1 balun or bifilar feed coil.

    Reference: “Transmission Line Transformers, Fourth Edition”
    Jerry Sevick, W2FMI, Noble Publishing, Atlanta, GA
    “Figure 2-1A can be recognized as the schematic of a 1:4 autotransformer. Although the analysis here is for the 1:4 unun, it can be shown that the 1:4 balun has the same result.”

    Reference: “4:1 VOLTAGE BALUN”
    Figure 1 Shows construction and shows that it is also an autotransformer.

    Numerous other references show the device as an autotransformer.

    The device is not a choke as configured in the push-pull outputs and will not keep RF out of the DC supply.
    First, the device is not wired correctly to perform as any kind of choke and is not a common mode choke. The currents that flow in the device are differential mode currents not common mode currents. So even if the device somehow performed as a common mode choke it would still not block RF from the power supply because common mode chokes only stop common mode currents, not differential mode currents as occur in the bifilar coil in push pull amps.
    Second, to perform as it does in all of the previous references as transformers and baluns, all three terminals including the center tap must conduct RF unimpeded, ie; with no choking action, otherwise, the device could not function as a transformer. In the push pull output the DC source is connected to the center tap and unless the device has as yet undiscovered properties, it can’t suddenly switch on inductance when it becomes “aware” that its center tap is connected to a DC source.

    Reference: Some thoughts on low cost, solid state, efficient, kilowatt-class HF linear amplifiers
    This is the article referenced by KG7SWP in an earlier thread that explains the operation of the DC feed coil. (Note that in the article that KG7SWP referenced, the author chose to represent the device as an autotransformer.)
    The author explains that on each half cycle of RF there will be an 8 amp current pulse coming out of the top of T1 and simultaneously an 8 amp pulse coming out of the bottom of T1. Therefore, there will be a 16 amp pulse going into the center tap from the DC supply. It’s that simple. The current coming from the supply is a series of 16 amp full wave rectified sine pulses. It is not a smooth constant DC current.

    “T2 is a relatively small ferrite toroid, on which a few bifilar turns are wound. This is connected as a center-tapped choke, with its two sides being very tightly coupled, thanks to the bifilar winding style.”
    Yeah, I know, he called it a choke in the article. However, when I contacted him, he explicitly stated that it will not keep RF out of the DC supply and agreed with my assessment that if connected to the DC supply without local capacitors, it will pull large full wave rectified sine pulses of current from the DC supply wreaking havoc upstream.
    Obviously, T2 is not keeping RF out of the DC supply, rather it is directly pulling 16 amp half sine pulses from the DC supply. The current waveform has a fundam ental frequency twice the RF frequency and is rich in harmonics. Unless there are capacitors to common or other low pass filtering between the transformer and the supply, this RF will propagate back up the supply line and will radiate from the supply line.
    Note that this author draws the bifilar coil T2 as an autotransformer.
    Note that the currents in T2 are always differential mode currents. So even if the device suddenly became self-aware and somehow turned itself into a true common mode choke but only when installed in push-pull circuits it still wouldn’t block RF from the DC supply because common mode chokes choke common mode currents, not differential mode currents.

    The difference between a choke and a transformer:
    The difference between the common mode choke and the bifilar feed coil or balun is simply how they are connected externally.
    The external connections determine whether the device acts as a choke or not.
    The external connections of the bifilar DC feed coil force the currents to be differential mode currents producing flux that cancels one another.
    [image removed]

    Other perspectives on whether or not the device is acting like a choke and blocking RF from the DC source.
    It’s been established that the same configuration is used as an antenna balun and as a 4:1 impedance transformer. In the balun or transformer configuration, RF must pass out the center tap as well as the other two connections, so obviously, these connections cannot act like a choke to RF. In push pull circuits the center tap is connected to the power supply. Since the center tap passes RF when used as a balun or transformer, it will also pass RF to the DC supply.
    Note that the coax center conductor in the diagrams below are connected to the center tap of the coil as is the power supply in a push-pull amp.


    Note that in the circuit diagrams below, the AC source is connected to the center tap of the device as is the power supply in push-push pull amplifiers.
    It’s been established that the device is an autotransformer. In push-pull circuits, the power supply is connected to the center-tap of the autotransformer. Obviously, the center tap of an autotransformer does not act like a choke or it could not be a transformer.
    [images removed]

    The following well documented statements with regard to the Ruthroff 4:1 Transformer also apply to the bifilar feed coil as used in push-pull output circuits as they are one and the same thing.
    As such, the following statements are true and verified in the literature and the references below and by an expert that I consulted.
    1. As typically configured in push-pull outputs, the device is acting as a 1:4 impedance transformer and is converting the unbalanced outputs of the transistors to a balanced output fed to the balanced primary of the final transformer.
    2. The device is providing an impedance transformation of 1:4. The transistors see 1/4th of the impedance presented by the final transformer primary.
    3. The device is an autotransformer.
    4. The device as configured in the push pull output is will not keep RF out of the DC supply. To the contrary, it more or less directly injects RF into the supply because the current it draws from the supply is large full wave rectified sinewave pulses of current, not a smooth DC current.
    5. It follows then, that the impedance transformation needs to be taken into account when designing the output stage and matching of a push pull amplifier.
    6. It follows then, that the inability of the device to prevent RF from propagating into the DC supply must be taken into account by providing separate low pass filtering, typically capacitors and inductors, between the center tap of the device and the DC supply in order to prevent the RF from propagating into the DC supply and causing undesirable feedback, interference, and oscillation.
    I contacted Manfred Mornhinweg, the author of the article that KG7SWP referenced that explains the operation of the bifilar coil, Some thoughts on low cost, solid state, efficient, kilowatt-class HF linear amplifiers. http://ludens.cl/Electron/mosfetamps/amps.html
    He explicitly agreed that it is a 1:4 impedance transformer in the push-pull configuration, that it is an autotransformer, and that it provides no isolation between the RF and the DC supply but, in fact, injects RF into the supply by drawing large unfiltered full wave rectified sinewave current pulses from the supply unless additional low pass filtering is provided between the coil’s center tap and the DC supply line.

    Excerpts from Patent US 5726603 A:
    “An input matching transformer T1 is a 4:1 impedance ratio step-down type with a 50 ohms input tap and a 12.5 ohms output over a range of 5 to 25 MHz.”
    “ … tied to an input of an output matching transformer T4. The latter transformer performs an impedance step-up function, matching 12.5 ohms to 50 ohms.”
    “A drain supply of 85 volts is fed via a reversing transformer T5 in which the magnetic flux is cancelled during each RF cycle. The net flux inside the transformer core is zero.” [And with no flux in the core, there can be no inductance or choking action.]

    Excerpts from Patent US 5420537 A
    “The drain electrodes of MOSFETs 114, 116, 118 and 120 are also connected through capacitor 142 to one coil of the balun transformer 144, the secondary coil of which provides the output signal to the output 146.”
    “Balun transformer 144 is preferably a 1:4 impedance transformer, the ratio being equal to the number of MOSFETs used. If the output of this amplifier is terminated by 50 ohms, the load resistance reflected at the drains of the MOSFETs will be 12.5 ohms.”
    [image removed]

    Excerpts from patent US 4042887 A:
    “In the embodiment of FIG. 4, … Transformer windings 70, 86, 88 and 90, along with resistor 92, … The transformer formed by windings 88 and 90 is a push-push bifilar transformer. If pins 3 and 3A of transformers T1 and T2 in FIG. 4 are at the same impedance of 2Z0 no voltage is dropped between these pins and transformer T3 formed by windings 88 and 90.
    Figure 13: “a two-way splitter can be constructed using a push-push bifilar transformer 84”
    [image removed]

    Google Image Searches:

    I searched Google images to see what kind of devices would show up with different terms.
    Search on Push-Pull DC feed choke: Got one hit, push pull circuit with the usual bifilar feed coil, author first calls it a 1:1 balun choke, then calls it a bias feed choke, then calls it a collector bias choke. http://amfone.net/Amforum/index.php?topic=34349.0
    Search on drain bias feed choke: Arnold Schwarzenegger, Donald Trump, Obama, and Bernie Sanders all show up but not the bifilar feed coil. (Gee, I wonder which terms triggered those guys to show up. :p)
    Search on Collector bias feed choke: Nothing related, not even Arnold or Donald show up this time. :(
    Search on balun choke: You get true common mode chokes, not the bifilar feed coil used in push pull amps.
    Search on Bias feed choke: Nothing
    Search for common mode choke:
    Not a single example that looks like the bifilar coil with the center tap.
    No matter how many times I asked Google to show more results it wouldn’t cough up even one example that resembled the construction of the bifilar drain/collector feed coil with the center tap.
    And I agree with Google. These are true common mode chokes and are not connected like the bifilar feed coil.
    [image removed]

    Search on push pull balanced feed choke operation and similar phrases:
    You get hits for baluns. When you follow the hits you get true feed choke baluns or standard feed chokes but you get very few hits anything close to the bifilar coil used in push pull amps and when you do, it’s called a 4:1 balun or a Ruthroff 4:1 transformer, etc.

    Do a Google search for balanced feed choke:
    I overwhelmingly get choke baluns (which are genuine common mode chokes), sleeve baluns, and true common mode chokes like the pictures below but they are not connected like nor do they have the center tap of the configuration of the push-pull bifilar drain/collector feed coil.
    [Image removed]
    If I do get the configuration of the push pull feed coil, it’s called a 4:1 balun or transformer, it’s not called a choke and the picture is that of a Ruthroff 4:1 transformer.
    [image removed]

    1. This article has cited and provided numerous technical references supporting the following conclusions.
    2. The bifilar coil depicted in many push pull amplifier output circuits is a Ruthroff 4:1 transformer.
    3. The configuration is analyzed extensively in the literature as a Ruthroff 4:1 transformer.
    4. The same configuration is also used as a 4:1 balun.
    5. The device provides a 4:1 impedance transformation and can convert an unbalanced line to balanced or vice-versa.
    6. In the push-pull configuration, the device is interfacing the unbalanced transistor outputs to the balanced primary of the final transformer and presenting the transistors with an impedance that is 1/4th of the impedance appearing at the final transformer primary.
    7. This impedance transformation must be taken into account when designing the matching circuitry and final transformer.
    8. The device can be depicted and analyzed as an autotransformer.
    9. The device as used in push-pull output circuits does not prevent RF from propagating into the DC supply.
    10. Local capacitors or other low pass filtering is needed between the coil and the DC supply in order to keep RF out of the supply.
    11. In the push-pull output configuration, calling this device a choke of any kind is inaccurate and misleading.
    12. If its windings were connected differently, the coil COULD function as a common mode choke, a 1:1 current balun, or even a common one to one isolation transformer.

    Attached Files:

    KI4LXB likes this.
  2. WA7PRC

    WA7PRC Ham Member QRZ Page

    Again??? o_O
  3. K7JEM

    K7JEM Ham Member QRZ Page

    The short answer is that your analysis is wrong. A DC source when analyzed in an AC circuit is always represented as a short circuit. It is not always that case in real life, so RF bypass capacitors are always located close to the feed choke. This essentially creates a short circuit path for any remaining AC current on the DC side of the choke. There is no "pulsing DC" going on in a properly designed amplifier. The bypass capacitors take care of that, so the ripple voltage at that point is very low, not at all as you have it represented.

    If you think about it, the "transformer" is acting as a choke, only. There is no AC present on the input winding, it is pure DC. You can't transform a DC voltage on one side to an AC voltage on the other. Your diagrams of the transformer show AC on both sides, when used as a transformer.

    I think your hang up is that the configuration of the transformer and choke are the same, which they are. But one is a choke, since it has DC on one side, and AC on the other. The other use is as a transformer, when AC is present on both the input and output.
  4. WW1WW

    WW1WW Ham Member QRZ Page

    This why Hams keep insisting tube based amps are better. This transistor stuff is just too complicated....
    AH7I likes this.
  5. WW1WW

    WW1WW Ham Member QRZ Page

    The OP did a great job of looking at a Ruthroff 4:1 transformer but that's not the only way to configure the output stage of a broadband SS amplifier. There are Equal Delay and Conventional transformer arraignments which both allow for the elimination of all DC chokes. The reason DC chokes are still used with conventional transformer outputs stages is to relive the transformer cores of the additional flux due to DC. If the proper material with a large enough cross section for the output transformer is used the choke can be eliminated.
  6. WA4SIX

    WA4SIX Ham Member QRZ Page

    AGAIN, if you look at it all, the Ruthoff 4:1 baluns.transformers are CONNECTED TO GROUND. The bifillar choke IS NOT !!!
    There is your difference. Please give it up.

  7. KD2NCU

    KD2NCU Ham Member QRZ Page

    K7JEM, you and I are saying the same thing on at least one point. My contention has been all along that bypass capacitors and/or other low pass filtering are needed locally where the DC supply connects to the drain drain feed coil. Others have contended that the coil itself blocks AC from entering the DC supply which would imply that local bypass connectors are not needed. I fully understand AC analysis and that the DC source will seem like a short circuit because of internal bypass capacitors and regulators, however, the DC source (AC short) may be inches or feet away and in that case I contend that the current pulled from the DC source connection by the center tap of the coil is a pulsed full wave rectified sinewave and if there are no local bypass capacitors those pulses of current will, in fact, be pulled from the DC bus and if the actual DC supply and its bypass capacitors are inches away, the bus radiates and other circuitry connected to the bus experiences these pulses as well. So we seem to agree that local bypass capacitors are needed.

    Now here's why the current from the DC supply WOULD be "pulses" if there were no LOCAL bypass capacitors. The current drawn by each transistors during its half cycle will be a half wave sine pulse. Since the drain currents come from the two coil windings, the currents in the windings of the "transformer" are full wave rectified sinewave pulses, alternate pulses coming from the alternate transistors turning on. They will not be smooth DC. If you don't agree that the currents in the two windings are full wave rectified sine pulses, then you don't understand how the circuit is working and you need to review the article cited by KG7SWP. Since the currents in the individual windings are in phase and both flow AWAY from the center tap connection to the DC supply lead, the current coming from the dc supply lead into the center tap MUST be equal to the sum of the two currents leaving and so will also be full wave rectified sine pulses twice as large as the ones in the two windings. So the current coming into the center tap or DC connection from the DC bus is not smooth DC it is rectified sine pulses. And that's why we need local bypass capacitors at this connection so that the DC supply does not have to supply these pulses. Maybe we agree on this too.

    I never said the DC transforms to the AC side. I said just the opposite, the AC on the AC side is transforming to the DC side in the form of these current pulses drawn from the DC connection through the center tap and that's why we need the local bypass capacitors.

    Regarding whether it is a choke or not, you are using circular logic. You are ASSUMING the device is acting like a choke, then reasoning that BECAUSE the coil is acting like a choke, there will be no AC on the DC side or input side and BECAUSE there are bypass capacitors on the DC there will be no AC on the DC connection. And then I think you're reasoning that because there is no AC on the DC side, the device MUST be a choke!

    If you think its acting like a choke, I'm asking you to explain how the two NON DC currents in the two windings leaving the center tap and flowing outward away from the center tap are somehow adding together to be a flat DC in the wire that feeds the center tap. I don't think you can explain this.

    In your last sentence you are using circular logic again. You are ASSUMING it has only DC on one side and AC on the other and then somehow reasoning that BECAUSE it has DC on one side and AC on the other, it must be a choke. The same physical coil can act as either a choke or a transformer or balun or autotransformer, etc., depending on the physical wiring of its four terminals. The external connections to the 4 terminals determine how the currents, voltages, and flux interacts inside the device which in turn determine whether it acts like a choke or a transformer. It's not the presence or absence of AC or DC that determine whether the device acts like a choke or transformer. The choke and the transformer/balun do not have the same external wiring. A bifilar wound coil wired as a choke cannot replace a bifilar would coil wired as a balun/transformer and they will not act the same way.
  8. KD2NCU

    KD2NCU Ham Member QRZ Page

    RICHS, what you are missing is that the Ruthroff Transformer/Balun and the bifilar drain feed coil used in many push pull configurations are NOT wired differently and are NOT used differently. They are fabricated and wired EXACTLY the same and both both are used to interface unbalanced devices to balanced devices (as well as other conversions) and that both provide a 4:1 impedance transformation. There are numerous references out there that explicitly state that the bifilar coil as connected in the push pull amp is providing a 1:4 impedance transformation and I cited such references stating that the device, for example, is providing a transformation from about 12 ohms to the 50 ohms of the final transformer primary. RICHS, you don't have a strong enough technical background to engage this conversation. In an earlier post, you said that the RF does not "pass through" this device because it's a choke. Well, the RF is definitely passing through this device. Additionally, you are ASSUMING that the device is acting a choke and then arguing that BECAUSE it's a choke, RF will not "pass through" it. Can you cite or provide a reference or analysis or derivation that explains analytically how this device acts like a choke to RF in this configuration? I don't think you can. I think you can only say that some people call it a choke so it must be a choke.
  9. KD2NCU

    KD2NCU Ham Member QRZ Page

    Well, KG7SWP, actually, it is grounded. If you knew anything at all about signal analysis you would know that for AC analysis, the DC source is a short circuit and that the transistors are treated as current sources. Heck, even K7JEM above seems to know this. Ask him. In many circuits, the negative of the DC source is common so for AC analysis the center tap of the coil IS GROUNDED.

    Now, where did you get the idea that Ruthroff 4:1 baluns or transformers have to be grounded? Some are, some are not. You seem to think that somehow the device is aware of whether its grounded or not and decides to act differently accordingly. I gave you plenty of references and if you looked for yourself, you would see that there are plenty of instances of the device not being grounded and if you read the references on the device, you'd find that the device is not "aware" and does not care whether or not it is grounded.

    So KG7SWP, can YOU provide any reference or analysis that truly derives the operation of the bifilar feed coil and demonstrates that it functions as a choke and keeps RF out of the supply. I mean textbook derivation that results in equations that demonstrate its operation. No you apparently can't or you would have by now. So once again, all you're bringing to the table is, "I heard people call it a choke so it has to be a choke, dude." "Someone told me that Ruthroff transformers are grounded so the bifilar coil can't be a Ruthroff dude." I, on the other hand have provided you references that show the exact device in the exact configuration in a push pull circuit wherein the author explicitly states that it provides a 1:4 impedance transformation and calls it a transformer and plenty of references that show the bifilar as well as Ruthroffs as autotransformers.

    In an earlier thread, you did a nice job citing an article that explains the operation of the bifilar feed coil pretty well. But then ...

    Funny how you referenced and quoted an article in my first thread that explains the operation of the bifilar coil. In this article, the author depicts the device as an autotransformer, yet you later argued with me that it is not an autotransformer.

    Funny that you copied and pasted the whole section of the article where the author shows that the voltage on the final transformer primary is twice that of the transistors during their on cycle and that the current of the transistors is twice that of the final transformer primary and then argued with me that the bifilar coil is not providing an impedance transformation of 1:4.

    Funny that you chose to quote and references an article that says, "Even a small net DC could saturate T2" then proceeded to argue with me that the core cannot saturate because it’s not grounded? Not grounded? The core doesn’t care or know whether it’s grounded. The current in a coil with a magnetic core with a fixed voltage forced across it will rise continuously until the core saturates or some resistance somewhere in the circuit limits the current regardless of whether the coil is grounded.

    Funny that I CONTACTED THE AUTHOR OF THE ARTICLE THAT YOU LIKED AND QUOTED and he confirmed all of my assertions regarding being an autotransformer, a 1:4 impedance transformation, providing ZERO choking action, and drawing full wave rectified sinewave current from the DC supply and wreaking havoc upstream if not low pass filtered as close as possible to the center tap of the transformer.

    Last question, why are some of you guys so emotional, frightened, and porky regarding this discussion? It's not like I tried to fool you into believing the earth is round or something.
  10. KD2NCU

    KD2NCU Ham Member QRZ Page

    Hey WW1WW, anything you and I could argue about regarding tubes? I don't want to discriminate here. I argue pretty good about tubes as well. Used to teach them many years ago in the Navy. :cool:

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