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Push-Pull Output Transformers

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Push-Pull Output Transformers Push-Pull Output Transformers - Part III, The Final Countdown: KD2NCU 28-Sep-2017 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. 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. T1 VDD Figure 5 · Ruthroff’s 1:4 transformer From: A Simplified Analysis of the Broadband Transmission Line Transformer By Jerry Sevick Typical configuration of Bifilar Feed Coil in push-pull amplifiers. T1 Figure 5 above redrawn to Transistor orient the transformer the output same way it appears in impedance push pull circuits, with Q2 conducting, Q1 off. Combination of transistor Q1 and DC T2 primary source producing impedance voltage across T1 windings. 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. UnBalanced Balanced UnBalanced Balanced UnBalanced Balanced I I VDD VDD VDD T1 T1 T1 Q1 and Q2 alternately apply the DC source across the low impedance unbalanced input of the Balun. 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 Figure 5 · Ruthroff’s 1:4 transformer Low frequency model 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? Add some EXTERNAL Start with an ordinary connections to make it a bifilar wound transformer “Bifilar DC Feed Coil” “Simple 1:1 “Bifilar Feed Transformer” Choke” (Drawn Differently) Center In Tap Out Center Center Tap Tap Draw it with the right hand coil under the center tap maintaining the same connections and it is obvious that it is an autotransformer. 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.” 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. 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. Center 4:1 Ruthroff “Bifilar Feed Tap Voltage Balun Choke” “Bifilar Feed Choke” Center Tap Z Ohms Center Same Connection in Push-Pull Output Circuit Tap “Bifilar Feed Choke” 4 x Z Ohms (4X impedance transformation) Bifilar Wound Coil Connected as Same Connection Without Coax 4:1 Ruthroth Voltage Balun Now it’s a “Bifilar Feed Choke” (4X impedance transformation) (4X impedance transformation) Same physical coil connected as Common Mode Choke. The Same physical coil Same as 1:1 Current Balun “Simple 1:1 connected as an ordinary “Common (No Impedance Transformation) 1:1 voltage transformer or (No center tap) Transformer” Mode Choke” isolation transformer. (No Impedance Change) In Out In Out In Out Out In 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. 3.125 Ω The impedance seen by each 50 Ω transistor will be 3.125 ohms 3.125 Ω 12.5 Ω 1:2 Turns Ratio, 1:2 Turns Ratio, 1:4 Impedance Ratio 1:4 Impedance Ratio 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.
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