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.

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.

Figure 5 · Ruthroff’s 1:4 transformerFrom: A Simplified Analysis of the Broadband Transmission Line Transformer By Jerry Sevick

VDD

T1

Typical configuration of Bifilar Feed Coil in push-pull amplifiers.

Figure 5 above redrawn to orient the transformer the same way it appears in push pull circuits, with Q2 conducting, Q1 off.

Combination of transistor Q1 and DC source producing voltage across T1 windings.

Transistor output impedance

T2 primary impedance

T1

Q1 and Q2 alternately apply the DC source across the low impedance unbalanced input of the Balun.

VDD

T1

VDD

T1

VDD

T1

BalancedUnBalanced BalancedUnBalancedBalancedUnBalanced

I I

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

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?

Figure 5 · Ruthroff’s 1:4 transformer Low frequency model

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.

In

Out

“Simple 1:1 Transformer”

Center Tap

“Bifilar Feed Choke”

(Drawn Differently)

Center 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.

Start with an ordinary bifilar wound transformer

Add some EXTERNAL connections to make it a

“Bifilar DC Feed Coil”

Center Tap

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.

Same Connection Without CoaxNow it’s a “Bifilar Feed Choke”(4X impedance transformation)

Bifilar Wound Coil Connected as4:1 Ruthroth Voltage Balun

(4X impedance transformation)

Z Ohms

4 x Z Ohms

Center Tap

Center Tap

Same Connection in Push-Pull Output Circuit“Bifilar Feed Choke”

(4X impedance transformation)

Center Tap4:1 Ruthroff

Voltage Balun“Bifilar Feed

Choke”“Bifilar Feed

Choke”

InOut

Same physical coil connected as Common Mode Choke.

Same as 1:1 Current Balun(No Impedance Transformation)

(No center tap)

The Same physical coil connected as an ordinary 1:1 voltage transformer or

isolation transformer.(No Impedance Change)

InOut

“Common Mode Choke”

“Simple 1:1 Transformer”

In Out In Out

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.

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.

The Balun and The UNUN, pg 1, 6

http://sk0qo.se/jo/images/bilder/Bilder-2016/Baluner/Balun_and_UnUn.pdf

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

RF Transformer Design

http://www.foxcomputer.se/RF%20Transformer%20Design.pdf

Analyzes 4:1 Ruthroff balun/transformer. Excerpt shown below.

The impedance seen by each transistor will be 3.125 ohms

50 Ω

1:2 Turns Ratio, 1:4 Impedance

Ratio

12.5 Ω

3.125 Ω

3.125 Ω

1:2 Turns Ratio, 1:4 Impedance Ratio

7 bifilar turns on a 2631803802 core.

This is a choke This is not

Low Cost 1000 Watt 300 Volt RF Power Amplifier for 27.12MHz

Advanced Power Technology Application Note APT9701

http://d3i5bpxkxvwmz.cloudfront.net/articles/2015/08/02/Low-Cost-1000-Watt-300-Volt-RF-Power-Amplifier-for-27.12-MHz-1438569631.pdf

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.

“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.”

“4:1 VOLTAGE BALUN”

http://vk6ysf.com/balun_4-1.htm

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.

Some thoughts on low cost, solid state, efficient, kilowatt-class HF linear amplifiers

http://ludens.cl/Electron/mosfetamps/amps.html

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.

Figure 1 Schematic of the 4:1 Ruthroff voltage balun.

“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 fundamental 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.

Current Drawn From DC Supply

16 Amps

The current drawn from the DC supply is a series of rectified sine “pulses”,

NOT a smooth DC.

8 amp pulses

16 amp pulses pulled from supply

DC Supply Bus

16 amp pulses pulled from DC supply bus

8 amp pulses

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.

“Common Mode Choke” “Bifilar Feed

Choke”

In the push pull configuration, the flux from the two currents are differential mode and cancel one another. There can be no inductance when the flux is cancelled like this.

In the common mode choke configuration, the flux from the two common mode currents is additive so they experience inductance. The differential mode currents flux cancels so the differential mode current does not experience inductance and passes through with minimal attenuation.

In Out

Differential Mode

Common Mode

True Common Mode Choke or 1:1 Current Balun

Ruthroff 4:1 Voltage Balunor Bifilar Feed Choke

Center Tap

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.

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.

Figure 5 · Ruthroff’s 1:4 transformer

Autotransformer lowfrequency model

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.”

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”

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.)

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.

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.

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.

Conclusions:

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.

KD2NCU