Discussion in 'Amateur Radio Amplifiers' started by KD2NCU, Sep 28, 2017.
Well PHASE MY FERRITES and PAINT 'EM PURPLE !!! I agree with RICHS on something. AGAIN!
My experience is anything you hang off the drain connection changes the output impedance. I tried a number of different configurations. Erbteck engineers had the best set up when they wound the choke with transmission lines. I couldn't get my lead lengths short enough to get good results. Yes a big FET will haul a lot of current through the choke but the shield of one or more transmission lines can handle the current. Remember, the only DC current is when there is no drive. Everything is AC past the drain bypass cap when it is making RF.
Now here is the kicker that got my mind right. Take another broadband amplifier in a different frequency range. A push pull transformer coupled audio amplifier. Dc feeds into the center tap of the primary and due to coupling AC comes out. So based on that you do not need a choke and transformer. A properly designed transformer does the job.
When you build an 1800 watt stage using the new FETs at super low drain impedance you will need large conductors to handle the impedance and current.
A drain choke is required when the TLT transformer ratio is 1:3 because there is no easy way to feed balanced DC into it. It is very easy using a 1:2 TLT or cascaded pair of 1:2 transformers as shown in Helge's book. I would use a 1:4 turns ratio with the big FETs. Frank WA1GFZ
Please don't confuse an unbalanced TLT with a balanced TLT. An unbalanced TLT will run hot with DC going through it. My simulation showed a 300 watt stage DC choke has about 1 amp of unbalance. Source of unbalance unknown but this is the reason it can be used as a feedback source with a third winding.
So if I am wrong why does Amplifier Research build BB amplifiers without DC chokes? Why does my 4800 watt amplifier work 160 through 6 meters with no core heating. Check out an old EB104. That had a DC choke that got so hot you couldn't touch it. BTW I have an article from the guys at the CERN who build amplifiers without DC chokes, Would you like a copy? gfz
BTW Erbteck wound their DC Chokes with a pair of 25 ohm coaxes in parallel to get the correct impedance match in a stage that ran a pair of MRF150s.
My case I had limited space and found a way to get both functions in 1 device. I didn't come up with the idea it is old information.
How’s everyone doing? Anyone feel like arguing today? Good. Me too! Let’s go.
Below is a juicy perspective we can argue over.
Let’s start with a simple push-pull circuit, get rid of the bypass capacitors and add a current probe because I know what the bypass caps are doing and I want to know if the bifilar coil is keeping AC, RF, harmonics, high frequency spectral content, whatever you want to call it out of the DC supply line. (Of course, I already know the answer.)
During Q1’s half cycle, the current or charge entering the red dashed boundary has to equal the current or charge leaving the boundary.
We know that the current through Q1 will be the positive 180 degrees of a sinewave.
So the current entering the center tap of the coil must be the positive 180 degrees of a sinewave, an exact replica of Q1 drain current.
During Q2’s half cycle, the current entering the center tap of the coil will again be a positive half sine pulse.
So the current entering the center tap is a full wave rectified sinewave of current exactly replicating the drain currents on alternate cycles.
The current coming from the DC bus into the center tap of the coil is an exact replica of the currents flowing into the drains of the transistors.
Anyone still think the coil is doing any smoothing, filtering, or choking of the current coming from the DC source or keeping AC components out of the DC supply bus?
Stay tuned. More to come in a while.
OK: Ready, Set, Argue!
My point is that you can get the function of T2 and T3 in one device. Yes DC current will flow if you don't ac couple the output which would be called a short circuit. Yes pulsing DC flows through the circuit and this pulsing with a little magic pixie dust turns into RF. I don't have a T2 in my amplifier so I AC couple the output at the 50 ohm port to reduce stress on the caps. So Richs do you have a ham call ? How about a picture of your solid state linear to show us how it is done. KD2NCU I struggled with your concept for a long time and simulated a final in LT spice so I could understand what was going on. I found in the end the deal is you need a balanced transformer like the T2 design big enough so the imbalance didn't cook the core. Then I found you could perform both functions in one device. I didn't invent it I just copied the approach used by other designers to help extend broadband range. A flux transformer runs hot when you start cranking power through it because the typical configuration is unbalanced. 2 ferrite sleeves with DC fed into the center tap generates big offsets that were cured by the band aide called T2. Oh BTW I agree with your analysis. gfz
Hello WA1GFZ and thanks: My postings are not advocating one approach over another. That would actually be a worthwhile discussion. We are bickering over something far more esoteric and petty than that! Come on, give us some credit here! We are arguing over how the balanced feed coil actually works, whether it provides a 1:4 impedance transformation, whether it does anything at all to keep AC or RF or harmonics out of the DC supply or not, whether it is the same thing as a Ruthroff 1:4 Balun/Transformer, whether or not a core can saturate if it's not grounded and all sorts of really important life-changing stuff like that! The way we've been at each other's throats, one might think we were arguing about global warming, Donald and Hillary, Hillary vs Bernie, etc. Been kinda fun though. Not done yet, still more fun to come.
This is simply not true. At the DC connection point of the transformer or choke there will always be a capacitor. The combination of the capacitor and choke keep DC on one side, and allow DC and superimposed AC on the other side. You won't see a waveform like what you are showing if you look at that DC feed point. You will see DC with perhaps a few millivolts of AC ripple. The same will happen with the DC current feed. It will be pure DC, with perhaps a very small amount of AC ripple. If that were not the case, the DC feedline would radiate like an antenna.
If you don't believe this, take a good scope and look at the DC feedpoint. If you want, insert a low resistance resistor in series with the DC feed so you can measure across it for current. Use your scope to see how much RF is actually there.
You seem to have a large misunderstanding of how these circuits work. Did you ever take any AC circuits courses, and if so, what college were they from?
I must have tried 20 different T2 configurations before I eliminated it. I found the design had a giant effect on second harmonic performance and broadband efficiency. I also found a small core as used in the EB104 produces nasty results because it isn't balanced. The only small T2 that came close to working was when I wound low impedance coax around a ferrite rod so it wouldn't saturate. My case the leads were still an inch long and hurt high frequency performance. The center tap of T2 is DC with pulsing current. The bypass makes it AC ground as K7JEM pointed out, but the ends are AC and if you install a third winding for feedback the phase imbalance becomes the feedback signal. gfz
Circuit analysis? You bet. It’s all on my bio page if you want to check it out. Let me know if you can’t get to it and I’ll email it.
But you haven’t struck me as a circuit analysis kind of guy. You seem to avoid circuit analysis like it was bubonic plague. Every time I use it, you ignore it, brush it aside, or just say “You’re wrong”, as you did above, never addressing the analysis itself, just disagreeing with the conclusion. But you don’t ever seem to try to refute it based on its technical merits or using circuit analysis.
And you never seem to use circuit analysis yourself. You seem to like to make assumptions and wave your hands rather than do real analysis.
So let's do some circuit analysis. You brought up circuit analysis so you stay with circuit analysis and tell me from a circuit analysis standpoint exactly what you disagree with below, (other than the conclusion).
During Q1’s half cycle, Q2 is an open circuit so it’s out of the picture.
The current coming into the center tap (Node 1) from the DC source must equal I1 + I2 at all times. (Kirchhoff's current law)
Note that at Node 2, I1 and I2 combine again and their sum must be equal to the drain current at all times.
So IQ1 = I1 + I2 = Is, the current coming from the dc power supply bus into the center tap.
So the transistor drain current is exactly equal to the dc source current as it enters the center tap at all times during Q1’s cycle.
During Q2’s cycle, the drain current of Q2 will equal exactly the current coming into the center tap from the dc bus.
If this were ever not true, there would be an excess or shortfall of electrons somewhere in the red boundary which is not possible.
So whatever the shape of the current is in Q1 during its half cycle, the current coming from the dc bus into the center tap will be an exact replica.
And since it's a linear amp, the shape of Q1's current will be the positive 180 degrees of a sine wave.
Now, from an AC circuits standpoint, what do you disagree with here?
Don’t bring up the capacitors. They’re off to the right somewhere and we’re looking at the current coming into the center tap.
If the drain current doesn’t exactly equal the current coming from the supply, then the charge and current entering the circuit does not equal the charge and current leaving the circuit. Where is this extra charge or current going or hiding or leaking to?
Remember, we’re doing circuit analysis here. Use circuit analysis to explain how the drain current of the conducting transistor could ever not equal exactly the current coming into the center tap from the DC supply and if they are ever not equal where is the mismatch in electrical charge hiding?
I'll be putting up another analysis later and you can use circuit analysis to try to refute it or find a flaw in the analysis or logic.