Coupling transformer phasing in class C rf amplifier

Discussion in 'Wireless & RF Design' started by Elerion, Mar 19, 2019.

  1. Elerion

    Thread Starter Member

    Sep 11, 2017
    57
    1
    Hi everyone.

    I built a simple amplifier to be used as a transmitter.
    There's a transistor oscillator circuit, coupled via a transformer to a class C rf amplifier (30 uH choke at the collector, and 68 ohm resistor from base to ground). That's all.
    rf_classC_phasing.png
    It is soldered neatly. No protoboard. Working at 7 MHz.

    For some reason, transformer's secondary phasing affects the circuit performance (in other words, swapping the secondary leads)
    With secondary in phase with primary:
    - I get more power.
    - The waveforms at the base of the rf amplifier are "noisier".
    - The collector voltage shows a second "peak" (this is later filtered by a pi filter, and the output to the load is sinusoidal).

    Attached, real measruments of both pashings.
    With some transistors, the power difference is small. For others, it is quite large.

    Could anyone explain why the secondary phasing (which, by common sense, should not have an impact) changes the base and collector waveforms and usually gets higher outputs?

    Thank you very much!
     
    Last edited by a moderator: Mar 23, 2019
  2. k7elp60

    AAC Fanatic!

    Nov 4, 2008
    555
    110
    I noticed that the second image has less distortion. Looking at the two images it is hard hard to tell which one has the greatest output as the scales are different.
     
  3. Elerion

    Thread Starter Member

    Sep 11, 2017
    57
    1
    That really depends on the particular transistor used. In the case of the data I posted, the difference on the output is quite small. I chose those just to illustrate the waveform shapes.
    On other cases, I measured two and even threefold difference in output amplitude (which is HUGE) for a circuit where the secondary phase should not have an impact, IMHO. This is why I'm asking, as I suspect that I'm missing some (maybe) non obvious issue here.
     
  4. Bordodynov

    Well-Known Member

    May 20, 2015
    2,175
    668
    There is an error in your scheme. Not enough separation capacitor between the collector of the first transistor and the primary winding of the transformer. Now you have a power supply short to a common point. If your circuit corresponds to reality, then one of the inductances should be burned.
     
  5. Elerion

    Thread Starter Member

    Sep 11, 2017
    57
    1
    Yes, I mixed by mistake two different setups.
    L5 is actually the primary (now renamed Lp), not directly attached to the supply, but through a 220 ohm resistor. Sorry for the mistake.
    This is the correct circuit from the experimental results

    classC_corrected.png
     
  6. Bordodynov

    Well-Known Member

    May 20, 2015
    2,175
    668
    Apparently between the inductance of the transformer and the inductance of 30 μH there is a parasitic connection. There is a parasitic capacitive coupling between the secondary winding and the inductance of 30 μH (collector capacity of the transistor). This capacitance has a variation that affects the phase relationships. Try to shield (or rotate 90 degrees) the inductance of 30 µH
     
  7. Elerion

    Thread Starter Member

    Sep 11, 2017
    57
    1
    I tried to rotate, but didn't see any change.
    I also tried to place the 30 μH inductance outside of the board, at a few centimeters, wired. This adds some parasitic inductance, but I think that is ok in this situation. Nothing changed either (or was not apparent).
     
  8. BR-549

    AAC Fanatic!

    Sep 22, 2013
    4,817
    1,313
    Have you got any paperwork on the transformer?
     
  9. Elerion

    Thread Starter Member

    Sep 11, 2017
    57
    1
    Do you mean how it is built?
    It is an iron powder toroid. Primary is wounded next to the secondary. 7:1 turns ratio.
    The inductances shown in the schematic match real values closely.
     
  10. bertus

    Administrator

    Apr 5, 2008
    19,507
    3,972
    Hello,

    What material is the toroid made of?
    There are several types of material, with each their specific frequency range.

    Bertus
     
  11. Elerion

    Thread Starter Member

    Sep 11, 2017
    57
    1
    Material 2
     
  12. bertus

    Administrator

    Apr 5, 2008
    19,507
    3,972
    Hello,

    How are the windings made on the toroid?
    Have a look at the attached PDF for some info.

    Bertus
     
    Elerion likes this.
  13. Elerion

    Thread Starter Member

    Sep 11, 2017
    57
    1
    Like the narrow band (page 2, Some practical aspects of Toroid) and the one attached.
    EXCEPT I didn't overlap the secondary.
    I wounded the secondary next to the primary (each of them "sitting" right on the toroid surface).
    In the same sense.

    EDIT: I attached an image of the actual toroid.

    Very nice articles, by the way. Learned a few things from them.
     
    Last edited: Mar 31, 2019
  14. Elerion

    Thread Starter Member

    Sep 11, 2017
    57
    1
    Rewinding the toroid to look more like the schematic view attached in my last post did make a difference. There's still some output power and waveform shape difference, but much closer now.

    Can anyone explain why such a difference between both ways of winding the toroid? Intuitively, parasitic capacitance should be higher now (as pri-sec are overlapped), but it works much better that way.
     
Loading...