EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: cadr on April 02, 2021, 05:06:18 pm
-
I am playing with an RF amplifier design, and I need to transform my 50 ohm load to something higher to make the transistors not need to work so hard. I was going to make a transformer with a bifilar wired toroid. I can set the turn ratio to do what I need, but I am confused on how the absolute number of turns matter. As long as I have the desired ratio of turns, how does the primary having 10 turns vs 20 turns affect things?
Thanks!
-
In general with transformers, the absolute number of turns affects the magnetizing inductance, one of the two important parasitic inductances in the model. At low frequencies, the magnetizing inductance shunts the ideal transformer inside the model, and steals AC current from the load. Of course, it also affects the leakage inductance, the other important parasitic inductance, which is important at high frequencies. The ideal transformer, governed only by the turns ratio, is inside the parasitic components.
-
Thanks!
Any suggestion about how I determine what impedance/# of primary turns I should aim for if I am using this at 7MHz?
-
You want the magnetizing inductance's reactance (essentially the inductance of the primary winding) to be much higher than the load impedance that the primary winding presents to the active devices at 7 MHz. However, the leakage inductance's absolute value will also increase with more turns, but the high coupling of the bifilar winding is good for low leakage. This is why such transformers have a relatively high bandwidth. For a simple 1:1 ratio model, the low-frequency cutoff (at -3 dB) is where the magnetizing inductance reactance equals the load impedance, and the high-frequency cutoff is where the leakage inductance reactance equals the load impedance (ignoring parasitic capacitance).
One of many articles in the literature: http://mwl.diet.uniroma1.it/people/pisa/SISTEMI_RF/MATERIALE%20INTEGRATIVO/Kikkert_RF_Electronics_Course/05-RF_Electronics_Kikkert_Ch3_RFTransformers.pdf (http://mwl.diet.uniroma1.it/people/pisa/SISTEMI_RF/MATERIALE%20INTEGRATIVO/Kikkert_RF_Electronics_Course/05-RF_Electronics_Kikkert_Ch3_RFTransformers.pdf)