Bifilar RF choke vs regular choke - Page 1
Forum section: electronics / rf-microwave

author: igor78
author_rank: Contributor
timestamp: March 06, 2023, 09:23:12 pm
content: I've seen this RF choke construction multiple times in the amplifier schematicHere is anotherYet, even more times, I see just a regular RF choke that decouples the power supply from the amplifier output.I understand the decoupling role of a regular choke.What is the purpose of the bifilar one?Bifilar winding is just a tap on a toroid, right?Essentially the amp output is taken from the tap i.e. middle of the choke. Why is it beneficial?

author: TimFox
author_rank: Super Contributor
timestamp: March 06, 2023, 10:01:42 pm
content: Electrically it may behave in a similar manner as a tapped solenoidal coil on a form, but the bifilar winding gives better coupling between "half-windings".Also, if DC loading of the core is important, a proper bifilar winding will cancel the DC flux.

author: richard.cs
author_rank: Super Contributor
timestamp: March 07, 2023, 02:22:37 pm
content: It's acting as a 1:2 transformer (in voltage, 1:4 in impedance). The bifilar arrangement improves the coupling between the two windings and lowers the leakage inductance.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: March 07, 2023, 06:46:44 pm
content: Consider the voltage between wires. Apply an ideal voltage step. It propagates along the two wires as a transmission line. When it reaches the other end, it is absorbed by a load or reflected off an open/short.When you apply that voltage step, some current flows into the pair. The ratio is given by the characteristic impedance of the transmission line, Zo.Now wire up the pair. A step change at the 2N3904 collector is coupled to the output side (2N3866 base) immediately through Zo (about 70 ohms for bifilar/twisted pair magnet wire). That is, it acts like 70 ohm in series between C and B. After one line delay, the wave propagates to the +V side, which is bypassed on one side (0.1uF) while the other side sinks current to the output. Thus the output sees double the input current, but in two phases, one immediate wave and one delayed wave. We get the doubling of current and halving of voltage as we expect from a 2:1 transformer. But we incur one delay, and therefore have a cutoff frequency corresponding to the electrical length of the TL.This is known as a Ruthroff transmission line transformer. See:http://k5tra.net/TechFiles/Transformers.pdfIf instead you use two twisted pairs (transmission lines), wire their input sides in series, and their output sides in parallel, and wrap enough ferrites around (or wrap them around ferrite) to raise the common-mode impedance to negligible levels, then you have a Guanella TLT. Note the 2N3904 collector in that case sees double Zo or 70*2 = 140 ohms, while the 2N3866 base sees half or Zo/2 = 35 ohms. As we expect for a 2:1 transformer, the impedance ratio is 4, but because the delays are matched, this remains true at all frequencies -- including multiple wavelengths through the transmission lines. (Upper cutoff is limited by how cleanly you can wire the TLs in series/parallel combinations, and their overall dimensions. Lower cutoff of course is given by the core size/turns.)Neither matters a lick for the frequencies this would run at (given the 2N3904, well under 100MHz?), but it is useful to know for broadband applications, and easy enough to do for little transformers like this.Tim

author: szoftveres
author_rank: Regular Contributor
timestamp: March 07, 2023, 07:46:34 pm
content: It's sometimes called "transmission line transformer"; the idea is that besides the obvious magnetic coupling, the two windings (side by side to each other) are also behaving like coupled lines. This helps at higher frequencies where the magnetic coupling -due to all kinds of physical properties of the core and the coil itself- would not perform adequately.

author: David Hess
author_rank: Super Contributor
timestamp: March 08, 2023, 03:06:57 am
content: In the case you give, the inductor is both providing the DC current to the transistor, but also performing a 4:1 impedance transformation of the RF feeding the next stage.