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To my knowledge, a conventional transformer is a balun - you simply connect a single-ended signal at the primary side, you get the balanced signal at the secondary side - magnetic coupling doesn't care where the ground is. It also automatically offers DC isolation.

A Balanced Circuit

But I see broadband RF mixers and amplifiers have a center-tap at the balanced side as well, for example Mini-Circuit offers a lot of baluns with center-tap. I often see the tap is grounded (or sometimes AC-coupled to ground), and seems to serve no purpose (*).

I found an application note from Mini-Circuit, AN-20-002: Application Note on Transformers, it says,

For Balun applications, choose a balun with center tap on balanced side as it provides excellent amplitude and phase balance.

In other words, a balun with an unconnected center-tap (or without one), has worse amplitude and phase balance than a center-tapped balun? Why? So a connection to ground can force the center to be at common ground potential, thus making the balanced side to have better balance?


(*) I know in double balanced diode mixers, a center-tap is needed to extract IF signal. I also read that a grounded center-tap can remove excessive common-mode voltage at the balanced side, that may degrade the performance of a differential amplifier. But I'm asking about the balun device, not its applications.

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For RF transformers/baluns, the inter-winding capacitance can play a big part in reducing common-mode signals between unbalanced input and balanced output. A 1 pF inter-winding capacitance may not seem that much but, at 1 GHz, it's an impedance of 159 ohms.

So, if you don't ground the centre-tap your balun won't be working as effectively as it can do because the balanced output will be strongly capacitively coupled to the unbalanced input. For most circuits this won't matter, but if you are relying on very low common-mode coupling then you need to ground the centre-tap.

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  • \$\begingroup\$ @andy_aka, can you please clarify how grounding the center tap reduces the capacitive coupling between the balanced output and the unbalanced input of the balun...Thank you. \$\endgroup\$
    – jrive
    Jul 27 at 15:36
  • \$\begingroup\$ Grounding the centre-tap ensures that capacitively coupled common-mode signals (on the unbalanced side) are more likely to be shunted to ground on the balanced side and, the fraction that gets through will be equalized on both hot wires of the balanced side. Maybe you should simulate this if you can't follow my explanation. \$\endgroup\$
    – Andy aka
    Jul 27 at 15:43
  • \$\begingroup\$ I"m actually working on that simulation, and I can't prove to myself why grounding the center tap is actually better. In fact, it is worse, in terms of cancelling out the common mode voltage on the balanced side--so, perhaps my simulation setup is incorrect. How can I share with you what I'm doing? Should I start a separate post? \$\endgroup\$
    – jrive
    Jul 27 at 15:50
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    \$\begingroup\$ @jrive yes, start a new question I think is the best idea - you can link this question as source for your motivation and it might enable me to get my head together (I mean it was back in 2019 when I answered this and I may not be paying as much attention now as I did back then). Go on - make me think harder and raise a new question!!! \$\endgroup\$
    – Andy aka
    Jul 27 at 17:14

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