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Design >> Analog Design >> Single-differential converter
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Message started by aaron_do on Nov 27^th, 2008, 11:27pm

Title: Single-differential converter
Post by aaron_do on Nov 27^th, 2008, 11:27pm

Hi all,

just wondering, what is the benefit of this kind of active balun? How is it better than a simple differential pair with one input grounded?

btw i forgot to add the VCC connection in between the resistors, but i don't think the resistors are necessary anyway...

thanks,
Aaron

Title: Re: Single-differential converter
Post by thechopper on Nov 29^th, 2008, 8:15am

Hi Aaron,

Not sure what the real advantages are since I´m not an expert, but concerning the resistors I guess the will be necessary (with the corresponding reference voltage in the mid-point) in order to guarantee the output CM of the circuit: since its output is differential you need to force the output common-mode.

Regards
Tosei

Title: Re: Single-differential converter
Post by aaron_do on Nov 29^th, 2008, 7:01pm

Hi Tosei,

thanks for your input.

the funny thing is the center point is tied to VCC. The resistors aren't sensing the CM value. Anyway it seems to me that this circuit is guaranteed to have a DC offset, and i doubt the AC balance can be any good since its obviously asymmetric.

the circuit has been patented which leads me to believe there is something good about it, but i just can't figure it out.

cheers,
Aaron

Title: Re: Single-differential converter
Post by loose-electron on Dec 5^th, 2008, 1:33pm

I actually tried using that structure a few years ago, except the MOS version.

It never made it into the final design, but I seem to remember it was a bit quieter than the differential pair (everything else being the same) but had nothing of significance to offer that I can remember.

Differential pairs that are driven with one side AC grounded dont give a perfectly complementary amplitude signal out, and If I recall that also offered better amplitude balance.

Patented? Why would that indicate that it has value or merit? (Sort of kidding!) Seriously, don't allow any presence of a patent ***ever*** fool you into thinking that an idea has merit or is unique. A lot of patents are pretty worthless. (and yes, I have patents that I consider worthless, but the company I was working for wanted the concept patented!)

Jerry

Title: Re: Single-differential converter
Post by rf-design on Dec 15^th, 2008, 2:08pm

Have seen that "differential combined common-base common-emitter LNA" in magazines in the early 80's. The patent is a zombie, def...

It have some advantage if you connect the output to differential integrated inductors. Also the matching more broadband through the common-base. The amplitude balance is getting critical, true.