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Design >> RF Design >> Which PA is advantageous, SingleEnded or Differential Output ?
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Message started by pancho_hideboo on Apr 20^th, 2009, 5:14am

Title: Which PA is advantageous, SingleEnded or Differential Output ?
Post by pancho_hideboo on Apr 20^th, 2009, 5:14am

Generally output of VCO or I/Q modulator is differential.
So driving PA(Power Amplifier) by differential signal is natural.
Then output signal of PA is also differential.
But TX load is generally single ended unless we do use loop antenna.
So we have to interface differeitial output to single-ended output.

Here I pick up the following important characteristics of PA.
(1) Availble Maximum Output Power Level
(2) Power Added Efficiency
(3) Second Order Harmonic Level

If we can use fairly good balun which is broad band enough to suppress Second Order Harmonic,
Differential output PA is superior than Single-Ended PA regarding the above three characteristics.

However if outputs of PA are open drain of differential pair
and we can only use very poor balun which is composed from discrete L and C,
I think there is no advantage in Differential output PA compared to Single-Ended output PA.

For example, we often use a balun of Figure-3 in the following.
http://www.zen118213.zen.co.uk/RFMicrowave_Circuits_Files/Balun%20Design.pdf

This balun is not broad band, so Second Order Harmonic can not be suppressed at all.
And power can not be doubled even if we double cosumption current.

My opinions are:
If we can't use good balun, Differential Output PA is
- Less efficiency regarding available maximum power level and consumption current
- Second Order Harmonic Suppression is not good at all

So if provided same consumption current, I think Single-Ended output PA is superior than Differential one regarding available maximum power level.
Here consumption currents include not only final stage of PA but also driver stages which might have differential input/output.

I know differential LNA inputs is preferable for RX sensitivity.
But I don't think differential output can be valuable for good performance for TX.

How do you think ? :-/
Is there any advantage in Differential Output PA ?

But I have to share TX pins with RX differential LNA inputs for low cost transceiver.
I'm always in very trouble to get best solution to accomodate best performance for both RX and TX.

Title: Re: Which PA is advantageous, SingleEnded or Differential Output ?
Post by aaron_do on Apr 20^th, 2009, 6:41pm

Hi,

I was interested in the answer to your question so I did a little snooping around online...I found a patent which has a pretty detailed description of some advantages of differential PA.

http://www.freepatentsonline.com/7031689.html

Most of it i'm not too familiar with, but I understand the point about the bonding pads.

cheers,
Aaron

Title: Re: Which PA is advantageous, SingleEnded or Differential Output ?
Post by pancho_hideboo on Apr 21^st, 2009, 4:57am

Thanks for reply.

aaron_do wrote on Apr 20^th, 2009, 6:41pm:

I found a patent which has a pretty detailed description of some advantages of differential PA.
http://www.freepatentsonline.com/7031689.html
Most of it i'm not too familiar with, but I understand the point about the bonding pads.

What do you mean by "I understand the point about the bonding pads" ?

http://v3.espacenet.com/publicationDetails/biblio?adjacent=true&KC=A1&date=20030515&NR=2003092408A1&DB=EPODOC&locale=en_EP&CC=US&FT=D

It seems that an inventor of this patent is people belonging to Avago Technologies.
This patent was applied to the Patent Office of our country too.
But an application was finally withdrawn by an applicant and has not been formed as a patent in our country.

This patent assumes a differential antenna, here balun is not required at all.
So there is no problem even though outputs of PA are differential.
If we can use a differential antenna like this patent, we can utilize full ability of differential output PA without any obstacle.

The followings are extractions from this patent.

Quote:

[BACKGROUND]
The antenna is inherently differential.

[DETAILED DESCRIPTION]
There are some specific advantages to having a differential PA.

For the same signal as used in the prior art, the output voltage across each device is half of the total voltage.
This effectively increases the headroom of the transistor enabling increased efficiency.

In addition, each device requires only half the current, so the output impedance is twice as high as for the equivalent prior art single ended PA. This results in reduced transformation losses at the output.

When driven differentially, the differential PA has a virtual ground, eliminating the required numerous grounding bond wires. High impedance to ground at the common node is advantageous, leading to high common mode rejection.
The many bond wires in the single ended design are required to reduce the common lead inductance which directly reduces the gain. Lower gain in the output stage requires more power in the driver stage immediately proceeding lowering the efficiency of the entire PA.

These are all very common knowledges.

If we can't use good balun, Differential Output PA is still advantageous compared to Single-Ended Output PA ?

Title: Re: Which PA is advantageous, SingleEnded or Differential Output ?
Post by aaron_do on Apr 21^st, 2009, 8:24am

Hi,

Quote:

What do you mean by "I understand the point about the bonding pads" ?

I was referring to this part:

"When driven differentially, the differential PA has a virtual ground, eliminating the required numerous grounding bond wires. High impedance to ground at the common node is advantageous, leading to high common mode rejection. The many bond wires in the single ended design are required to reduce the common lead impedance, which directly reduces the gain."

actually you quoted it too so I guess you already know...

By the way, regarding this part:

"High impedance to ground at the common node is advantageous, leading to high common mode rejection."

If the second harmonic is the major concern, wouldn't it be possible to resonate (parallel) this bond-wire inductance at the second harmonic in order to reduce the second harmonic gain?

cheers,
Aaron

Title: Re: Which PA is advantageous, SingleEnded or Differential Output ?
Post by pancho_hideboo on Apr 21^st, 2009, 9:45am

aaron_do wrote on Apr 21^st, 2009, 8:24am:

"High impedance to ground at the common node is advantageous, leading to high common mode rejection."
If the second harmonic is the major concern, wouldn't it be possible to resonate (parallel) this bond-wire inductance at the second harmonic in order to reduce the second harmonic gain?

Maybe it could be possible if we can realize very sharp resonance at 2*f0 and enough low impedance for f0.

Title: Re: Which PA is advantageous, SingleEnded or Differential Output ?
Post by aaron_do on Apr 21^st, 2009, 6:43pm

Quote:

Maybe it could be possible if we can realize very sharp resonance at 2*f0 and enough low impedance for f0.

Does it matter at f0? My thinking is that this impedance is common-mode and so it will lower the common-mode gain at 2f0, but it won't affect the differential-mode gain at f0. Correct me if i'm wrong...

cheers,
Aaron

Title: Re: Which PA is advantageous, SingleEnded or Differential Output ?
Post by pancho_hideboo on Apr 21^st, 2009, 6:50pm

aaron_do wrote on Apr 21^st, 2009, 6:43pm:

Quote:

Maybe it could be possible if we can realize very sharp resonance at 2*f0 and enough low impedance for f0.

I was thinking Single-Ended PA.

If PA is differential, impedance of common node is virtual ground for f0. So impedance of common node is very low for f0.

Title: Re: Which PA is advantageous, SingleEnded or Differential Output ?
Post by aaron_do on Apr 21^st, 2009, 7:45pm

So for differential PA, you might be able to resonate the bond-wire at 2f0 and get good second harmonic suppression, but you sacrifice common-mode rejection at f0.

However, when you add the off-chip balun, it should reject common-mode at f0.

cheers,
Aaron

Title: Re: Which PA is advantageous, SingleEnded or Differential Output ?
Post by RFICDUDE on Apr 27^th, 2009, 8:08pm

I think the real benefits of pseudo differential versus single ended PA are

1. class B push pull operation may be more efficient than class AB for the same linearity requirements

2. differential operation divides the output power requirement across two devices instead of one (this is helpful for low voltage applications)

Title: Re: Which PA is advantageous, SingleEnded or Differential Output ?
Post by pancho_hideboo on Apr 28^th, 2009, 1:00am

Thanks for always valuable comments.

RFICDUDE wrote on Apr 27^th, 2009, 8:08pm:

1. class B push pull operation may be more efficient than class AB for the same linearity requirements

If we can't use good balun,
can we still realize good class B push pull operation which is more efficient compared to Single-Ended Output PA ?

Title: Re: Which PA is advantageous, SingleEnded or Differential Output ?
Post by RFICDUDE on Apr 28^th, 2009, 7:42am

You must have some way to combine the push-pull signals at the single-ended load (I am not considering differential loads), so some sort of balun or coupling network is required to do this.

Lumped element networks might be able to synthesize a combining/matching structure, but what you specifically need depends on the mode of operation.

If both amplifiers conduct (class A) then a balun type structure is absolutely needed to perform phase rotation, combining and matching functions. If the amplifiers conduct over a half cycle (class B) then you may need a network that appropriately couples each output to the load, but doesn't (somehow) mess up the match when one device is off.

I am much more familiar with the former situation than the later. I have combined two class AB 180 degree outputs using lumped element networks to rotate the phase, combine the signals and provide 50 ohm match to the load.