whats the relation between bandwidth of a single ended linear PA vs diffrential PA .. There is no specific LC tank in the diffrential PA like class B as compared in single ended design..
is diffrental offers twice the BW as compared to single to single ended????????i dont get it?

purplewolf wrote on Jul 19^th, 2010, 2:23am:
What do you mean by "classical formulas for LC tank design" ?

purplewolf wrote on Jul 19^th, 2010, 2:23am:
Actual transfomer is no more than mutual coupled inductors which forms double resonator, that is, coupled LC-tank.
So it is same as "classical formulas for LC tank design", although I don't know what you mean by "classical formulas for LC tank design".

Also see http://www.designers-guide.org/Forum/YaBB.pl?num=1240229676

purplewolf wrote on Jul 19^th, 2010, 11:59am:
What on earth do you mean by these ?

In your formula, Q is proportional to R. It is very funny.

yups... Ken is absolutely correct

Your original question asked about single ended versus differential impedance as it applies to amplifier matching to some desired load (I assume this is what you meant?).

The only difference between differential and single ended matching is that under the same conditions the differential parasitic capacitance of the amplifier may be reduced compared to the single ended amplifier. Why? because a differential amplifier looks like two single ended amps driven differentially. The size of the two devices is likely to be smaller than the single ended equivalent (although it does not have to be the case). The shunt capacitances of the two single ended amplifiers appear to be in series for the differential amplifier thus making the capacitance look smaller for the diff amp. There is no real advantage to this over the single ended amplifier.

The resistive losses may be a little less too because the resistance of each half of the diff amp looks like they are in series for the differential output. So, it may be that the resistance across the tuned tank is less for the differential case and perhaps this is why some may consider the Q to be higher for the diff amp?

The paradox seems to be that BW decreases as Q increases for a resonant circuit, so I too am perplexed by the assertion that BW is wider for a differential amp versus single ended.

However, there can be a LC tank for class B PA. Most textbook differential class B circuits are pseudo differential with a center tapped transformer. The center tapped transformer represents an equivalent half circuit for the class B amp, so the circuit really looks like two single ended circuits driven differentially. The transformer usually offers some RF common mode reject such that there still is some difference between the differential and common mode gain of the amp.

I'm not sure if this helps answer your brief question  :-/

for individual inductor the Q = wL/R, where R account for the losses of inducto..
but the loaded quality factor of tank is Q = f/BW =R/wL=50/wL
wL =wC  <=>  1/(ω0*L)=ω0*C <=> 1/(ω0*C)=ω0*L. .. makes no difference.

Dont mix up the two Q's...

I just assume that the bandwidth is either twice (or may be half) incase of diffrential as compared to single ended since it is combination of two single ended..I havent found any mathematical proof about its BW in literature  .. Long story short ,I have absolutely no idea about it.

.

purplewolf wrote on Jul 21^st, 2010, 2:15am:
No, you don't understand Q of resonators.

Consider a definition of resonator's Q.
Resonator's Q is defined as ratio of reactive energy and loss energy at resonance for any resonator.

Q of parallel LC-tank resonator is Q=ω₀*L/R, here R means loss of L. This Q is not inductor's Q.
Q of parallel LCR-tank resonator is Q=ω₀*C/G, here G means parallel conductance of LC-tank.

purplewolf wrote on Jul 21^st, 2010, 2:15am:
Again see your description surely.

Do you think "wL =wC" is surely correct ?
I think your "wL =wC" makes clear difference compared to 1/(ω₀*L)=ω₀*C <=> 1/(ω₀*C)=ω₀*L.

Your "diffrential.jpg" uses ideal transfomers which are not physical.
So your "diffrential.jpg" should show lowpass characteristics, since there are no DC-Block capacitors and no resonators.

But actual transfomer is mutual coupled inductors which forms double resonator, that is, coupled LC-tank.

purplewolf wrote on Jul 21^st, 2010, 2:15am:
Again see http://www.designers-guide.org/Forum/YaBB.pl?num=1278414763/9#9

Study characteristics or behaviors of double resonators or coupled resonators
Generally BW will be narrower if you use double resonators.
But if you permit two peak characteristic over passband, you can increase BW using double resonators.

purplewolf wrote on Jul 21^st, 2010, 2:15am:
You seems to be new comer in RF-engineering world.
Such things are decribed in any RF books.
Sorry I don't have international books which are written in English.

Read true classical-RF circuit text books which are oriented for discrete RF circuits not RF-IC's.
Or search old application notes on discrete PA by Motorola(=Currently On-Semi).