JFET input, MOSFET VAS, LATERAL output = Perfect!!

[keantoken]

I've sen many circuits that primarily use an N-Jfet input transistor. Ultimately I think it is a decision of which transistor needs the N-silicon more, input or VAS.

SWF, more filtering should be better. A separate supply or heavy filtering for the frontend is better. I thought at first the filter caps were resonating with something but it turns out that is not the case. In fact, if there is a problem it's that there is not enough filtering. Giving the frontend a separate supply or filtering from a capacitance multiplier will go a great way towards lowering the distortion of the amplifier without increasing OLG or adding more active devices in the signal path. It will also decrease the contribution of the filter caps to the output signal, as I mentioned before. I have attached a simulation showing THD and FFT with separate rails (with stability compensation removed).

Although the harmonic profile looks less appealing, the 98% criteria is still fulfilled. Regulating the frontend removes a lot of the input stage distortion, which reveals the distortion of the output stage, which is primarily odd-harmonics but still has little high-order content.

EDIT: Attachment attached!

- keantoken

[keantoken]

Wait a sec, I made a mistake. The distortion benefit doesn't occur at 7KHz, because the vast majority of input Jfet loading is from the capacitance of the VAS and output MOSFETs. 7KHz distortion benefits little from filtering. It is distortion below 4KHz that benefits from filtering, because this is when low PSRR begins to outweigh capacitive loading as the main source of distortion.

500Hz distortion is drastically reduced. Harmonic profile is the same as in the above photo, but distortion and output impedance are both reduced by about a factor of 10.

- keantoken

[juma]

Originally Posted by swordfishy

...What I meant was what a the pros/cons of using a npn input and pnp vas versus a pnp input and npn vas? ...

I don't see that NPN BC550C is more linear than PNP BC560C. Maybe you mean N-ch MOSFETs versus P-ch ?

[mikelm]

yes - I was referring to mosfets

[mikelm]

swordfishy

I looked again at 2 x zvp4424 - seems to work very well.

I wonder did you ever try 2 x 3310A's ?

perhaps it could be the perfect solution

[juma]

Originally Posted by mikelm

yes - I was referring to mosfets

Yes, transfer characteristic shows that N-ch (IRF610) seems more linear at small currents. ZVN3310 look even better, but to my ears the BC550c takes the cake...

[keantoken]

The BC560 is PNP which means it will most likely have worse Early effect, which will decrease it's gain and linearity. According to the Fairchild datasheets Hfe is less linear then the BC550. They also confirm that Early effect is worse.

- keantoken

[keantoken]

BTW, has anyone compared the Fairchild and Phillips BC5x0 or does it matter? Because in the datasheets they are different in terms of Beta and junction capacitances. The Phillips version seem to be inferior.

- keantoken

[juma]

Originally Posted by keantoken

The BC560 is PNP which means it will most likely have worse Early effect, which will decrease it's gain and linearity. According to the Fairchild datasheets Hfe is less linear then the BC550. They also confirm that Early effect is worse.

- keantoken

Keantoken,
those are valid facts but you have to look at them in perspective:

Early effect (AKA base-width modulation) is when the voltages applied to the base-emitter and base-collector junctions are changed, the depletion layer widths and the quasi-neutral regions vary as well. This causes the collector current to vary with the collector-emitter voltage.
So how big is the Vce variation of Q1 (BC560) ? We are talking tenths of mVs here - negligible...

Also, the hfe linearity (i.e. its dependance of Ic) is non-issue because Q1's Ic is pretty much constant at 1mA.

[mikelm]

swordfishy,

Just a thought.

Thinking about your comments and spicing what you have stated as your favourite versions you seems to like very high feedback designs but it is worth bearing in mind that this extra feedback has two different effects. One is to reduce distortion but the other is to increase power supply noise rejection ratio.

If you have a noisy power supply you are very likely always going to choose the higher feedback designs because lower noise almost always sounds better, but if you have a whisper quiet PSU arrangement already, it may be that you choose differently regarding the feedback levels.

mike