Massive 2nd harmonic reduction, why does it happened?

[aparatusonitus]

Hi,

Could someone explain why do I get something like 30dB less 2nd harmonic with the differential on the right?

[jechentau]

because the voltage swing over the jfets is way smaller, due to their d/s voltage being way more constant (still minus the rBE of the bipolars though)

and, depending on the R's values, having a properly large voltage over them period possibly.

[Gordy]

My theory: on the right hand side...
the sources are shorted together
therefore R3 and R4 have no impact on the gain (they are just part of the 'long tail')
therefore gain is affected by internal resistance of fets, not internal plus R3 and R4
therefore open loop gain is much higher
therefore your loop feedback is effectively much higher
therefore the feedback reduces the distortion

[AKSA]

Gordy is on the money.....

By connecting the sources, you raise OLG hugely and therefore increase loop gain, and with it, error correction. The LTP degeneration becomes an irrelevance....

Hugh

[jechentau]

ah ok yes good point,
but threadstarter: could you post the schematic of the entire circuit and values?

in my amp i sortof in- or exclude the input j-fet more or less inside the feedback loop,
to generate just the right amount of 2hd to (partially) cancel the tube's 2hd in antiphase.
the j-fet has about 10% of the total feedback so to say.

[aparatusonitus]

Thank you guys for your input...I think that you are both right, but I'll have to check the difference in open loop response in each case to see if it's all about more available negative feedback. Circuit is based on AD797 schematic.

[aparatusonitus]

I checked the OL in both configuration and here are simulated response: First image is for left hand configuration, second one is for right hand configuration.

As you can see difference in OL gain is about 5-6dB, so if sim is not playing tricks with me, I think there is more to it then higher OL/more usable negative feedback for right hand configuration.

Also, I'll try to check explanation by jechentau latter tonight.