The Cross Quad diff

[mirlo]

What's the claimed advantage of the crossquad? Is it supposed to linearize the diffpair? The gain improvement can be acheved with a simple cascode ...

[davidsrsb]

The circuit seems very touchy . Q1 is working as an emitter follower. Q2 is the gain stage and Q3 runs in coomon base.
As Q2 turns on more. Q4 turns off and Q1 also acts as a common base stage. This is why the current in Q1 decreases as the base goes more positive. So this is positive feedback but not enough to oscillate.

The circuit simulates as being much more linear than the LTP with far more abrupt limiting

[Eva]

The cross quad LTP is just a low part count version of this, I think:



It's a darlington LTP with the inner transistors cascoded to operate at a fixed 0.7V Vce. However, the collectors of the outer transistors are cross-connected to the load resistors and this ruins the gain and phase characteristics at high frequencies because the base currents of the inner transistors contribute to the wrong output leg (a plain darlington would behave better).

I don't see any advantage in that circuit other than part count reduction, because the higher gain, the 2nd order roll-off and the cumbersome latch-up are absolutely undesirable characteristics. We all have been adding degeneration resistors to our LTPs for a long time now in order to reduce their gain, so why would it seem attractive to add 30dB of gain now?

[davidsrsb]

Not really
At audio frequencies the input transistors collector currents are nearly constant and modulating the bases of the output transistors has little effect.

[mirlo]

It is different from the darlington LTP because the emitter follower/cascode that drives the base of the other side, carries current of the opposite side driving it. So it has Vbe that varies opposite to the Vbe variation of the diffpair transistors.

In other words the Vbe's of the emitter followers adjust slightly to predistort the signal that goes in to the emitter coupled pair. This enhances linearity.

It seems like a really elegant solution, would work with enhancement mode MOSFETs just as well, to reduce input stage nonlinearity, and MOSFETs wouldn't suffer the saturation/reverse operation problem when overdriven that can cause latchup.

[ilimzn]

Originally posted by mirlo

It seems like a really elegant solution, would work with enhancement mode MOSFETs just as well, to reduce input stage nonlinearity, and MOSFETs wouldn't suffer the saturation/reverse operation problem when overdriven that can cause latchup.

That was exactly what I wrote about in my post. Unfortunately, simulating it with the MOSFET models I have at hand is even more touchy than with BJTs. Also, if you use all the same MOSFETs in tbe simulator (impossible in real life, of course), the simulated distortion goes down to the order of the sim numeric error

[Eva]

Originally posted by davidsrsb
Not really
At audio frequencies the input transistors collector currents are nearly constant and modulating the bases of the output transistors has little effect.

At RF nothing is constant and base currents are no longer negligible in comparison with collector currents. Good routing of base currents makes the defference between poor and good stability.

Originally posted by mirlo
It is different from the darlington LTP because the emitter follower/cascode that drives the base of the other side, carries current of the opposite side driving it. So it has Vbe that varies opposite to the Vbe variation of the diffpair transistors.

In other words the Vbe's of the emitter followers adjust slightly to predistort the signal that goes in to the emitter coupled pair. This enhances linearity.

It seems like a really elegant solution, would work with enhancement mode MOSFETs just as well, to reduce input stage nonlinearity, and MOSFETs wouldn't suffer the saturation/reverse operation problem when overdriven that can cause latchup.

MOSFETs would require both transconductance and Vgs matching in 4 devices, this is not a trivial task at all.

I think darlington-like approach still wins:



This works the same or better provided that the model of PNP and NPN devices is chosen to have similar transconductance around 1mA. Furthermore, the current mirrors with high value resistors work only for audio frequencies and roll off quickly at RF, thus improving phase margin. As a bonus, this circuit should not suffer from latching or phase reversal issues, altough turn-on after a overdrive period relies on leakage currents and positive feedback, so it may probably take some hundreds of nanoseconds. Two additional 1uA current sources may be added to ensure class A operation of the PNP devices and proper recovery after overdrive.

Note that ultra-precise current sources are not required, because what matters here are just current slopes.

[Eva]

A funnier version:



It should be also latching-free and phase-reversal free. The four lower transistors and their emitter resistors would require some matching for best results, though.

[mirlo]

For the version with the PNP emitter followers, it looks like you might need another output leg on the current mirrors on top to actually have an output (?)

I have a bad habit of presuming matching is no problem.

Does anyone make monolithic MOSFET arrays? Like MAT-04 except MOS?

[mirlo]

Oh, silly me, I see you are using the current mirror degen resistors on top to double as load resistors. Cool.