The Cross Quad diff

[Terry Demol]

Hi all,

I have the cross coupled pair sussed out completely. Sorry, have
to go to bed will reply in morning, it's been a big week.

Cheers,

Terry

[Eva]

Humm...

From now on, I think that I'm going to use the last circuit in all my projects. It has another nice feature that makes it superior to the 4 transistor version: The Vbe corrected transistors are operated all at a similar Vce. This is a great thing because d(transconductance)/d(Ic) "slope" changes a lot with Vce, and a transistor operating at 1.4V Vce is not going to match at all with another one operating at 40V or so.

[Elso Kwak]

The cross-quad circuit was discussed here in connection with a moving coil cartridge preamp:
http://www.diyaudio.com/forums/showt...099#post377099

[Terry Demol]

Originally posted by Elso Kwak
The cross-quad circuit was discussed here in connection with a moving coil cartridge preamp:
http://www.diyaudio.com/forums/showt...099#post377099

where

[Elso Kwak]

Originally posted by Terry Demol


where

further on in that thread

[darkfenriz]

Originally posted by Eva



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.

Yes, after any type of overdrive it can severely oscillate, therefore this a nice tim generator, or be positively fed back (lack current thru one leg implies switching off of its 'driver') so that everything turns into smoke.

[darkfenriz]

Originally posted by Eva
A funnier version:

It should be also latching-free and phase-reversal free.

Are you sure?

[Eva]

Originally posted by darkfenriz



Are you sure?

Try it, it simulates fine. According to Pspice, it behaves well no matter how much overdrive is applied, as opposed to the original cross quad that latches badly.

Also, according to Pspice, it has slightly better phase margin than the original circuit, that has a peak at 22Mhz (BC556B) that never disappears (those naughty base currents badly routed), no matter how much compensation is added (in all circuits that I tried Pspice requires 10pf capacitors between each input and output node for stability). THD and gain figures are similar.

[davidsrsb]

Originally posted by darkfenriz

Are you sure?

Behaves OK on my sim, but what is the advantage? The gain is just x10 and 20MHz -3dB

One thing about the basic crossed quad is relatively poor supply rejection. Further cascoding is needed, but we probably need a higher voltage stage anyway.

[Eva]

The advantage is not the gain but the Vbe linearisation. Anyway, classic amplifier circuits have no room for more gain.