The Cross Quad diff

[darkfenriz]

Eva
Yes, I got it! some of bjt's do saturate at overdrive, but this does not affect output.
Good job.
This can be useful in projects with differential stage having huge effect on overal harmonic distortion.
regards

 

[Eva]

I have the 6 transistor version working in breadboard now, and indeed it works. Effectively, it has negative input impedance, and this necessarily causes oscillation at HF. My prototype is made with On-Semi BC556B (these feature very low Vbe change with Ic) and oscillated at 16Mhz like crazy until I added 22pF miller capacitors to make the input impedance back positive at HF. I'm also considering resistive impedance cancellation.

The real prototype confirmed that my version does not latch up and does not suffer from phase reversal when overdriven.

I'm using this input stage in a N-channel rail to rail project where the output HEXFETs are part of a CFP. Such a circuit is very hard to stabilise, but in return for the effort it does not require any thermal compensation, it does not suffer undesired cross-conduction, and the chore of matching output devices is avoided by using one CFP cell for each one. I'm going to open a thread to show it one of these days since I've just got to work a preliminary version with +-18V rails and one pair of IRF640

 

[Elso Kwak]

I have the 6 transistor version working in breadboard now, and indeed it works. Effectively, it has negative input impedance, and this necessarily causes oscillation at HF. My prototype is made with On-Semi BC556B (these feature very low Vbe change with Ic) and oscillated at 16Mhz like crazy until I added 22pF miller capacitors to make the input impedance back positive at HF. I'm also considering resistive impedance cancellation.

The real prototype confirmed that my version does not latch up and does not suffer from phase reversal when overdriven.

I'm using this input stage in a N-channel rail to rail project where the output HEXFETs are part of a CFP. Such a circuit is very hard to stabilise, but in return for the effort it does not require any thermal compensation, it does not suffer undesired cross-conduction, and the chore of matching output devices is avoided by using one CFP cell for each one. I'm going to open a thread to show it one of these days since I've just got to work a preliminary version with +-18V rails and one pair of IRF640

Why not begin with building the circuit in the MAT04 datasheet and go from there???? BC556B not matched transistors

 

[Eva]

Of course these are not matched, but I got 100 BC546B and 100 BC556B (both On-Semi) for less than 10 euro Do you know how much does a MAT04 cost?

 

[Elso Kwak]

Of course these are not matched, but I got 100 BC546B and 100 BC556B (both On-Semi) for less than 10 euro Do you know how much does a MAT04 cost?

Ever heard of samples Eva? Are you a woman?

 

[Terry Demol]

I have the 6 transistor version working in breadboard now, and indeed it works. Effectively, it has negative input impedance, and this necessarily causes oscillation at HF. My prototype is made with On-Semi BC556B (these feature very low Vbe change with Ic) and oscillated at 16Mhz like crazy until I added 22pF miller capacitors to make the input impedance back positive at HF. I'm also considering resistive impedance cancellation.

The real prototype confirmed that my version does not latch up and does not suffer from phase reversal when overdriven.

I'm using this input stage in a N-channel rail to rail project where the output HEXFETs are part of a CFP. Such a circuit is very hard to stabilise, but in return for the effort it does not require any thermal compensation, it does not suffer undesired cross-conduction, and the chore of matching output devices is avoided by using one CFP cell for each one. I'm going to open a thread to show it one of these days since I've just got to work a preliminary version with +-18V rails and one pair of IRF640

Eva,

The original croos coupled circuit has fewer components and
theoreticall perfect linearisation of VBE voltage, especially when
used with an almost ideal and perfect matched device such as
MAT04.

Non of these will have that degree of inearisation, however I can
see #2 (post 17) has the advantage that it can swing significant
IP voltage. I like this one and have experimented with it myself.

However I found that at the end of the day the non linearity
due to VBE is only 1 factor amongst many that add up to
front end linearity.

#3 appears to me to be pretty much identical to Hawksford IP
arrangement from paper "distortion correction circuits for audio
amplifiers'- see fig 9, and cannot swing significant IP voltage
so the cross couple pair would be superior if implemented
correctly.

But as always YMMV so have fun.

I'd be interested to see how all these circuits measure in real life,
sims tell little to me, and how theory translates to practice.

Cheers,

Terry

 

[AKSA]

Stability might be improved if the two cross coupled bases were interconnected with three resistors each of 1K.

Cross coupling would then be between resistors 1/2 and 2/3. This dilutes the level of crosscoupling , molifying the extreme effects.


Cheers,

Hugh

 

[Eva]

According to the datasheet, for Ic=1mA my BC546B show unity gain at 100Mhz. On the other hand, a 22pF capacitor shows approx 68ohms impedance at 100Mhz (the value of emitter resistors that I'm using). So I tried a 22pF between both inputs of the 6 transistor version of the quad LTP, and it eliminated all the oscillation.

I think that compensating it this way is much better than using miller capacitors. Furthermore, in my amplifier the negative rail is bootstrapped to ensure rail to rail output, and this was causing me trouble because the 22pF miller capacitors that I was previously using had to be necessarily tied to that negative rail. The circuit turned unstable everytime the negative rail was bootstrapped and followed the output of the amplifier. The new compensation solved it.

Also, I've placed a 15K resistor between both inputs of the LTP. Considering that my transistors have a typical (measured) hFE of 250, and that 68*250=17000, that resistor should produce a good degree of cancellation of the otherwise negative input currents in the audio band, thus making input current almost constant and input impedance huge.

 

[mirlo]

This is getting interesting. I hope you will post a new schematic when you are finished making adjustments, Eva ...

I guess a next step might be to try to invent a similarly absurdly linear VAS or perhaps output buffer stage.