Cascoded LTP input stages

[Mindaugas]

Has anyone tryied to use it, instead of tradicional ltp, and which advantages this approach has.

I found interesting site and different and interesting input stage approches.

Input stage variants

Now in my power amp, I am using tradicional LTP, but I'm planning to try B example. But I' m concerned about CFP LTP stability.

Maybe someone has more experience . And is it worth cascoding ltp at all.

[AndrewT]

Hi,
I would not call any of those variations cascodes.
They all appear to be complex transistor multiples.

None have the cascoding base/gate referenced to a fixed voltage.

[Mindaugas]

Hi Andrew,

Thanks for correcting me. Sorry for mistake. I'm interested in LTP-CFP input stage. Is it better than a traditional diff. input stage and why. I used only tradicional diff. input stage in my amp with constant current source and current mirrors.

[aguantesoda]

hello! how is the functioning in "b" configuration?

*Hola! Como es el funcionamiento en la configuración "b"?

[forr]

AndrewT,
C and D are real cascodes, they could be called "cascode followers" (like John Linsley-Hood did) or "floating cascodes" : the bias voltage of the common grid is referenced to the base of the input transistor, it could also be to the emitter.
The aim is to maintain the Vcb and Vce voltage of this input transistor nearly constant, this is all the beauty of this kind of cascodes which is lost when using a fixed voltage referenced to 0V or power supply unless there is no emitter degeneration.

[AndrewT]

Hi Forr,
thanks for the clarification.
I had seen similar but never thought of them as cascodes. They seemed too different from the classic cascode that I was used to.

[forr]

Hi AndrewT,

I must a bit more clear about emitter degeneration : in the case of a LTP pair, as shown in the schematics (which come, I think, from Peufeu's site), the input transistors without emitter degeneration suffer from voltage variations if the common grid transistor has its grid bias referenced to a fixed voltage.
An often seen cascode scheme is bipolar transistors in common base used as outputs of the cascode circuit with their base voltage referenced to the tail of the input pair.

[Chucko]

Your figure C is a variant of the usual scheme, where the gates of the FETs are connected to the R7/R8 junction. Biasing the FETs from this point has the advantage of reduced input capacitance. Your scheme trades increased input capacitance (due to Miller effect) for constant Vce on the input bipolars.

[jcx]

more diff pair hacks in:

cross-quad and cascomp circuits

Originally posted by jcx
http://db.audioasylum.com/cgi/m.mpl?...rophead&n=3169


http://www.analog.com/UploadedFiles/...917MAT04_d.pdf

(fig 6)


Hawksford also explored some related diff pair input linearization techniques

J4 DISTORTION CORRECTION CIRCUITS FOR AUDIO AMPLIFIERS, M.O.J. Hawksford, JAES, vol.29, no.7, 8, July/August 1981

At http://www.essex.ac.uk/ese/research/...lications.html

Somewhere you might also find Gilbert’s “multi-tanh” proposal

Its not clear that these techniques are necessary in audio amplifier design - high loop gain, emitter degen and bootstrapped cascode appear to be enough for super low distortion in audio amp inputs

do you really need to do better than ExtremA's input stage? (or the converse of the question - is the rest of your amp already better than this)
ExtremA, class-A strikes back?

[thanh]

Hi !
If I can understand Sefl's book, CFP LTP is only good for analysing input stage. In the whole amp, CFP has no any improvement.

A good paragraph
"With the 1mA current chosen for the cfp stage the gain will be increasing with current, and this will add to the distortion from other sources of non-linearity. What we need is a device with either constant current gain at 1mA +/- 10% or even better a reducing gain with increased current so that other non-linearity can be partly cancelled. The best I found so far is the BC650 (or the higher voltage BC651) which has maximum d.c. current gain around 1mA, but unfortunately appears to no longer be available. Another alternative is the 2SC2911 which has an almost flat current gain from under 1mA up to 30mA, but this is a medium power device with lower gain"
(from http://www.angelfire.com/ab3/mjramp/ampdes1.html)

MikeB said that CFP boost gain--> compensate. Oh! I fear it