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In Reply to: OPamplifiers, Replacements posted by groman on January 05, 2002 at 02:37:11:
Hi,I have linked below to a real interresting article from 1957, which is the first extensive analysis of transient distortion and overload in feedback Amp's I have come across. The fact that it is applied to Valve Amp's makes it not less relevant, but moreso, as Valve Amp's operate with much less feedback and more inherent linearity than Solid State Amp's.
Later T
PS it's from this general site:
http://web.mit.edu/cheever/www/main.htm
Follow Ups:
Hi Thorsten,
Do you do this on purpose or do you have such a bad scanner? I can't read it; my eyeballs pop out!
ELSO
Hi,I didn't scan this, it's on someone elses web-site. I printed it out many moons ago and yes, reading it suX.
Ciao T
Good work. Norman Crowhurst was a 'visionary' in the audio field. Today , it is easier to measure and explain his vision. I keep and read virtually everything that I can find that he wrote, and have been doing this for decades.
I do not like global feedback
Separate voltage amp and output buffer
Output buffer: Bipol transistor, if needed 2T in compl foldback
this is feed by a constant current source.Regarding diffrential longtail pair input.
I prefer them feed by a resistor at emitters,
instead of const current source. Beacause of the
fact that current increases in T1 while decreases in T2
That is not the case with resistor in emittrrs.
And as I said local feedback.
I never want more than 40dB feedback, gain= 100*gain of closed loop
so resistors in emitters is standard.I right now favour single power supply.
For regulation, if nessesary, TL431, adjustable zenerdiod,
amplified with output transistor.I am intrested to try and use JFET (2SK117) in
input stages.
Have also begun study different TUBE-preamps-solutions.
???
Hi gronam,
The answer is better CMRR with the active constant current source. CMRR is short for Common Mode Rejection Ratio.
You can even go as far as Peufeu in France and cascode this current source again to cancel thermal distortion and the Early effect.
Its all in the conjuring-box of tricks of the solid state engineer-magician!
ELSO
You did not quite understand my question.I point out one possible benefit
when using resistor to feed the emitters,
In "my philosphy in design of amps".Obviosly it had skipped your attention,
when you read it.So I ask you comment on resistor feed
emitters in differential input-stages,
once again.
The CMRR was already known by me.questioning
groman
Hi groman,
You mean emittor degeneration? Or simply put resistors in series with the emittors of the input differential pair? Yes it provides local feedback and it is done all the time by everyone.
Its not a new trick in the conjuring-box........
ELSO
...be bad.
That is what happens in a resistor feed longtailed pair,
when feedback is applied.And that is certainly not the case in a stage feed
from transistor generated
constant current source, which you uses!So there I might be better off
although I loose some CMMR,
which I can create in another way.Thank you
I am still ahead
thinks
groman
Hi Groman,
I am not still sure I understand what you mean.
Do you mean by the title of your post that the cutrrent through T1 can not be equal to the current through T1??? By "current" I mean the curent from collector to emittor(bipolar transistor) or from drain to source(FET)
I just found a nice schematic on DIYaudio.com, just click on the link below and this makes it more easy to talk about the circuit.
I do use the resistors R1 and R2 in the source leads AND a active constant current source. If you want the said currents to be equal all things have to be equal. That includes the two FET's and the two resistors.
This goal can only be achieved, for the DIYer with a monolitic Dual JFET such as the 2SK389 or the industry standard 2N5912. If the two halves of the input differential pair are equal the current from the current source will split up equally between the two halves.
Anyway the differential pair only works by virtue of the exact equality of the two halves. Only James Bongiorno will argue that the matching of the inut transistors is not necessary, but that's a other story.
I also have practical experience in matching transistors for input differential pairs, matched does sound a lot better! And Duals even more!
ELSO
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