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-   -   active plate load v.s. constant current... (http://www.diyaudio.com/forums/tubes-valves/5043-active-plate-load-v-s-constant-current.html)

mwmitchell 6th August 2002 01:41 AM

active plate load v.s. constant current...
 
Hi everyone. I have been looking into constant current sourced, simple gain stages and also active load stages. I am some what of a beginner here, so i'm sorry if i'm not making any sense. Would'nt an active load present a constant current, and also the same, the other way around? I mean, don't they accomplish the same thing?

Also, how exactly do you "design" a C.C.S. or an A.L.

Thanks for any info at all!!!!!!!!

- matt

dice45 7th August 2002 07:42 PM

Matt,

CCS and AL do the same, they enforce a specific current, no matter what voltage is across the CCS. Within limits, of course. They simulate (almost) infinite plate resistance and so the tube is running at the maximum gain possible. Any decrease in plate load decreases the actual gain of the stage. A : Gain, G_m : transconductance, r_p : plate resistance, R_L : load resistance
A = G_m/(1/r_p +1/R_L).
A becomes biggest if R_L is appearing infinite ac-wise.

How to design a decent CCS? Better ask the solidstate gurus, i have some ideas but untested. I prefer plate chokes for that purpose. AC-wise they do the same.

A constant current device has another beauty: it forms infinite resisitance between power supply and tube. This improves power supply ripple rejection.

mwmitchell 8th August 2002 07:12 PM

More ?'s on CCS and other things...
 
Thanks for the info! That helps a lot. And it makes sense to me now. I was wondering if you could just show me you ideas for current sources. I would be very interested. And anymore info would be great.

I guess I don't understand how the CCS keeps a constant current even though a.c. is being amplified. Wouldn't the a.c. (audio input) change the voltage to the CCS and then the CCS would then too change it's current?

Also, do you now exactly what happens when a resistor becomes dynamic impedance? i mean, how does the plate resistor become higher in value (for a.c.) than it's resistive value?

Does a cascode stage (where there is a FET or other SS device driving the cathode - the audio input - grounded grid?) benefiting from CC at all?

Thanks for any info at all on this stuff! Anyone!

- Matt Mitchell

dice45 8th August 2002 09:34 PM

Matt,
sorry, i cannot give advice on a topic i am trying to learn and understand myself.

A simple CCS is a MOSFET with a source resistor adjusting the current and the gate sensing the voltage across that resistor. the whole circuit is floating, current flows from source to drain, no ground reference necessary.
It works but i have been informed that high frequency performance is less than satisfactory.

CCS gurus, please cut in.


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