Questions About This Local NFB Scheme

[jan.didden]

 

[20to20]

I'm not sure why there is so much debate over something that has been defined for almost a century.

I don't think there really has been a definition. It's just been assumed local means a shorter loop than global. But if you want to get some terminology better suited for different versions then you could say INTRAstage FB, for plate to plate (which is really plate to grid of the output through its coupling cap) and say INTERstage FB for FB going back to include one or more prior stages but not all stages, and then GFB for a system loop.

 

[jan.didden]

If you think 'I don't think there is ...' doesn't mean there isn't.

I am again surprised why people want to invent all kinds of terms that nobody else except they understand? Where does this obsessive behavior come from ??

From output to input is global. Everything else is local. What's so hard to understand about that?

It is abundantly clear if you want to see it, that the original title of this thread results from limited understanding of the term 'local feedback'. Local feedback does NOT mean that you return the feedback from output to some 'local' point.
The solution to limited understanding is NOT to invent other terms in an attempt to cover up the limited understanding. Rather, the solution is to read up and Get It.

Jan

 

[20to20]

That's your definition. Imprecise. Nebulous. Vague.

 

[jan.didden]

Haha! Is that a trick to let me do your legwork? Will not work, if you are happy with what do or don't know, I'm not going to force you to your happiness.

Jan

 

[kodabmx]

№ 4 — for resistors P = E² / R … = 8² V ÷ 150 = 64 ÷ 150 → 0.43 W. Why the 5 W model? I'm not against it generally, but still … a solid 2 W would be smaller. [/indent]Nothing really matters as long as the P is well below P_MAX

⋅-=≡ GoatGuy ✓ ≡=-⋅

Here's a picture of a 3W 47k resistor used as a dropper for an LED. It dissipates 1.5W... It's also in a chassis that is basically an oven. As a rule, I build with the largest resistors practical now! Those 5k6 resistors are series/parallel connected for Pd of 12W with a circuit that needed 2.8W I had them already though, so why not?

Since the burning resistor is just a dropper for an LED, I don't care at all that it's discoloured If it explodes and takes the LED with it, J'men câlice

 

[6A3sUMMER]

Take a typical triode:

Use a Regulated grid power supply and a Regulated plate power supply.

Put the grid at -10V.
Put the plate at + 100V.
Put the cathode at 0 Volts
Wait for 1 day for the current to stabilize (are we there yet?).
Then measure the plate current (Ip1)

Put the grid at -10V.
Put the plate at +200V.
Put the cathode at 0 Volts.
Wait for 1 day for the current to stabilize (are we there yet?).
Measure the plate current (Ip2)

200V - 100V = 100V (the change in plate voltage).
Ip2 - Ip1 = Delta Ip (the change in plate current).
100V / Delta Ip = the plate resistance, rp.

Would you call that negative feedback?

After 1 day, the Miller Effect capacitive current is much less than KTB.

 

[Ketje]

Take a typical resistance:

Regulated plate power supply.


Put one end of the resistor at + 100V.
Put the other end at 0 Volts
Wait for 1 day for the current to stabilize (are we there yet?).
Then measure the resistor current (Ir1)

Put one end of the resistor at +200V.
Put the other end at 0 Volts.
Wait for 1 day for the current to stabilize (are we there yet?).
Measure the resistor current (Ir2)

200V - 100V = 100V (the change in resistor voltage).
Ir2 - Ir1 = Delta Ir (the change in resistor current).
100V / Delta Ir = the resistor resistance, r.

Would you call that negative feedback?

After 1 day, there is no capacitive current, test is done with DC .

Mona

 

[FlaCharlie]

I'm not concerned with semantics so I probably shouldn't have mentioned local vs global thing.

I am not splitting hairs, it is not about semantics. It is about accurately describing something so that anyone anywhere in the world understands what you mean. Modifying definitions and making them internally inconsistent as well because you have this urge to add something personal to our heritage collapses the community to just you and your buddies. A huge step backward.

I'm not technically astute enough to offer an opinion in the Local vs Global debate. Whatever the correct nomenclature is, I'm enjoying the amp. So thanks to all who helped.

I apologize if my reference to this as Local is hastening the end of civilization as we know it.

 

[6A3sUMMER]

Ketje,

Creative!

You "quoted" me, but you changed my words.

I guess that you were saying the triode (rp) was acting just like a resistor.
And, most people will not argue that a resistor has negative feedback.

Thanks!

 

[kodabmx]

Isn't it true that a triode is effectively an electrically controlled resistance vs a current source like all others but SITs?

 

[TonyTecson]

i can live with that....a triode is more like a Thevenin source with rp as internal resistance....