5v333

Average plate/transfer characteristics
« on: June 27, 2020, 10:48:01 AM »
im quite familir with the plate characteristics for making loadlines and calculating 2nd 3d and 4th harmonic distortions.
but recently ive started to have fun with the transfer characteristics. i guess the curves found there is a gm curve. does that sound right?

something that i noted (if im doing this the right way) is that if i do a thd analysis of the curve in the transfer charecteristics i can get obvious anomalies depending on the bias point while in the plate characteristics i can get very nice linearity doing loadlines with the same bias points?

Is the gm curve showing intrinsic thd or even something that translates to IMD?

Is it good pratice to find a bias and load that shows good linearity on both plots?

Have a nice day!


abbey road d enfer

Re: Average plate/transfer characteristics
« Reply #1 on: June 27, 2020, 02:26:57 PM »
Gm is the expression of plate current vs. grid voltage. It increases as the current increases. At the same time the plate resistance decreases. Since we are generally considering a voltage-to-voltage transfer, it is more adequate to consider the variations of Mu. But in fact, it's really the overall circuit, including the plate load, that must be considered.
There is a sweet spot where the Gm variation is compensated by the inverse variation in equivalent plate load.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
Star ground is for electricians.

5v333

Re: Average plate/transfer characteristics
« Reply #2 on: June 27, 2020, 03:17:04 PM »
your reponse made me put mA/Vg upside down and now it looks like what im thinking about is related to cathode impedance..
which - against a stiff cathode resistor - would make the final amplification wooble/modulate a bit.
in one bias point the curve in the transfer characteristics looks like plain 2nd harmonics while with a slightly lower mA or gm, the curve starts to look like an S or whatever is the proper term.

i havnt made a thd comprison between the two variants yet.


 

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