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Aikido Zo Calculation

the formula in the pdf is this:

Zo = [rp/(mu +1) + R8] || R13 || [rp + (mu + 1)Rk]

I'm pretty good at building things, but math is not my strong suit. I'd like to figure out the output impedance of this 12BH7A LV Aikido but I'm stumped by 1) figuring out Rp. How do I get the value? And 2) Parallel lines - Er, what do they and 'in paralell with' mean?:
 
Rp you get from the data sheet, at your chosen bias. There may be a chart of Rp, or use Rp=mu/gm so get mu and gm from the chart.

Parallel lines used in a formula in pairs like that usually mean resistances in parallel. The first term is the bare cathode follower output impedance, the last term is the active load impedance, and the middle is an actual physical resistor. Work out each term, then calculate their parallel combination. You will probably find that you can ignore the last term as it will be much higher than the others.
 
To get a more or less accurate figure through calculation requires a good deal of working with the plate curves and testing your particular examples at the OP of interest. For me, it's easier just to test the finished circuit.

Use a pretty high-value ground referencing resistor on the output (say, half a meg) to simulate zero load, and adjust the circuit with a generator to give about a tenth of a volt on the output. Then put a pot (20K or so) across the output terminals, and dial the resistance down until you get half the original voltage. Measure the R across the output, that's your Zout. No speculating, no **, no theorizing. Amen.

Be prepared to be a little disappointed; it probably won't be as low as webgeeks and manufacturers claim.

Aloha,

Poinz
AudioTropic
 
It can be difficult to test the finished circuit before you have built it. By then it may be too late to redesign whatever follows it. The best thing to do is estimate the output impedance from 1/gm (or Rp/mu - same thing). If this is close enough, then fine. In most cases the exact figure doesn't matter too much anyway.
 

regal

Member
2004-01-04 8:41 pm
MD
the formula in the pdf is this:

Zo = [rp/(mu +1) + R8] || R13 || [rp + (mu + 1)Rk]

I'm pretty good at building things, but math is not my strong suit. I'd like to figure out the output impedance of this 12BH7A LV Aikido but I'm stumped by 1) figuring out Rp. How do I get the value? And 2) Parallel lines - Er, what do they and 'in paralell with' mean?:


A || B = (A * B) / A + B