Maths Problem?

[ms660]

Never been good at maths, anyways here goes, two formulas from the link below can be found under the heading Operating Conditions:

http://www.r-type.org/articles/art-167.htm

I've also replicated them here, first formula......Pout = (Vin(RMS) * gm)squared * RL

2nd formula....Vin(RMS) = (1/gm) * (Pout/RL)0.5.

I've used the following figures for the 1st formula:

Vin(RMS)=7 Volts

gm = 7mA/V

RL = 2,400 Ohms.

Punching those into the 1st formula gives Pout as 5.76 Watts.

Entering 5.76 Watts into the second formula and still using 7mA/V and 2,400 Ohms doesn't return a Vin of 7 Volts(RMS) Well not the way I'm doing it

Anyone know what's wrong or where I've gone wrong?

Lawrence.

[Jon_G4MDC]

You are taking the 0.5 in 2nd formula as meaning square root?
(Pout/RL)**0.5 is meant I think

[ms660]

I took it as multiply by 0.5.

Lawrence.

[Jon_G4MDC]

You squared in the first formula so it need sqrt in the second.
They are the same thing rearranged.

[ms660]

Ok, thanks, so just to confirm I only square root Pout/RL?

Lawrence.

[Jon_G4MDC]

Yes worked for me 5.76/2400=2.4E-03 sqrt of that 0.489
0.489/7E-03=6.998
close enough < he wasn't meant to be rolling his eyes - just smiling!

[ms660]

Yes, I just tried that and got the same result as you did but why 6.998 Watts instead of 5.76 Watts?

Lawrence.

[G8HQP Dave]

6.998 volts?

[ms660]

Bingo, cheers Dave, all ok now.

Lawrence.