# Zen circuit theory

#### ding

##### Member
Ok I give up. Who can tell me how a 10W output figure is calculated for the zen amp. With a Vds of 17V and Ids of 2A why doesn't the Pout = VI = 34W. Has this got something to do with the DC filtering Capacitors C3 & C4 or maybe resistor R2 for bleeding DC to ground. Can someone please clear this up for me.

Cheers

Dan

#### Keithj

##### Member
Try...
Power in a load = V^2/R
Where V is the RMS output voltage and
R is the load impedance.
(I know, I know, this says noting at all
about phase angels and other A.C things)
but it is indicative.
regards.

#### ding

##### Member
This may be it

Correct me if I'm wrong.

The voltage Vds = 17V is the Vpp of a sine wave output. To convert this to Vrms = 17/(2*sqrt(2)) = 6V. RMS power should be Prms = VrmsIds = 6*2 = 12Wrms.

If this method is correct why do I get 12Wrms output and not 10Wrms as described. If I am way out can someone please set me straight.

Cheers

Dan

#### GRollins

##### Member
Dan,
Subtract the 4-5V to the gate (the bias voltage) from the available voltage.
17V-4V= 13V (available [peak] voltage swing)
13V*.707= 9.19V (RMS voltage)
9.19V/8ohms= 1.15A (current through 8 ohm load)
9.19V*1.15A= 10.6W (close enough for government work)

Grey

Just the ticket

Thanks Grey