#### Nomi 1114

Joined Dec 4, 2017
124
Dear All,

Hope you all are doing well with good health. Here i would like to ask one question that i have designed RF power amplifier by using LTSpice, where at output i can obtain about 35V and -16W power level, according to my requirement it's fine. But i am afraid why this power is in negative (-) while i need positive (+) 16W ...

please somebody help me in this regard. I shall be highly grateful to you for this act of kindness.

Regards

#### ericgibbs

Joined Jan 29, 2010
12,270
hi Nomi,
In LTS, the polarity of current direction in a resistor is dependent upon the resistors orientation.

E

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#### Nomi 1114

Joined Dec 4, 2017
124
hi Nomi,
In LTS, the polarity of current direction in a resistor is dependent upon the resistors orientation.

E
Dear Respected ericgibbs,

Thank you thank you very much for your kind help and suggestion. I rotate the Resistance now i get the positive signal ... Highly appreciated. Bundle of Thanks ... !!

Regards

#### Papabravo

Joined Feb 24, 2006
15,523
Dear Respected ericgibbs,

Thank you thank you very much for your kind help and suggestion. I rotate the Resistance now i get the positive signal ... Highly appreciated. Bundle of Thanks ... !!

Regards
In a real circuit there is no such thing as "negative power". If you think that, you are chasing a chimera. The power in a resistor is:

$P_R\;=\;I^2R\;=\;\frac{E^2}{R}$

It is the squaring that gets rid of the minus sign. In the LTspice waveform viewer you can also legitimately take the absolute value of current times voltage. That is:

$P_R\;=\;|V\times I|$

#### ericgibbs

Joined Jan 29, 2010
12,270
hi N1114.
Clip from LTS Wiki.
E
Why is the current going the wrong way in a resistor?

LTspice has to pick a reference current direction, and it can only be one way. Think of it like the current convention used in Kirchhoff's circuit laws. Simply rotate your resistor 180 degrees in the schematic and the current will be to your liking. Alternate: Download File:Res.asy and place in your /lib/sym folder. This symbol has a small arrow indicating the defined way current flows.

#### Papabravo

Joined Feb 24, 2006
15,523
They physics doesn't change because you picked the wrong reference direction. Current is a scalar, not a vector.

#### KeepItSimpleStupid

Joined Mar 4, 2014
4,566
By convention, positive power is power dissipated and negative power is power generated when doing analysis. You don't have a -4 MW power plant, but when you analyze systems you have to give it a - sign.

Solar cells have either Jsc or Isc. they are negative quantities. We always drop the sign except in print. Isc is short-circuit current. Jsc is short-circuit Current / Unit area). When I was involved in research Jsc was the important quantity. We assumed a nominal area and when it was time to have a reportable result, the area was measured. if we were taling about a particular device, Jsc would be something like 10mA/sqcm.

The only time we did not do this was with our 8-10 calibration cells. The area was always 1 cm^2. A few were traceable to NREL and then we calibrated our own. We always calibrated with two cells. One that was used the previous calibration and a new one and we rotated the 10 or so cells.

There's a lot of nuances depending on the discipline. Examples. Thermocouples: Red is always negative; Electrician: White=Neutral, Black Hot; Optician: Screws are always measured by overall length. A new one learned this week: Automotive wire is measure by OD of wire+insulation. Refrigeration: A 7/8 refrigeration elbow is a 3/4 plumbing elbow. Construction: a 2x4 is not 2"x4". Tubing OD is very different from "refrigeration tubing".

A wire moving in air on earth generates a current - Usually no, but the answer is yes. Three 1K resistors is equlivelent to a 3K resistor - Usually yes, but the answer is no.

The letter "I" is typically not used except for current. The electronic alphabet skips "I".