# Question on current density for layout

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#### suria3

##### Full Member level 5 Hi guys,

I have a question here. I have designed a single stage common source amplifier. Now
i'm going to do layout on that circuit. I have some doubt on allocating the current density for the metal line. Attached is the transient current from the common source amplifier. So, how much current density i suppose to draw for the metal line. Since it is said the the current density is calculated with dc and AC current, then in my case, is it correct to say that the dc is 200uA and ac is 600uA density or we take the max current, in this case 800uA as the max current density. How should I differentiate this?. Then another question is, since the resistor sizing is depend on the current density is should take, what refernce current i should take in consideration, is it 200uA, 800uA or 500uA, I'm new in layout, please explain.

Thanks.

#### sharpsheep

##### Junior Member level 3 I think you should consider the rms current, [(700^2+200^2)/2]^0.5
Because we choose the W of the metal line accroding to the heat(Q) which is produced by the current. And the formula to calculate the heat is the integral of (I^2)*R, from 0 to T. T is the period of the signal. So in one period, you can get the whole Q: [(700^2)*(T/2)+(500^2)*(T/2)]*R. Then you can get the rms current I have told you.

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