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Thermal resistance of different amplifiers

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konrad_gom

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Hello,
I have two amplifiers:
https://www.skyworksinc.com/uploads/documents/SKY67100_396LF_201265I.pdf
https://www.asb.co.kr/datasheet/ASW114.pdf
1. Why thermal resistance differ so much? SKY67100_396LF - 80 0C/W; ASW114 - 321 0C/W.
2. According to calculations for 25 0C ambient temperature, Tj for
* SKY67100_396LF will be Tj=25 0C + 80 0C/W*4V*56mA=43 0C;
* ASW114 will be Tj=25 0C + 321 0C/W*3,2V*37mA=63 0C.
Do I understand proper, that in this case:
3. Tc (case temperature) will be about Tj?
4. just switched on ASW114 will be so hot (63 0C) even without any RF signal?
 
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It's pretty obvious. The one with the lower thermal resistance has a metal pad on the bottom, the other one doesn't. Metal is a much better conductor of heat compared to plastic.
 
barry said:
It's pretty obvious. The one with the lower thermal resistance has a metal pad on the bottom, the other one doesn't. Metal is a much better conductor of heat compared to plastic.
Click to expand...

Oh, right, I did not notice.
And what with point 3 and 4 from my first post?
 

3) You need to know case-to-junction thermal resistance to answer that.
4) depends on idle current. It's probably a class A amplifier, so, yes, it will draw current even with no input.
 

For item 4 you can measure the ground or Iq current with no signal on your bench using a current probe as barry is suggesting. If it's very low you're device should stay relatively close to ambient temperature. But if it's running current your junction will be greater than ambient.
 

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