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# Power dissipation of BJT in linear operation

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#### mixmaestromark

##### Newbie level 6
Hi all

I am trying to calculate the power dissipation of a high-frequency npn BJT (NE461M02) which is acting as a small-signal amplifier in CB configuration. This is for reliability prediction and I need to calculate the stress on the component.

I have done some googling but I cannot seem to find any useful information on this. If somebody could explain the physics behind power dissipation in a BJT and how I can calculate it I would be very greatful!

Regards
Mark

You would need to calculate the sum of the power dissipations caused by the DC bias point and the rms value of the AC signal

You would need to calculate the sum of the power dissipations caused by the DC bias point and the rms value of the AC signal

Yes I understand that the dissipation will be a combination of dc and ac, but I would like to know how exactly this is calculated and how the power is actually dissipated within the transistor?

In amplifier operation your collector to emittor votlage and collector current will keep on changing. You need to integrate the multiplication of instantaneous voltage and current and find the average over a full cycle.

mixmaestromark

### mixmaestromark

Points: 2
You have asked for a small-signal amplifier. Then Pv = Vce*Ic + Vbe*Ib.

Points: 2

### umesh49

Points: 2
Thanks guys. I guess the overall dissipation calculation would involve umesh49's integration of ac voltage & current as well as FvM's dc operating point dissipation?

The dissipation of an ideal class A amplifier is independent of the AC signal, as implied by FvM's answer. If the amplifier is non-linear, then the dissipation can change with signal. In this case, the calculation is not straight forward, and a simulation would be your best approach to calculating the dissipation.

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