# Reconciling power dissipation and output characteristic of BJT

From the datasheet of 2SC1384 I got the following characteristics. I plan to use this transistor in an RF amplifier.

The power dissipation characteristic sets a limit on the dissipated power of the device of 1 W at 25 degrees ambient temperature for the TO92 package. The other characteristic shows the collector emitter voltage and collector current for different values of base current. For example, the curve for Ib=10mA shows Ic=800mA for Vce=5V. For this condition the power dissipated by the device is 4W, which is obviously in excess of the 1W limit.

I planned to draw a load line on the characteric and noticed the power dissipated in the device is very high according to these curves. I draw the maximum power dissipation hyperbola in the characteristic (not shown on screen), which makes the better part of the characteristic look unusable.

My question is how these two seemingly contradicting characteristics can be reconciled. How can the manufacturer even measure the properties of this transistor well above it's maximum ratings?

My question is how these two seemingly contradicting characteristics can be reconciled.

It's all covered or implied in another curve called the safe area of operation and this curve factors in time or duration: -

So, for example, with a current of 1 amp and a collector emitter voltage of 10 volts the power dissipation is clearly 10 watts but this can't be indefinitely applied - notice how the curve above has time limits of 1 second and 10 ms shown.

This is how the characteristic is produced in your question; voltage and current are applied for a short period of time. See the red boxes below: -

I plan to use this transistor in an RF amplifier.

See the purple boxes above.

It's not a very good RF amplifier. $$\F_T\$$ is 200 MHz and that's the frequency where $$\\beta\$$ drops to unity. At 2 MHz, $$\\beta\$$ will be 100 but drop linearly to unity with frequency hence, at 10 MHz $$\\beta\$$ will be 20 and at 100 MHz $$\\beta\$$ will be 2 etc..

If your RF frequency is below about 5 MHz then it should be OK.

Be aware

• This particular BJT, together with the 2SC1383 was used in the 500 mW output stage in 27MHz CB equipment of the 80's. I don't intend to use it above 30 MHz at no more than 200 mW, so I should be fine with this one.
– Bart
Mar 17, 2020 at 14:40
• The foot note about pulse measurement does not occur in the datasheet that I have (Unisonic), but it does have the SOA graph you mentioned. I just overlooked the pulsed aspects of it. It doesn't apply for me anyway, because I will use the device in continuous mode.
– Bart
Mar 17, 2020 at 14:45