need help BFG135A

Probably the transistor's full rated 150mA. But your question is garbled and I can't tell. What's the voltage across the transistor?

Wrong, at least for the specified 2% duty cycle. Ptot = 1W is the continuous power rating, and if the thermal pulses are short enough (the OP specified 10ns) the dissipated power can be much higher, as shown in the transient thermal-resistance curves (Infineon calls this Permissible Pulse Load). For example, the maximum-rated voltage and current = 25V * 150mA = 3.75W, which is well under 50W implied by 2%, or about 25W implied by the 2.8K/W thermal-resistance-plot's value for 100ns 2% duty-cycle pulses.

Looking at the transistor's beta spec at 100mA (Infineon) or beta plot (Philips), it's likely this part can be operated well above 150mA for short pulses. Transistors that have integrated emitter- ballast resistors can often give truly amazing performance.

I assume that you mean the signal at the collector is 5Vpp ?

Depending on the load and the waveform shape, the current may be Icmax or Ptot limited.

Graham

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Is my following statement correct, for calculating the max collector current, referring to your example 100ns 2% ? Looking at thermal resistance plot, Ptot is about 25W, as you described. From the S21-f plot at pag 6, we can assume beta is about 30 at 2Mhz. Beta = Ic/Ib, so if i have a base current of 20mA, i can calculate an IC current of 30*20mA = 600mA. Is it correct ?

Thanks,

mauri

I don't think it's very wise to drive transistors with currents, because current gain is an uncontrolled parameter. With respect to the s21 plot, that's at 100mA and we can assume beta will be lower at higher currents. It's better IMHO to use voltage drive with emitter degeneration, or balanced differential pairs with emitter current sources, etc., for predictable performance. BTW, an emitter-follower is a voltage-drive application. Of course, I don't know what you're trying to do. For example, is there a 50-ohm coax cable involved? Is it terminated?

this unit gain stage (emitter follower), drives a laser diode. I need to know if the BFG135A, that i'm using to drive the laser, is "strong" enough to ensure high power laser beam, or if I have to choose a different RF transistor to design the unit gain stage. So i would like to understand the ICmax i can obtain with a periodic impluse (ampliture

Thanks a lot,

mauri