SOA - What is true and what is wrong ?

Dear Friends ,
This is my first thread on this forum. First of all,please forgive my bad english .

I have experience in solid state amplifiers design but something is not clear (yet) for me.

I have a very good amplifier QSC USA1310 with 16 outpout drivers MJL21194/MJL21193 per channel , and it work very
hard for 5 years (550w rms/4ohms)(+/- 80v).

I have another very good amplifier SWR SM-900 with only 4 outpout drivers 2SC3264/2SA1295 from SANKEN and it work very hard for 4 years (500w rms/2ohms)(+/- 67V)

In first case we have Imax = 16.58A (Peak Current)
In second case we have Imax = 22.36A (Peak Current)

I just saw the SOA for 2SC3264 and isn't OK becouse this driver have just 3A on 67V in DC and 10A for 10ms , single puls.

Can somebody tell me why my SWR amp don't blow yet ?

Best regards !


2001-11-08 12:49 pm

Hi,first of all i assume that your amplifier is a class AB type.
If the output voltage raises,the C-E Voltage of the active transistor decreaseses by the amount of the output voltage.
That means,if the transistor is fully open,it "sees"only 2or3 volts.
In addition the supply voltage decreaseses under load.
The other transistor "sees" almost the double rail voltage,but only at idling current.