Since Vbe splits amplifier power supply voltage in half, can you get by using transistors rated at half of that voltage?
For example, let’s assume power supply voltage -37V 0V 37V (dual voltage). Now, can I get by using 40V VAS transistor?
Thanks!
For example, let’s assume power supply voltage -37V 0V 37V (dual voltage). Now, can I get by using 40V VAS transistor?
Thanks!
VAS swings rail to rail. Needs full 80v. You probably want to go higher, although most any “80 V” part will physically take more.
Well, the lower part of a cascode wouldn’t need much, but the upper part would.
There are crazy voltage division schemes, but they aren’t commonly used.
There are crazy voltage division schemes, but they aren’t commonly used.
As long as the amplifier does not use the middle point of the supply (ground), all VAS devices would have to be rated for full voltage and maybe a little extra.
If that +/- 37 V is not regulated, it can also be 10 % higher, so 80 V is too tight. With bootstrapping, it may be even higher than 1.1 times 74 V. A 100 V transistor should be enough.
With bootstrapping, the actual collector voltage of the VAS never gets more than one diode drop above the supply with bipolar outputs. The collector-base diode of the driver clamps it. With mosfet you only go above by the 12 or 15 volts that you allow with your gate-source zener (and you will use one after your first blown set).
100 or even 160 volt transistors, even fast complementary pairs, are common as dirt these days. The only reason to cling to 80 volt parts (ie, BD139) is the lack of quasi-saturation. It is a valid reason, and much harder to find that characteristic in higher voltage parts.
100 or even 160 volt transistors, even fast complementary pairs, are common as dirt these days. The only reason to cling to 80 volt parts (ie, BD139) is the lack of quasi-saturation. It is a valid reason, and much harder to find that characteristic in higher voltage parts.
Is it to generate 2nd order harmonic?
Quasi-saturation means more distortion, does it?
Just to make the assumptions explicit: I think you are assuming a complementary double emitter follower or a complementary source follower output stage.
Some sort of a follower, yes. In some topologies, the output stage has voltage gain. You don’t normally think of those as having a “vas”. Something like a Tiger sort of does - but that stage still needs to tolerate swinging to its rails in the case of overdrive.
Quasi saturation is the loss of hFE (gain) at low vce. The lower the supply voltage the bigger a deal it is. With high power amps, you never swing really close to the rails anyway. When you’re only running +/-18 volt rails keeping high gain down to 2 volts is useful. When you’re dropping 5 or 6 volts in your output stage (and half of that in an emitter resistor) hFE below 5 volts in the VAS is meaningless.