Low-voltage rail for transistor bases?

[Lauri Alanko]

Hello.

When power transistors are used as switches, the base current can be
significant, as the saturation beta may be as low as 10. On the other
hand, the base _voltage_ need only be at Vbe(sat), and any additional
voltage needs to be dropped by e.g. a resistor. Given the high
currents, this voltage drop may be a significant loss.

This suggests to me that ideally the base of a low-side NPN switch
would be powered by a low-voltage rail, only slightly higher than
Vbe(sat). This way the conduction loss would be minimized.

Has such a rail been used with bipolar switches? If not, what's wrong
with my thinking?

(Please don't tell me to use MOSFETs because they are better. I know
that already. I'm looking for understanding, not solutions.)


Lauri

 

[Phil Allison]

"Lauri Alanko" <

When power transistors are used as switches, the base current can be
significant, as the saturation beta may be as low as 10. On the other
hand, the base _voltage_ need only be at Vbe(sat), and any additional
voltage needs to be dropped by e.g. a resistor. Given the high
currents, this voltage drop may be a significant loss.

This suggests to me that ideally the base of a low-side NPN switch
would be powered by a low-voltage rail, only slightly higher than
Vbe(sat). This way the conduction loss would be minimized.

Has such a rail been used with bipolar switches?

** No doubt - but it would still have been a 5 or 12 V rail.

But the DC voltage being switched might be several hundred.


.... Phil

 

[whit3rd]

When power transistors are used as switches, the base current can be
significant, as the saturation beta may be as low as 10. On the other
hand, the base _voltage_ need only be at Vbe(sat)...

This suggests to me that ideally the base of a low-side NPN switch
would be powered by a low-voltage rail...
Has such a rail been used with bipolar switches? If not, what's wrong
with my thinking?

Yes, of course. Old NIM (nuclear instrumentation module) power for switching
transistors included +24, +12, +6, -12, -24V power supplies, and
it was common to make use of whatever was most convenient and
efficient from the plethora.

Vacuum tube radios typically had A battery for filaments (1 to 6 V), B battery for
tube bias (20 to 30V) and C battery for plate supplies (as high as you can get).

Sometimes, you see Vbb for base drive voltage supplies, Vcc for collector.