# Low-voltage rail for transistor bases?

L

#### Lauri Alanko

Jan 1, 1970
0
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
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

P

#### Phil Allison

Jan 1, 1970
0
"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
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

W

#### whit3rd

Jan 1, 1970
0
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
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.

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