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What is advantage of high negative supply for amplifier?

throbscottle

Well-Known Member
When I got my hands on the manuals to repair my Cossor CDU 150 oscilloscope, one of the things I found a bit strange was the use of +12 and -50v supplies, even for the low voltage circuits. It's even brought out to the front panel to supply an amplifying probe. I'm assuming there is a good reason they did this, but can't really put my finger on why. Can anyone shed any light on what the thinking might have been?
 

OBW0549

Active Member
The X and Y deflection plates on a CRT usually require a fairly high voltage to do their job of sweeping the electron beam, and this is probably one reason for the high supply voltage. The fact that the 50V is negative suggests it may also be involved in biasing one or more of the CRT grids, assuming the cathode is somewhere around circuit ground potential.
 

BobW

Active Member
Could also be part of a constant current sink used by some differential transistor pairs. The high voltage could provide extra headroom in the amplifier stage.
 

throbscottle

Well-Known Member
CRT doesn't explain what +12 and -50v is doing powering a pre-amp. It has separate bias supply. IIRC the plates do actually run off +50v and -50v though. I think Bob might have it, at least partially - the timebase has a constant current source "below ground" running off -50v. (though, that said, it might be -12) But the pre-amps?
Could it be to do with the noise floor? (I got this idea from reading about emitter-coupled logic, which is otherwise not relevant to this)
Could it simply be that 50 years ago, negative supplies were a lot more relevant than now and designers were happier working with them?
Shocking experience, hurr hurr... shockingly bad joke more like ;)
 

Nigel Goodwin

Super Moderator
Most Helpful Member
My vote would be for a 'constant' current source as well - by using a higher voltage you can simply use a resistor to give a relatively 'constant' current.

It's also quite likely that there was a -50V supply already there, so it makes sense to utilise it.
 

dr pepper

Well-Known Member
Most Helpful Member
With a high voltage you can get plenty of gain from just 1 or 2 stages.
 

unclejed613

Well-Known Member
Most Helpful Member
Could also be part of a constant current sink used by some differential transistor pairs. The high voltage could provide extra headroom in the amplifier stage.
i think you are right, and it would make sense to have the same voltage source for active probes if they are using similar diff amps. it's not "really" constant current if the diff amp is supplied with just a resistor, but with a 50V supply instead of a 12V supply for instance, the current feeding a diff amp will vary a lot less with the higher voltage supply as the voltage on the input changes.

early op amps were used for doing math in analog computers, and had +/-300V supply rails... [Philbrick vacuum tube op amp]
 

Nigel Goodwin

Super Moderator
Most Helpful Member
i think you are right, and it would make sense to have the same voltage source for active probes if they are using similar diff amps. it's not "really" constant current if the diff amp is supplied with just a resistor, but with a 50V supply instead of a 12V supply for instance, the current feeding a diff amp will vary a lot less with the higher voltage supply as the voltage on the input changes.
As I said in post #6, a high voltage and a high value resistor were the classic way of getting a 'constant' current.

And as Dr Pepper said, it gives you high gain as well.
 

throbscottle

Well-Known Member
I'm just in the process of reading this: keith-snook.info/wireless-world-magazine/Wireless-World-1968/High%20Input-Impedance%20Amplifier%20Circuits.pdf It looks like they may have done it get higher input impedances as well.
 

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