Hi NaN :)
In old schematics I remember that the ground was the positive pole.
Then, in the more recent period of electronics, the convention to use as ground the negative pole won on the past one.

Why was one chosen versus the other in the first place, and why then, finally, the negative ground won? (this assuming that my premise is correct, and that I wasn't the victim of too many coincidences while looking at ancient transistor radios' schematics)

I've built some theory in my mind.. but it may be completely off: I think that the reasons that made decide for the negative as ground as related to ions and oxidation.. that makes it a bit safer.
I.e. if we e.g. put a 9V battery on our tongue, the nasty chemicals (ions? salts?) will appear on the positive pin.

Or maybe the reason may be simpler.. i.e. since the Earth can be considered at 0V potential (by convention), then it's easier to deal with positive voltages than negative ones.

Perhaps the old "standard" of using positive grounds was even safer, and the modern one was born for the above latter consideration.

As I said, probably I'm completely off though.. but I'd like to know the real reasons, so I stop misinforming myself with my stupid theories. :grin:
Posted on 2003-04-15 11:43:53 by Maverick
I've seen old transitor tube schematics once or twice before.. but i never recall them having ground on the positive voltage source.

That being said, tubs have special biasing concerns, dont remember them all to clearly. However, it seems to me that there was often a "control" plate (like the base of a transistor) that would require a strong NEGATIVE voltage. To produce the negative voltage, there would be a power source with its positive end to ground, yielding the inverse voltage. Ie, use a transformer to produce 30 V and then connect the positive to ground, wich with respect to ground now, offers a -30V potential at the negative end.

I hope you see what im trying to explain here.

Often in old radios as well you will have a transformer with a center tap to ground. By tapin and placing ground at the center of the coil, its like splitting a bar magnet in to: you get N->S N->S in series. Except now you have decaired electrically that the middle S N (or -/+) is to be 0V. Doing so makes one end positive (N) and the other negative (S). The amount in which they are +/- is the the voltage the coil would produce entirely divided by 2.

This is a good solid approach to supplying a positive and negative power source for things like stereo amplifiers. Of course you still have to rectify each half as well ;)

:NaN:
Posted on 2003-04-15 15:12:03 by NaN

Hi pal :)

I've seen old transitor tube schematics once or twice before.. but i never recall them having ground on the positive voltage source.
Oki.. so I saw the wrong schematics. Effectively they were all from the same old book.. some it definitely was THAT designer that got things in reverse. ;)

That being said, tubs have special biasing concerns, dont remember them all to clearly. However, it seems to me that there was often a "control" plate (like the base of a transistor) that would require a strong NEGATIVE voltage. To produce the negative voltage, there would be a power source with its positive end to ground, yielding the inverse voltage. Ie, use a transformer to produce 30 V and then connect the positive to ground, wich with respect to ground now, offers a -30V potential at the negative end.
I used to be a bit into tubes, years ago though..
Triodes (the tube most similar to a typical bipolar transistor or, better, FET) have a cathode, an anode and a grid (and a heating filament, of course.. which is often in common with the cathode). There's usually a high voltage between catode and anode (400V aren't uncommon at all), and the grid controlles the amount of current flowing between cathode and anode (inhibiting it, i.e. no grid voltage means current flowing between cathode and anode). Tubes like the 12AX7 (a typical high gain tube used in audio preamps), IIRC saturated at something like -3 V. They are very similar to FETs, but with a different transform function.. which makes them quite ideal for e.g. electric guitar preamps, since their distortion products are mostly even harmonics, which are musically pleasant. The -3 V of the 12AX7 may seem little, but it's a high gain tube, though (gain is about 100, again IIRC.. it's been many years since I've wrapped my brain on this kind of devices).

Tubes are very interesting devices, IMHO.. too bad that they need some time to heat, and that they're vulnerable to physical shock, etc..
There's supposed to be no current flowing on the grid (i.e. they're extremely high impedance devices), but a positive voltage applied on the grid will mean grid current flowing.

I hope you see what im trying to explain here.
I do, don't worry. :)
Usually, though, in tube schematics you will see that the required grid bias is obtained by using a resistor between the tube cathode and the (negative) ground, instead.


This is a good solid approach to supplying a positive and negative power source for things like stereo amplifiers.
Yup, class AB, for example. :)

Of course you still have to rectify each half as well ;)
Unless you like that buzzin' 50/60 Hz sound. ;)

Anyway, thank you for clearing that doubt for me.. positive grounds is something that may disturb my sleep. ;)
Posted on 2003-04-15 15:50:45 by Maverick
Thanks for the refresher ;)

I guess -30 V is too high then <lol>, but you obviously were with me there. Im told tubes are making a come-back if it helps you sleep at night. There is some companies making lower voltage tubes and sockets for special applications. I've looked beyond this bit of information tho :)

However your text might have been written by a physics guy.. i dunno really, but as far as i can remember ground was always on the negative terminal (for + voltages).

Oh ya one more thing... i ment tubes in the opening line, not "transistor tubes" (Whatever that would be <lol> )

PS: You should post what you know about Tubes in the Theory section...

:NaN:
Posted on 2003-04-15 16:10:51 by NaN

PS: You should post what you know about Tubes in the Theory section...
It has been a long time.. I may have some wrong memories.
But as soon as I put my hands back on my tube guitar-amp simulator and refresh my mind, I'll definitely post some detailed info. :)
It's in my ToDo.TXT file.. it may take the end of the year, though. Sure I'd like to contribute, and sure++ with this topic.. so let me find the time for it. :)
Posted on 2003-04-15 16:38:06 by Maverick
Where I've remembered positive grounds were in old transistor circuits that used PNP transistors.

To reverse bias the base-collector junction, the collector needed to be tied to negative voltage.
Posted on 2003-04-15 19:00:05 by tenkey
Groiund can be either the most positive point, or the most negative point, or something in between.

Its your choice as the circuit designer to pick where you find it most appropiate. Often that means following the same convention as the thing you're plugging into.

There is nothing magical about circuit ground, its just the point picked to be called zero volts.

Chassis ground, earth ground have different meanings, and you best not stick a potential on those.
Posted on 2003-04-15 20:34:37 by Ernie

Where I've remembered positive grounds were in old transistor circuits that used PNP transistors.

To reverse bias the base-collector junction, the collector needed to be tied to negative voltage.
Right, in those schematics that I saw they were PNP transistors too. It can't be otherwise, though, in class-A amplifiers.. as is most common to see in those portable radios.
Posted on 2003-04-16 04:19:20 by Maverick
I follow now. Intersting to see this is how it was done in the old days...

Ernie does make a good point too..
:NaN:
Posted on 2003-04-16 11:50:47 by NaN
Yes, as Ernie says, ground can be somewhere between the most negative and the most positive voltages. The old RS-232 interface that's used for COM ports uses voltages that are above and below the Signal Ground.
Posted on 2003-04-17 03:04:44 by tenkey
It seems to me a legitimate reason for using a positive ground had something to do with noise in certain situations if that's any kind of hint if you want to research it.

ECL logic used a positive ground but they also used a lot of differential circuitry for passing signals around.
Posted on 2003-04-17 16:41:13 by drhowarddrfine
If i remember RS232 uses both + and - signals such that, though an op-amp circuit, it can reject any noise picked up by EMI (op-amps common mode rejection).

Singal (to send)

+ line = Signal / 2
- Line = - Signal/2

Transmit: Noise is picked up.

+ line = Signal/2 + Noise
- line = - Signal/2 + Noise

Receiver Op-amp CMR: (V+) - (V-) = Output

Output = (Signal/2 + Noise) - (-Signal/2 + Noise)
Output = (Signal/2 + Signal/2) + (Noise - Noise)
Output = Signal

Noise is rejected, since the amount is common to both lines (in an ideal world).
:NaN:
Posted on 2003-04-17 17:54:05 by NaN

Yes, this "balanced" scheme is typical also in Ethernet, hi-fi preamps and.. surprise surprise, the ancient telephone line!
The wires are twisted, so most of the EMI is common to both wires and thus gets nullified.

They are two different things, but close enough for me to classify them together. :)
Posted on 2003-04-18 03:28:59 by Maverick
Ya i was surprised to learn reciently that in the case of Ethernet, they also twist each pair of wires at different twist rates through out the bundle of wires. This is to minimize cross talk EMI, since each pair will not be in phase with each other all the way done the line, and preserving the 100MB bandwidth.

I also discovered this 100MB limitation is not due to the wires resistance, but the protocal itself. The size of packets are restricted to a minimum size. This means they have to travel to the furthest point, and if collision happens here, the collision signal needs to travel back to the source before it finishes sending the packet (min sized). If not, it would never realize its data collided, and not resend. This is why a 100m limitation is imposed on Bluetube Ethernet wire at 100Mb. ;)

Well so i have learnt anyways ;)

:NaN:
Posted on 2003-04-18 09:24:58 by NaN
Hi, Maverick,

First of all, the ground is a reference point, a point with respect to which all other signals are referenced. Therefore the ground is said to be at "zero volts" (0V).
Now let's see what "positive" and "negative" ground means.

You are absolutely correct: in old transistor schematics, the ground was "positive".

That was because most transistors were PNP. And that is because the old transistors were manufactured using Germanium. With Germanium, PNP transistors were easier to produce, with better characteristics than NPN's.
For the PNP transistors to work in their active region, you need to have the emitter connected to the "+" side of a voltage source (battery) and the collector to the "-" side of that source.
Emitters are connected to "ground" (through resistors or directly) since generally the amplified signals are taken from the collector (that way you have voltage amplification). So the "ground", being connected to that "+"side of the battery, is said to be "positive".

With Silicon, the opposite is true. PNP's need more silicon, are slower (poorer HF response). Therefore the NPN's are preferred, and IC's employ mostly NPN's inside, avoiding silicon consuming PNP's.
And so the ground is "negative", since the emitters now must be connected to the negative rail.

With tubes, the cathode emits electrons, which are negatively charged particles. They can only be attracted by a positively charged body. That is the anode of the tube. Of course, to make it positive with respect to the cathode it must be connected to the positive side of a voltage source and the cathode must be connected to the negative side of that source. The anode is the equivalent of the collector, the cathode the equivalent of the emitter of a NPN transistor. So the ground, with the cathode, was "negative".

It is true that in some schematics there is a resistor in series with the cathode. The voltage drop across it (due to the cathode current = anode current for a triode) makes the cathode slightly more positive (a few volts) than the ground. That is because the grid has to be negative with repect to the cathode , so it repels electrons. This keeps the anode current under control.

The grid is actually connected to ground by a high-ohm resistor (1Meg or more), but since there is no grid current, the grid is at ground potential (0V), so a few volts more negative than the cathode. However, since the anode voltage is high (hundreds of volts) the cathode is still more negative than the anode.

Regarding the RS-232 signals, again the ground is a reference point. However, it is known that if one of the signals is connected to earth, the other one should be negative with respect to it to prevent corrosion. That is why in old telephones one wire was earthed and the other negative with repect to it.
I suspect the signals were chosen the way they were because when the line is idle, with the ground connected to earth, the signal line is negative with repect to it and so corrosion is prevented. Of course, when "0"s are transmitted that doesn't happen, but most of the time it's true.

Hope my little dissertation here helps.
Posted on 2003-04-18 12:37:43 by VVV

Hello VVV, you wrote:
You are absolutely correct: in old transistor schematics, the ground was "positive".

That was because most transistors were PNP. And that is because the old transistors were manufactured using Germanium. With Germanium, PNP transistors were easier to produce, with better characteristics than NPN's.

[..]
Yup, I suspected it was some sort of "commercial vs physics" comprimise.. thanks for clearing that up. :)

You gave some nice physics details, BTW.. very interesting read. :alright:
Posted on 2003-04-18 12:47:29 by Maverick

If i remember RS232 uses both + and - signals such that, though an op-amp circuit, it can reject any noise picked up by EMI (op-amps common mode rejection).
RS-232 does not use differential signal lines. The minimal bidirectional configuration is three wires (Rx, Tx, and Signal Ground) and no shielding. The Rx and Tx lines are + for data bit 1 and - for data bit 0. The Maxim RS-232 level converter chip I'm using on one my dev boards generates approx +9V and -9V from a +5V source.
Posted on 2003-04-18 16:05:13 by tenkey

ECL logic used a positive ground but they also used a lot of differential circuitry for passing signals around.
On output, the logic signals were tapped from the emitter side, rather than the collector side (TTL). With NPN transistors, this would be the more negative voltage. Apparently, this maintains the notion that the primary signal side is the power side.
Posted on 2003-04-19 18:39:28 by tenkey
at one time I worked in electronics
the ground we used was both +and -
like this

+battery- + battery-

at the -+ connection between the two batterys is what was used as ground

this way we could get +and - voltage swings with a half bridge
Posted on 2003-04-26 19:29:43 by rob.rice