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Neg. Resistance Amps

Help!

I'm trying to build a NEGATIVE RESISTANCE amplifier to play with by putting it in a simulated long line telephone circuit, the way I believe the phone company did many years ago. Bell Tell needed an amplifier that they could put somewhere in the MIDDLE of a connection between two phones many miles apart.

Bell used the neg. res. amp in a TWO WIRE CIRCUIT, the talk signals from EACH phone were shared & were carried on the same wire pair. That is, this was NOT a 4 wire circuit with each phone having a pair to send its talk signal.

So, the negative resistance amplifier does not have a fixed input and output, it just sits there handling both signals, both directions simultaneously.

I'm not sure how this is actually done. For example, is the neg. res. amp IN SERIES with the line or IN SHUNT? How would I write the circuit equations to describe this? etc.

Negative resistance amps are explained in some texts, I hope that someone reading this knows of such a text. I DON'T WANT a description of neg. res. amps for high or microwave frequencies, I just want something good at audio frequencies for my two phone hookup.

I think the Bell Tell boys used a Tetrode as their neg. res. amp., it can be biased to produce a neg resistance. This must have been written about in the Bell System Tech. Journal. I would also love to find out what issue.

Thanks for any help. Puzzled in Portland

Reply to
Jess Hanchrow
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Hmm, both ought to work. Parallel would use bias current to directly amplify a signal voltage, while series would use bias voltage to amplify the signal current. In either case, you have some delta (change in) V or I.

You must be *very* careful about reactances around a negative resistance, because they tend to oscillate. Such happens when your output is your input and they are in phase! ;-)

Well, anything that has a negative resistance characteristic will do, you could set up an op-amp to do it. You could use a tunnel diode, if you're adventurous (and can find one!).

No idea, but on tetrodes, if you go to a website like this:

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Then browse around for tetrode sheets, like this:
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And scroll down to the plate curves, you'll notice a juicy negative resistance region.

I've often contemplated using a 24A in parallel with a variable resistor as a constant current source with perfect output impedance (since it can be nulled to somewhere between very close to say, +/-infinity ohms).

As I recall from past contemplations, said plate curves top out at -40kohms, in the 15-60V region. Screen current is way over the rated limit, but oddly, there is no power rating for either electrode, mmrrrph...

Tim

--
Deep Fryer: a very philosophical monk.
Website: http://webpages.charter.net/dawill/tmoranwms
Reply to
Tim Williams

Either will work or both are better.

As I recall triodes were used and they were coupled into the line with a transformer type of hybrid. I don't remember the circuit but I do know that the gain was limited by the impedance match on the telephone line. Also they were powered locally and were not powered from the telephone line.

At one time I could have drawn the schematic from memory. Oh the joys of getting old :-)

Bill K7NOM

Reply to
Bill Janssen

Also look for Western Electric "E" repeater. They may have been made by others also.

Bill k7NOM

Reply to
Bill Janssen

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