RadioBanter - View Single Post - Bi-directional amplifiers

May 24th 04, 09:01 PM

In article ,
(Ashhar Farhan) writes:

are there any resources available on design of bidirectional amps on
the net?

Telephone (wired) repeaters are an example. Those have been used
at rather wide bandwidths but none at RF that I know of.

i have read the few paragraphs in SSD and EMRFD. I am looking for some
better material.

What are "SSD" and "EMRFD" acronyms?

In particular, I have the following questions:
1) consider that we have two broad-band amps based on a single npn
transistor with a Feedback resistor of Rf and emitter degeneration of
Re. consider, further, that we simply connect them back to back and
power one or the other, depending upon with direction you might want
to amplify. Now, what is the total feedback resistance? is it Rf or
Rf/2 (the Rf is between the inputs and the outputs and they are cross
coupled, paralleling Rf resistors.

Analytically model it.

You are going to have to measure the OFF transistor characteristics
yourself since few parameters other than ESD (Electro-Static
Discharge) are given on datasheets.

But...the "off" transistor might get turned partly on by the other,
"on" amplifier output signal.

2) Suppose that the output load of a bidirectional amplifier is RL and
resistor from collector to Vcc is Rc. Now, does Rc have to same as RL?
I guess this is required for best power transfer. But, when the
amplifier is amplifying in the opposite direction, then the amp input
is paralleled by the Rc. Thus, won't the input impedance of the amp
drop by two?

Yes and no. :-)

3) Will substituting a pi type attentuator for the load and feeding
the input in the opposite direction through the same attenuator solve
the problem?

A very few bidirectional amplifiers have been built at RF, all based
on using wideband "hybrid" transformer coupling to a line, both of
them "on." The amount of amplification is limited to just less than
the best directivity of the transformer network (connected as a
directional coupler)...and the relative phase shift.

"Directivity" is the relative difference (usually expressed in db) between
forward and reverse direction coupling.

The limiting problem in "hybrid" transformers is the resulting phase
shifts over any frequency range. That's a separate problem which
can be identified, measured, and used to model the repeater
amplifier. The reason such types of bidirectional amplifiers are NOT
production items is: (1) Need; (2) Ability to control the phase shift
over quantity.

About the only source I know of for bidirectional amplifiers, besides
the telephone industry (begun before 1920), is VHF-UHF coaxial
cable TV service which has "up and down" path communication,
both from the "head end" and to the "head end" (for subscriber
signalling for such things as pay-per-view showings or for Internet
broadband access).

Len Anderson
retired (from regular hours) electronic engineer person