Transistors switch input impedance?

[SISKO]

Hi! Im given this problem at class and i cant seem to understand it or  im seeing the stuff wrong, so im looking for some help here. Sorry if this OT, please, tell me.
Im suppose to: design a transistor stage to convert a current loop (4 - 20)mA to voltage loop (0 - 10)V. Input impedance must be lower than 10ohm.
Now, this last sentence is killing me!

Considering a common emmiter config:
If i by any reason have 10ohm of impedance and current from 4 to 20 mA, doesnt this mean 40mV to 200mV range of volatge? This voltage cant turn on a transistor.
I had the idea of making an amplifier to amplify 200mV to say, 5V. But i think that my control signal will interfier with the biasing. Ive not given much time to this idea..

The question is: should i consider a large or  a small signal model? Is impedance considered on large model signal?

If i can use the large signal model i can happily solve it, as the B-E impedance  is 0ohm so the input impedance is Rb.

Ill apreciate any response.. hope i made myself clear!

[SISKO]

solved this  by using a discrete differential amp

[PRR]

Current-loop is old-fashioned.

Current-loop should NOT have any tight specification on series impedance. That's kinda the point: you can have 10 feet or 10,000 feet of cable from sensor to monitor zero Ohms to many Ohms, and the monitor may be several types of things (needle-meter, amplifier, sensitive relay, maybe all in series).

Yes, given the problem as stated, a 10 Ohm resistor and an amp with DC gain of 62.5 (deduct the 4mA "zero" current) will work, and should be simple (depending what they have taught you already).

In real life you may have only one power rail, the 4mA-20mA loop is probably not grounded, the 0V-10V may want to be grounded....

[SISKO]

Thanks PRR

Current-loop is old-fashioned.

While looking for information, i read that it was indrustry-standart these days. Maybe "these days" is 20-30 years behind today?

Current-loop should NOT have any tight specification on series impedance. That's kinda the point: you can have 10 feet or 10,000 feet of cable from sensor to monitor zero Ohms to many Ohms, and the monitor may be several types of things (needle-meter, amplifier, sensitive relay, maybe all in series).

Yes, it makes sense!

Yes, given the problem as stated, a 10 Ohm resistor and an amp with DC gain of 62.5 (deduct the 4mA "zero" current) will work, and should be simple (depending what they have taught you already).

Indeed, we should be able to "design" a differential amp. Is there any other DC amplifier?

In real life you may have only one power rail, the 4mA-20mA loop is probably not grounded, the 0V-10V may want to be grounded....

Thats where octocouplers come in the game, no? ´Cause i saw many many examples on the net with octos