The AM Forum

This article describes an advancement in IC application. I can not find this product today, but maybe don't know what I am looking for. There are ICs that take a voltage and make a 4-20mA current, but these in the attached article do + and - polarities in and out. So what are they and where can they be bought?

Electronics-Today-1977-01:

SOME MONTHS AGO we ran a short
item about a device totally unknown
in this country at that time - the VCT
or Voltage to Current Transactor. Well
now we hope to make it considerably
less unknown. The VCT will get get its
commercial launch from Texas Instruments
early in 1977...

the microcircuit design was
breadboarded up for the first time
with discrete components, and early in
1975 the first I.C.s rolled out of the
ovens. The first vast improvement over
the op -amp to become apparent was
the slewing rate, up to 20V per
microsecond, as compared to
0.5V/ microsecond for the 741
The offset on these prototypes was
^10mV due to the layout not being
totally symmetrical. Production
models, when they appear, will have a
much much lower offset....

For a voltage
input, we get a pure constant current
output. Both input and output
impedances are very high, around 10s
of Megohms in the production devices.
There is a fixed ratio between Vin
and lo, which is set by one fixed resistor
R. i.e. lo = k 1/R Vin. The constant
k can be designed to be any value - it
will be four in the Texas (Instruments) VCTs. A bias
current is applied down BR, and the
device can only output twice as much
current as it draws through BR. Early' -R/k
devices will be 20mA output VCTs,
but later marks will be up in the amps
range. A ±15V rail is used with the
VCTs, and a 13V signal is quite permissable!

The application we found initially
most amazing is the (pair of) VCT's ability to
replace a transformer, better than a
transformer! All transformers exhibit
some power loss, but this circuit has a
selectable loss factor, which naturally
can become a gain if so desired...

This appears to be what we now call a "current amplifier" or "transimpedance amplifier".

Probably the first one made (at least IC and solid state - note it apparently was NOT patented?).

It's neat in that the circuit is a bridged amplfier.
They say in the text that if you need a voltage, drop a resistor at the output.
Same as the devices named above that we have today.

                  _-_-bear

Yeah it was the first, or very early type according to the article and no orderable part number was included.

One of the coolest things I saw was using a pair of them in place of an audio transformer. It could be useful for the audio chain although common sense says that connections to the outside world should not be direct to small ICs without protection.

I was thinking more along the lines of a mike input that could be adjusted to give a good match on various mikes, like an adjustable ratio transformer. It would be nice to switch from a lo-Z to hi-Z input by adjusting a knob or using a switch, and no expensive transformer required. I didn't really understand from the article how this worked but if it is one of the two modern products you mentioned, there ought be some appnotes.

Well, you might provide any R value (within the range of currents that can be sunk) and so have a fixed output Z of some value.

But in reality there is no benefit to that since driving a high Z from a low Z source works just fine as long as you have the proper level...

If ur really interested in more background on this, I can post it up to another forum and likely get a boatload of information.

 

It's not that important, just saw the article and wondered if the design had been successful.