# Current - vs - Voltage

Discussion in 'Electronic Basics' started by Scott Brehler, Jun 2, 2007.

1. ### Scott BrehlerGuest

Hi all,

This has been burning in the back of my mind for quite some time, and I've
not been able to come up with an obvious/simple answer.

What is the difference between something being described as voltage
controlled as opposed to current controlled? I.E. FET's(voltage
controlled) - vs - BJT's(current controlled). If current and voltage are
directly proportional to each other (given a constant resistance)why is one
device described one way and the other another way? It seems that you could
always use voltage (or current) to describe either device and come up with
the same calculations.
Is it because the resistance of the device is variable that one is used
instead of the other? I refer to the transistors, but I've heard of other
devices described as current controlled or voltage controlled too. What
makes this distinction?

Thanks in advance for any input.

Scott

3. ### BobGuest

In general terms, something that is a voltage controlled device usually (but
not always) has a high input impedance and the "controlled" quantity is
most-easily described by the input voltage. A current controlled device has
the opposite characteristics -- that is (usually) a low input impedance and
the controlled thing is most-easily described by its input current. That's
not to say that you couldn't describe a voltage controlled device by its
input current, but it usually doesn't make sense to do this (see below).

When describing a FET with a simple model, its output current (drain-source)
is typically related to the input voltage (gate-source). The gate to source
impedance is extremely high so it's not appropriate to talk about its input
current. The output vs input quantities that are the most constant (from
unit to unit) are its output current vs its input voltage. This concept is
key in the characterization of a FET's transfer function. The ratio between
output current and input voltage is called the transconductance of the FET.

When describing a bipolar junction transistor (again a simplified model),
its output current (collector-emitter) is characterized as a function of the
input current (base-emitter). The base-emitter impedance is fairly low. The
ouptut vs input quantities that are most constant and linear are its output
current vs. its input current. The ratio between the output current and the
input current is called the Hfe and/or beta of the BJT.

Hope this helps.

Bob

4. ### John PopelishGuest

Any real device requires both voltage and current to control
it. But usually, one of those input variables generally has
a more linear relationship to the controlled output than the
other, so it is easier to think about that being the
controlling variable. This has little to do with the
physics of the situation, and more to do with simplified
mental models of what is going on.

For example, BJTs require a voltage base to emitter to
enable collector current, but there is an exponential
relationship between base to emitter voltage and collector
current, so it takes a higher level of mathematics to deal
with that relationship than it takes to deal with the more
linear base current to collector current relationship, even
though that relationship is less precise than the voltage to
current relationship is. So the simplest description of a
BJT is that it is a current controlled device. and you have
to add that the base to emitter voltage varies much less
than the current and has a nonlinear relationship to the
current (sort of as an after thought).

5. ### Charlie SiegristGuest

Circa Sat, 02 Jun 2007 03:09:05 GMT recorded as
<lb58i.17192\$> looks like "Scott
Aye, there's the rub. You are used to thinking in a linear manner, and
semiconductors are non-linear devices. John's response about users
preferring to examine the most linear relationship between control and
output is quite excellent. We all like to think in linear terms, because
the math - and the imagining of the operation - is simpler that way. Fewer
curvy lines means easier algebra.

6. ### Jonathan KirwanGuest

linearly with voltage, some with current. Some respond linearly to
either. By 'respond,' I mean that the desired outcome behaves
linearly to changes in the input. This doesn't mean, in any way, that
the internals in the black box being controlled must all operate that
way. Just the observable, desired behavior.

It is one thing to use 'x' and control the value of 'y' with y=k*x and
quite another to try and control the value of 'y' with y=A*e^(kx). If
you have a choice, you'll go with the first equation. It's just
easier.

Jon

7. ### Jonathan KirwanGuest

It's also just plain easier to control precisely, in practice, too.
Not only in one's imagination.

Jon

8. ### Scott BrehlerGuest

Thank you for the GREAT replys. I had sneaking suspicions, but wasn't able
to put it quite so elegantly as those of you. I am greatly enlightened now!

Scott