# Current - vs - Voltage

• posted

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

• posted

"Scott Brehler"

** Yawn - not this boring one again ....

In a nut shell:

Current controlled = you need to keep suppling current into it = BJT etc.

Voltage controlled = presence of a control voltage required only = Mosfet etc

........ Phil

• posted

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

• posted

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).

• posted

Circa Sat, 02 Jun 2007 03:09:05 GMT recorded as looks like "Scott Brehler" sounds like:

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.

• posted

A way of thinking about this is that some things respond relatively 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

• posted

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

Jon

• posted

one

could

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

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