Here's a basic question: what does it mean to say that a device is current controlled, and how does it contrast with something that is voltage controlled?
If we look at a device as a system that has an input/output relation y = T(x), with T being a function of some controlling variable p,
do we say that the device is: "Current controlled", if p is a current, and "Voltage controlled", if p is a voltage?
In that case, if I take a T which is a function of a current p (which means T is a current controlled device), and if I'm able to express the p as a function of a voltage, doesn't T become a function of that voltage(and thereby a voltage controlled device)?
This sort of description usually means that it is simpler to describe the relationship between the specified input variable and the output variable. There are exceptions, however.
For instance, a relay is easier to describe, accurately, as current controlled, because the magnetic forces that are needed to operate the relay are almost exclusively related to the coil current. However, in this case, since most relays operate in essentially constant voltage environments, the coil current requirements are combined with the range of coil resistance over the operating temperature range, and converted to an operating voltage specification, for the convenience of the user. But if you are trying to predict at exactly what voltage the relay will pull in or drop out, knowing the current through it is more useful than knowing the voltage across it, especially if temperature variation can swing the coil resistance around quite a bit.
Bipolar junction transistors have a fairly linear relationship between base current and collector current (assuming collector to emitter voltage is larger than base to emitter voltage, and the temperature doesn't change much, and the transistors have been sorted into narrow range current gain groups), and people like linear math, so BJTs are often referred to as current operated devices. However, a more accurate way to describe BJT operation involves the very nonlinear relationship between base to emitter voltage and collector current, with a second very nonlinear expression for the base to emitter leakage current that takes place while the voltage is controlling the collector current. So beginners, who like linear math, use the more approximate current controlled model, while chip designers use the more complicated, but more accurate voltage controlled model.
Field effect transistors draw so little DC gate current that they are almost universally described as voltage controlled devices, even though they can require very significant current, if you want to change their output current very quickly. This is because the gate acts as a nonlinear capacitance during operation. Modeling the transients requires taking into account, the effect of both the drain voltage changes across the changing gate to drain capacitance and the gate to source voltage changes across the changing gate to source capacitance.
Hi, Pradyumna. Phil and John have pretty much covered it.
You're probably just overthinking this. Assuming you've got some background in op amps, here's an example of current controlled and voltage controlled devices which aren't transistors (view in fixed font or cut and paste to Notepad):
They're just descriptive terms. Now, it's possible the input to the current controlled amplifier is actually a 4-20mA transducer measuring pressure, or the voltage controlled amplifier input is a temperature sensor. So, the final terms might be mV/degree C or V/psi.
The important thing is, the terms are meant to help you understand how the device works, not meant to further confuse. Let it settle, have a beer and think about it. It's a lot easier than you're making it.
A couple of other examples to help re One I am working with now - LED backlights. The linear transfer function of an LED is basically current -> emitted photons (very simplistically), so my drive for these is a current source. The voltage is the variable here; it varies up and down depending on applied current and LED forward voltage which varies with temperature. Can an LED be described by it's forward voltage vs. output light? Sure, but it's not as clean as describing it by the current through it.
Another one was where I designed a simple current mirror (decades ago now) to measure the current used on a piece of equipment under test. In that case, I fed the mirrored *current* as input to the scaling amplifier. Here the *only* description was Vout = Iin x scaling factor. (Known as a transresistance device, incidentally, for the trivia minded).
Phil, John, Chris and PeteS, Many thanks for the insights. I'm clear about it now. I understand now that it's all in the model that I choose to use. While daffodils may look like they're swaying in the breeze, it doesn't hurt if one chooses to look at their motion as complicated manifestations of pressure variations ;-) Thanks again, Pradyumna
As an afterthought, the BJT is not a current controlled device, a MOSFET is not a voltage controlled device, just the way no resistor obeys V=RI.
It's true, however, that a ""current controlled device" model" is a good model to describe the BJT, a ""voltage controlled device" model" is a good and tangible approximation to a MOSFET, and the ohm's law is a fine approximation to the behaviour of the resistor in normal, peaceful conditions. (Does the ohm's law hold if the resistor is up in flames, is a BJT still current-controlled if it's junctions are under uncontrolled avalanche?)
Well, some kind of a spring, dashpot and mass model and a force, probably three dimensional, may describe the daffodils quite well in the breeze, but the daffodils aren't simple enough to be captured by such simple linear equations.. Just wait for a tornado, and look at the daffodils :-D
ElectronDepot website is not affiliated with any of the manufacturers or service providers discussed here.
All logos and trade names are the property of their respective owners.