Differance in bipolor mosfet and jfets? - Page 1
Forum section: electronics / beginners

author: Beamin
author_rank: Super Contributor
timestamp: October 10, 2017, 12:04:35 am
content: I was trying to figure out: Is there a transistor that lets current pass when there is no voltage applied to its "base" and stops when it get voltage? Is this what a jfet does? And a mosfet just needs a pulse to turn it on unlike a bipolar that needs constant current and that current is proportional to the output?So whats saturation? Is there a way to control voltage by adjusting input current or vice versa? When ever I try to figure out current controlled vs voltage I usually read about a debate to which is which. But there must be a clear solution?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 10, 2017, 12:12:59 am
content: What do you mean by "gets voltage": do you mean positive voltage (for N type device)? Or do you mean any magnitude of voltage at all?If the latter, then yes: JFETs, depletion MOS, and vacuum tubes all are "on" when the gate-source (or equivalent) voltage is zero. They turn off when the voltage is negative.The direction of operation is always the same: a common source (or emitter or cathode) amplifier is always inverting. There are no transistors with a positive gain, common source characteristic.(There are, however, magnetic amplifiers, which are most easily realized as voltage followers. These are not transistors, though!)Tim

author: RJFreeman
author_rank: Regular Contributor
timestamp: October 10, 2017, 12:57:17 am
content: A Bipolar Transistor is current controlled. That is the current flow between Base and emitter controls the current flow between collector and emitter.So; if you have an NPN Transistor with a gain (aka Hfe) of 100, then if you have 1 mA flowing from Base to Emitter, you would have 100mA flowing from Collector to Emitter.But say you have a 10 V supply, and a 200 ohm resistor between the power supply and the collector of the Transistor.Then with 1mA flowing from Base to Emitter, but with only 50mA (i.e. 10/200=50mA) available, the Transistor is said to be in saturation, as it would like to pass 100mA, but can't as there is only 50mA available.So Saturation basically means the Transistor is on as hard as it can, but there is not as much current as it could be switching.A Bipolar Transistor will require at least 0.6V between Base and Emitter before it turns onFETS (mosfets and Jfets are different kinds of FETs) are Voltage controlled. That is a voltage between Gate and Source (VGS) controls the current flow between Source and Drain (IDS).As well as N and P channel, Mosfets generally come in two types, Enhancement mode and depletion mode.For an enhancement mode N channel Mosfet, a Positive voltage between Gate and Source will turn the Mosfet on, so current flows between Drain and Source. While to turn it off, you need zero Volts between Gate and SourceFor a Depletion mode N channel Mosfet, zero volts between Gate and Source will turn the Mosfet on, so current flows between Drain and Source. While to turn it off you need a Negative Voltage between Gate and Source.Probably the easiest way to summarise is this table (which I pinched fromhttp://www.electronics-tutorials.ws/transistor/tran_7.html);MOSFET TypeVGS (+ve)VGS (0v)VGS (-ve)N-channel EnhancementONOFFOFFN-channel DepletionONONOFFP-channel EnhancementOFFOFFONP-channel DepletionOFFONONJFETs use Depletion mode, so act similarly to Depletion mode Mosfets (there are other subtle differences).

author: strawberry
author_rank: Super Contributor
timestamp: October 10, 2017, 01:19:45 am
content: Quote from: T3sl4co1l on October 10, 2017, 12:12:59 amIsn't they 3state devices like gate controlled resistors? positive gate decrease resistance, neutral gate steady state, negative gate increase resistance

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 10, 2017, 04:04:15 am
content: Quote from: strawberry on October 10, 2017, 01:19:45 amA JFET, or to a lesser extent, MOSFET too, at low Vds, is effectively a gate dependent resistor, yes. It's not tristate because those are positions on a continuum of on-ness and off-ness. It's analog, not digital.Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 10, 2017, 04:13:11 am
content: Quote from: RJFreeman on October 10, 2017, 12:57:17 amWithout rehashing this old troll subject . . .BJTs are voltage controlled devices.Regardless of material: germanium, silicon, SiGe HBT, or SiC: BJTs all very closely follow the Ebers-Moll equation,Ic = Is * exp(Vbe / Vth)Where Vth = N * k_B * T / q_e,when in forward bias, in the linear range.This is true, completely and utterly regardless of what the device's hFE is, or how it varies with temperature (of course, Vth and Is vary with temperature as well, but it's a better known variation, arising from absolute temperature, and the density of states, respectively).Better to understand the BJT as an ideal transconductance amplifier, with the given gain (as above), with the downside that it happens to consume an input current of Ic / hFE, whatever hFE happens to be (which varies, especially in saturation).That way, you capture both input and output characteristics: the input acts like a diode junction, and most of that diode's current goes to the collector pin, with a little going to the base.QuoteIn saturated operation, the hFE limited design approach is perfectly acceptable -- and preferred. The thread subject is about linear range operation, however, which demands a different approach!Tim

author: RJFreeman
author_rank: Regular Contributor
timestamp: October 10, 2017, 05:02:43 am
content: QuoteI suspect that is the discussion that confused the OP.

author: Beamin
author_rank: Super Contributor
timestamp: October 10, 2017, 06:59:23 pm
content: Quote from: RJFreeman on October 10, 2017, 05:02:43 amSo if some one says BJT's are current controlled devices can I call them an idiot? Or am I the idiot? Lets get to the bottom of this using math.If you put 1 volt 100ma into a transistor with 10v 1a at the collector what happens? What is the formula in lay terms?.Lets hook our BJT up to a bench power supply: What happens if you turn the voltage up and down and feed it constant current? What happens if you turn the current down?What happens if I put a mosfet or jfet in its place?How would I build this circuit; as in what should I use as a load (I don't have a fancy dummy load machine just resistors and don't want to make burns in the carpet/hands/bread board)? I have a feeling there is an eevblog video on this but can't find it. Would be a cool video to see with the dummy load machine connected.

author: GreggD
author_rank: Regular Contributor
timestamp: October 10, 2017, 07:30:39 pm
content: Try a DN3525N8-G N-Ch mosfet 250VIt is on with 60 ohms with zero gate voltage (yah there not high power)Off with gate voltage.Have a nice day storming the castle.

author: rfeecs
author_rank: Frequent Contributor
timestamp: October 10, 2017, 07:32:27 pm
content: Quote from: Beamin on October 10, 2017, 06:59:23 pmIf you had a curve tracer you could do exactly this.With a bipolar you get a family of curves for different base current.With a FET you get a family of curves for different gate-source voltage.Please forget current controlled or voltage controlled. It's all semantics.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 10, 2017, 08:55:00 pm
content: Quote from: Beamin on October 10, 2017, 06:59:23 pmYou say they're using a risky method, because hFE varies by a considerable margin across several variables, whereas transconductance is simple and given.Semantics? Sure, why not. Semantics exist because they're useful.The best example, I think, is a circuit which should be perfectly fine with a true current-input device, but which falls flat on its face when built with a BJT. In this circuit,The group wrapping around from top-right to bottom-left has a pull-up resistor (22k) charging a capacitor (0.0033 uF) into a base junction.The 10k pull-down on the base is required for operation. If the BJT were a current controlled device, the 10k would not be necessary.Does that sound like a reasonable statement to make?The actual effect is that, without the 10k, yes, the circuit does still work: but it works very sloppily. The purpose of this element is to terminate a square-wave pulse if its duration is too long. That is, the 22k charges the capacitor, and when the capacitor is fully charged, base current ceases, the transistor turns off, and the output is turned on again. This happens very slowly (~20us), because the supplied current is relatively excessive (22k delivers peak 0.2mA, whereas the transistor is switching 0.5mA so should only need 0.005mA), and is due as much to the RC time constant as it is due to recombination in the transistor junction (storage time). The time delay is very raggedy, noisy, because this process is very sloppy. It's also very susceptible to noise, because the current is small (the impedance is high).Increasing the 22k resistor to, say, 1M, and reducing the capacitor accordingly, is not a feasible solution. The dV/dt at the driving transistor will not be high enough to deliver enough current through the capacitor, and the second transistor won't turn on at all! The second transistor also has even more capacitance than the collector, so you're dealing with a capacitor divider, which attenuates the drive signal.Using a voltage mode design approach, all constraints can be neatly met: the signal is fast (~100s ns risetime), the timing is crisp, and the RC time constant is well defined.This would all be easier to explain with waveforms, but I don't think I have any handy. This circuit is years old.QuoteFirst, define your conditions: what is a "1V 100mA" source? Is it a constant voltage source, with a nominal 100mA capacity (the most common use of this type of specification)? Or is it a source that is current-limited to 100mA under some specified condition -- in which case, what condition is it? And how does it go between those conditions (voltage limit to current limit) -- is it straight line like a bench supply, or resistive?The transistor also matters, because some have hFE so low that you won't exercise the full collector current range, and see what you might've expected to see (namely, saturation). Others have it so large that you'll see nothing but. When it happens to be in the linear range, the Ebers-Moll equation will still hold (within its accuracy, because it too is just another model, of course), and hFE will hold as well.It's not exclusive, but which one is more useful.QuoteA MOSFET of course has very little current consumption on its gate terminal, so that the only thing you will observe is the gate voltage ramping up or down when a positive or negative current is applied. Similarly with a JFET, except the G-S junction is also a diode, so it will limit at ~0.7V (plus series resistance).There is possibly the knock-on effect of JFET enhancement. When the gate is forward biased, free charges are injected into the channel (the gate is actually an emitter, too, if that makes any sense), which can reduce the channel resistance (Rds(on)) below the resistance set by its doping level.This effect was exaggerated and harnessed in another obsolete device, the UJT (unijunction transistor). As its name suggests, it transists (positive voltage on the input terminal results in current flow in the output terminal), and it has only one junction. It's designed as a JFET with a really long, resistive channel. The channel is too wide, so it does not pinch off; instead, when the emitter (formerly gate) is reverse biased, current still flows. The resistance above and below the emitter puts the junction in the middle of a voltage divider, typically a few kohms. Thus, when the emitter voltage rises above the divider voltage, current flows, enhancing the channel, reducing its resistance; this both acts to short out the emitter voltage to ground (drawing even more current, enhancing conduction even further -- a runaway (negative resistance) condition), and causes the channel to draw current through the remaining terminal.Thus, a UJT was very easy to use for relaxation oscillators, seeing use in SCR/TRIAC phase controllers, timing applications (who needs a 555 -- when they're not even invented yet?), tone generators, maybe even some computing uses (negative resistance can be used for logic operations; see also tunnel diodes and neon lights!).UJTs are long obsolete, but one can still be constructed from discrete components:https://youtu.be/Iscf6UJF3MY?t=463Tim

author: LvW
author_rank: Frequent Contributor
timestamp: October 11, 2017, 08:42:19 am
content: Quote from: rfeecs on October 10, 2017, 07:32:27 pmAs an engineer, I do not understand such a position.For a good understanding of active circuits it is absolutely necessary to know if the ouput of this device can be controlled by a voltage or a current.This is important, in particular, because both explanations can be found in the various text books.Everybody who is involved in teaching electronics knows that students are confused because of the two different "explanations" (even in this thread!!) - and they ask for clarification.[By the way: The famous Barrie Gilbert (...the base current is just a defect") has explained the background for this surprising phenomenon of two different views - several decades after the BJT was invented].Finally: There are (a) many observable effects (Early-effect, tempco of -2mV/K,...) as well as many BJT-based circuits (low-resistive base voltage divider, voltage feedback due to emitter resistor Re, current mirror, translinear loops,...) which work because of (and which can be explained with)voltage controlONLY!I wonder if there is somebody who is denying the validity of Shockleys equation describing the relation between voltage and current for a pn junction.

author: Zero999
author_rank: Super Contributor
timestamp: October 11, 2017, 12:55:15 pm
content: With, N-MOSFETs, JFETs and BJTs, the drain or collector current is positive and increases as the gate or base voltage increases.With, P-MOSFETs, JFETS and BJTs, the drain or collector current is negative and increases as the gate or base voltage decreases.The difference is with depletion mode devices the drain current flows, when the gate voltage is zero. To stop the drain current, the gate voltage has to be taken below the source voltage (N-channel) or above the source voltage (P-channel).When using a BJT as a linear amplifier, it's best to consider it to be a voltage controlled current souce. When using a BJT as a switch, it's best to consider it to be a current controlled switch.

author: rfeecs
author_rank: Frequent Contributor
timestamp: October 11, 2017, 04:25:47 pm
content: Deja vu.I see the same people gathering up for a repeat of this unfortunate thread:https://www.eevblog.com/forum/beginners/current-controlled-device-vs-voltage-controlled/

author: LvW
author_rank: Frequent Contributor
timestamp: October 11, 2017, 06:46:43 pm
content: Quote from: rfeecs on October 11, 2017, 04:25:47 pmWhy unfortunate? Don`t you like real discussions ?“Religion is a culture of faith; science is a culture of doubt.” (R.P. Feynman)"Many errors, of a truth, consist merely in the application of the wrong names of things." (Spinoza)"The most erroneous stories are those we think we know best--and therefore never scrutinize or question".(Stephen Jay Gould)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: October 11, 2017, 08:28:02 pm
content: Quote from: LvW on October 11, 2017, 06:46:43 pmAn interesting use of quotes...The discussion has been had; you are welcome to read the arguments, made in the link cited above. Once you affirm your knowledge and understanding of all those arguments, if you havenewarguments, feel welcome to air them.The request for more discussion, for the sake of discussion, implies the desire for religious repetition of mantra, the drilling of wrote information that each camp repeats mindlessly in an attempt to drown out the din of the opposing camp.It's a call for tribal politics, the very same force which is being ever so expertly conducted by our present world masters, in the new, and more influential than ever, battlefield of "social media".(Conspiracy enough for 'ya? Or too real?)Anyway, I digress. I probably should've stopped at "discussion for the sake of discussion".Tim

author: Beamin
author_rank: Super Contributor
timestamp: October 12, 2017, 03:15:12 am
content: Quote from: LvW on October 11, 2017, 06:46:43 pmI think its because its important and when you ask anyone they say its definitely one or the other with such authority. Then someone says the complete opposite.Does eevblog Dave have a video putting this topic to rest?

author: Zero999
author_rank: Super Contributor
timestamp: October 12, 2017, 07:59:12 am
content: Quote from: Beamin on October 12, 2017, 03:15:12 amI believe Dave has done a video where he suggests that BJT devices are current controlled, which started one of those discussions in the first place.The truth is, saying a BJT is current or voltage controlled is a simplification. It's true that when the BJT is biased into its linear region and the temperature is constant, the collector current can be more easily predicted, by looking at the voltage between the base and emitter, than the current in to the base junction. An argument could easily be made about BJTs being temperature controlled: keep the base-emitter voltage constant and the collector current can be predicted by measuring the temperature of the transistor junction.

author: LvW
author_rank: Frequent Contributor
timestamp: October 12, 2017, 08:04:56 am
content: Quote from: Beamin on October 12, 2017, 03:15:12 amFortunately, we are not forced to "believe" somebody because the subject of discussion is not a matter of anyones opinion or view.It is simply a fact that can be (and is) proved.

author: Neomys Sapiens
author_rank: Super Contributor
timestamp: October 12, 2017, 10:58:06 pm
content: The discussion can be finally forced into religious or political realms when we take notice that current, and not voltage, is at the base of our electrical system of units!?!

author: KL27x
author_rank: Super Contributor
timestamp: October 12, 2017, 11:07:44 pm
content: QuoteAdditionally, a transistor isn't tri state because it can be high Z, but it can't switch between both sinking and sourcing. NPN can be used as open collector, for instance. It can sink or be at high impedance (plus anything between). But sourcing isn't on this contiuum. 2 out of 3 ain't bad, though.I/O pins require at least 2 transistors in the pin circuitry to achieve tristate (at least in the traditional way).