MOSFETs for current. - Page 1
Forum section: electronics / projects

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 28, 2023, 11:56:38 am
content: As you can use bipolar junction transistors to deliver current and power in a linear power supply, is it possible to use MOSFETs for the same purpose. I know MOSFETs are either on or of, and bipolar transistors have a linear functional ability. I wanted to know if FETs can do the same job as a BJT in the same application. So turn on when regulator current reaches a certain level set by the gate resistor. I have seen a couple of circuit diagrams in searches, but the application text is a bit vague on using MOSFETs in a linear power supply. I guess if it is possible, it's either the FETs are on or off, and no linear characteristics with them. I'm interested in your thoughts on this. Thanks for reading.

author: wasedadoc
author_rank: Super Contributor
timestamp: August 28, 2023, 12:06:12 pm
content: Quote from: davelectronic on August 28, 2023, 11:56:38 amYour knowledge is faulty. (As is your spelling under your avatar.)

author: BrokenYugo
author_rank: Super Contributor
timestamp: August 28, 2023, 12:34:54 pm
content: It isn't that MOSFETs are incapable of linear operation as a whole, it's just that most are optimized for switch use to a point they aren't even specified for linear, as they tend to self destruct.The search term you're missing is "DC safe operating area", it's part of the safe operating area graph on the datasheet. Many MOSFETs are only specified for switching, but some are specified for linear (DC) use. If you search around you'll find discussions about suitable parts, additional thermal derating etc.

author: xavier60
author_rank: Super Contributor
timestamp: August 28, 2023, 12:42:12 pm
content: There are MOSFETs suited to high power linear applications. They tend to be ones with highish voltage ratings. Then look for the Safe Operating Area charts to decide the suitability. I chose the PSMN020-150W,127 for a 20A post regulator, now EOL.I recently chose an ST MOSFET for a linear application, yet to be tested.

author: moffy
author_rank: Super Contributor
timestamp: August 28, 2023, 12:48:57 pm
content: MOSFETs have been used successfully in the output stages of linear amplifiers for quite a while. They require different biasing to BJTs and are a bit more of a challenge, but they do have low drop out voltages from D-S because at low D-S voltages they look like a resistor. You can't use them like the classic current monitoring BJT with a resistor between B-E monitoring the current as the turn on voltage is generally higher than Vbe for enhancement mode MOSFETs and can be quite variable from device to device (I am excluding low voltage ICs from this, just talking about common discreets) Please excuse the simplified explanation as I am just trying to provide a brief overview to what is a vast topic.

author: David Hess
author_rank: Super Contributor
timestamp: August 28, 2023, 06:11:09 pm
content: Both bipolar transistors and MOSFETs are voltage in current out devices with a transconductance (dCurrent/dVoltage) characteristic. Both can be used in their linear range but most MOSFETs are designed for saturated switching. Nothing prevents using these MOSFETs in their linear range but they often lack specifications for linear operation.MOSFETs can be less advantageous in linear applications for a couple of reasons. For a given power capability, which is determined by die size, they are more expensive. MOSFETs also have lower transconductance than bipolar transistors. The threshold voltage of a MOSFET is higher than the Vbe of a bipolar transistor so biasing requires higher voltage. Matching between MOSFETs is worse than the matching between bipolar transistors.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 28, 2023, 06:48:28 pm
content: Quote from: wasedadoc on August 28, 2023, 12:06:12 pmNot very helpful, but who cares. I have read an old post that popped up in a search, I suppose what I meant either on or off was a MOSFETs switching ability. I should have said that, I can see from all your helpful explanations, there not well suited to linear current use, not whilst there are good BJT transistors available. I was Just curious if MOSFETs could be used for the same purpose as BJT's.

author: rhodges
author_rank: Frequent Contributor
timestamp: August 28, 2023, 06:50:14 pm
content: Many years ago, I used a pair of IRF540 "Hexfets" in the linear mode to drive a variable flow hydraulic valve. The gate voltages were set with precision screw potentiometers.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 28, 2023, 07:51:38 pm
content: That's interesting, I'm guessing the hydraulic valve was an inductive load possibly. Although I don't know how FETs are affected with inductive or resistive loads, or a combination of both. I need to read a lot more about the different types of MOSFETs. I'm not going to run out and buy a load of them to try. But I need to find out more how MOSFETs behave when there in the linear region. I can see it's not straight forward to use MOSFETs in this way, in a transformer driven linear power supply.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 28, 2023, 08:05:20 pm
content: Quote from: davelectronic on August 28, 2023, 06:48:28 pmMany LDO use MOSFETS, so they certainly are used.MOSFETS are widely used in audio power amps.Linear use at high power is rare, which brings another issue mentioned above : EOL & package choiceThe TO247 package at low voltages is now rare.MOSFETS with very low RDS are now cheap and available, so SMD packages are fine, preferred by the high volume customers.

author: ajb
author_rank: Super Contributor
timestamp: August 28, 2023, 08:40:15 pm
content: Quote from: davelectronic on August 28, 2023, 07:51:38 pmUsing a FET in place of a BJT+resistor current source isn't straightforward, but throw an op amp in there and things are a lot simpler. There are still a few differences to consider: in particular the gate capacitance causes a lagging response in the control loop which needs to be compensated for, and with high slew rates you have to manage parasitics in similar ways as in a switching circuit to avoid oscillation. Whether the load is resistive/capacitive/inductive is a matter for the control loop to be designed around. But those are generally surmountable problems, and may be well worth the trouble to deal with. Using a FET may result in lower power dissipation versus a BJT, since Vds may be lower than Vce (this depends on the load characteristics as well as the control mode), and in steady state there's no significant gate current, versus some continuous base current with a BJT.The biggest thing to watch out for, as has been mentioned, is the safe operating area of the particular part. Many FETs aren't very well spec'ed for DC operation, and when they are the current/voltage ratings are SIGNIFICANTLY reduced versus the headline numbers in the datasheet. You may well end up selecting a "20A" part to handle a 1A load. There are a small number of parts advertised for linear operation, but there are many more general purpose parts that will also work--you just have to sift them out of all of the parts that won't. Once you've got a candidate with a suitable SOA you want to look at the gate characteristics, which will give an idea of how challenging it will be to drive.

author: langwadt
author_rank: Super Contributor
timestamp: August 28, 2023, 08:47:24 pm
content: Quote from: PCB.Wiz on August 28, 2023, 08:05:20 pmLDMOS still used for RF power amps

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 28, 2023, 09:26:42 pm
content: I'm looking for many ten's of amps, it doesn't sound like I am going to find that in a MOSFET, I have RF linear amplifiers that have MOSFETs in them. But in that application they get driven quite hard, and I know if over driven they will fail. I have read about the safe area of operation, but if it's like, a 20A part to gain 1 Amp, it's not worth trying to use them. Another down side at the moment is I don't have an ocillascope to look at the behaviour of a MOSFET in a psu circuit. If I can get a reasonable scope I might try to find a suitable FET to use. Anyway I need to try and get a scope just as a much needed tool.

author: nctnico
author_rank: Super Contributor
timestamp: August 28, 2023, 09:41:06 pm
content: Quote from: BrokenYugo on August 28, 2023, 12:34:54 pmIndeed. There are quite a few MOSFETs out there that are optimised for audio purposes. Actually, MOSFETs and transistors are pretty similar in that way. Some are designed for switching, others are designed for linear operation. If you look for some teardowns of DC loads you'll often find these use MOSFETs intended for switching though. So things aren't as bad as the OP may think.It all comes down the the DC safe operating area. And keep in mind that MOSFETs have a negative tempco in their linear region just like transistors do. So hotter means more current!

author: DavidAlfa
author_rank: Super Contributor
timestamp: August 28, 2023, 09:44:10 pm
content: Quote from: davelectronic on August 28, 2023, 11:56:38 amNope, not at all! It like saying BJTs are either off or saturated.Mosfets have their own active region, called ohmic region.Look at mosfets like some sort of voltage-controlled potentiometers, the higher the gate voltage, the lower the channel resistance.You have mosfets capable of 100Amps everywhere.Plus fets can be easilly connected in parallel, as they don't have a bandgap voltage like bjts.Put 10x 50Amp fets in parallel, now it's a 500Amp fet!Of course it needs a bit of extra circuitry to drive the gates fast enough, as each gate adds capacitance, 10 power fets can easily sum 100nF, which is not easy to switch at several KHz.

author: nctnico
author_rank: Super Contributor
timestamp: August 28, 2023, 09:46:31 pm
content: Quote from: DavidAlfa on August 28, 2023, 09:44:10 pmNot when used in their linear region! You'll find MOSFETs have a far worse spread compared to transistors. Thats why DC loads using MOSFETs typically have a control loop circuit for each individual MOSFET.

author: DavidAlfa
author_rank: Super Contributor
timestamp: August 28, 2023, 09:47:37 pm
content: Quote from: nctnico on August 28, 2023, 09:46:31 pmTotally true, my bad! I'm too used at using fets as switching devices

author: TimFox
author_rank: Super Contributor
timestamp: August 28, 2023, 09:53:25 pm
content: If you look at the characteristic curves of the JFET, MOSFET (similar to JFET curves), BJT, and vacuum pentode, they are superficially similar.Each of these curve sets is a family of curves ofI(drain, collector, or plate) vs.V(drain-source, collector-emitter, or plate-cathode).The JFET, MOSFET, and pentode curves are each for a givenV(gate-source or grid-cathode), while the BJT curves are each for a givenI(base).They look similar, because at relatively high voltage they are all roughly horizontal (high impedance).At the left edge, these devices' curves are a bit different.Another device, the vacuum triode, has different looking curves since each curve is at a finite slope (medium impedance).The SOA discussed here is the region inside these I-V graphs where the device can operate safely, as a function of conduction time.Of course, for switching applications, you try to stay at the bottom (current off, high voltage) or left edge (current on, low voltage) to minimize the power dissipation and keep the device temperature (junction or plate) within safe limits.Over the rest of the graph, the power dissipated in the device is substantial.

author: ajb
author_rank: Super Contributor
timestamp: August 28, 2023, 09:57:51 pm
content: Quote from: davelectronic on August 28, 2023, 09:26:42 pmOverdrive ANYTHING and it will fail. That's just a reason to not overdrive things, it isn't a reason to categorically discard a whole class of components.QuoteThis actually isn't much different from BJTs, because in steady state, ultimately you're concerned with power dissipation, which is limited by package type and heatsinking. My point was that switching-optimized (or at least switching-advertised) FETs will be spec'ed for a very high current, because in that operating mode the current is high when the voltage is low and vice versa, which limits power dissipation, except during the transition between off and on. If you put 10V across and 10A through a transistor, you're going to be dissipating 100W whether it's a BJT or FET. Big chunky transistor territory either way!

author: TimFox
author_rank: Super Contributor
timestamp: August 28, 2023, 09:59:41 pm
content: Yes, normally the SOA for very short time of power dissipation is larger than at longer times.There are other limits on the maximum current that are included in a proper SOA.

author: MIS42N
author_rank: Frequent Contributor
timestamp: August 28, 2023, 10:10:18 pm
content: @davelectronicWhat wasedadoc was pointing out is your poor grammar. Life'stooshort. If it's broke,fix it

author: langwadt
author_rank: Super Contributor
timestamp: August 28, 2023, 10:12:44 pm
content: Quote from: ajb on August 28, 2023, 09:57:51 pmbut many mosfets are much worse than you'd think, rated for hundreds of Amp and several 100 watt and then you look at the DC SOA, 1A at 20V

author: TimFox
author_rank: Super Contributor
timestamp: August 28, 2023, 10:22:08 pm
content: See TI app note:https://www.ti.com/lit/an/sluaao2/sluaao2.pdf?ts=1693228620419Like many parts, a MOSFET might be advertised for a voltage rating and a current rating, but that does not imply that you can apply that voltage and current simultaneously.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 28, 2023, 10:35:22 pm
content: Quote from: MIS42N on August 28, 2023, 10:10:18 pmI thought that was a play on words : life was too short, to add the extra o !

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 28, 2023, 10:56:18 pm
content: Quote from: davelectronic on August 28, 2023, 09:26:42 pmThere are parts designed for better SOAhttps://www.nexperia.com/products/mosfets/application-specific-mosfets/asfets-for-hotswap-and-soft-start/PSMNR56-25YLE.html- but notice thepackage and thermal problemswill be more of a practical ceiling, than the device curves.Even BJT have second breakdown limits too, eg a 20A part with nice HFE like 2SA2210, has a practical 30W rating, but at 30V it is ~300mA maxIt can tolerate 2A at 30V for 1ms, or ~ 4.5A at 20V for 1ms so any linear design will need foldback current limiting.Anything needing 20A linear is going to needmany packages spread over a large heatsink, probably with decent tunnel airflow.Quote from: davelectronic on August 28, 2023, 09:26:42 pmA good scope is essential, as like Audio Amps, linear regulators can oscillate in the many 10's of MHzA decent Audio amp is also a useful too, to sweep the load current, via a power resistor, as you need to check stability at all currents.

author: xavier60
author_rank: Super Contributor
timestamp: August 28, 2023, 11:27:07 pm
content: The PSMN020-150W,127 I mentioned earlier is good for ~15V, 20A @ TC = 25°C. It's still available on the grey market.The FDH055N15A I use in motor drives is ~10V, 20A @ TC = 25°C. I'm certain there is many suitable parts out there.Keep in mind that the ratings in the SOA charts are only for TC = 25°C, so needs to be derated for higher temperatures.Use of a mica washer will add tens of degrees to TC. Direct mounting to a heat spreader can help or use multiple devices to spread out the dissipation.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 29, 2023, 03:10:12 am
content: Quote from: davelectronic on August 28, 2023, 06:48:28 pmIf you want to study voltage regulator examples, ADi have Discrete Pass Element Linear Regulators (LDO) : some that can support N-FETs and some PNP.https://www.analog.com/en/parametricsearch/11478#/

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 29, 2023, 11:54:01 am
content: Don't know what has happened to this thread, looks like some of my posts have disappeared, very strange. Thank you for all your help and the links.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 29, 2023, 12:27:55 pm
content: Both tapes can perform the same functions. The have pros and cons of course.Note that mosfets are voltage controlled devices whereas bipolar transistors are current controlled.You cant generally swap one for another.In a simple linear regulator MOSFETS can be a headache because of the higher headroom needed.Bipolars are more prone to meltdown.You can have the best (or worst) of both worlds with IGBTsStart by understanding how a Bipolar single and Darlingtons work in these circuits before moving on to mosfets.

author: David Hess
author_rank: Super Contributor
timestamp: August 29, 2023, 01:24:00 pm
content: Quote from: Terry Bites on August 29, 2023, 12:27:55 pmThey are both transconductance devices. The base-emitter current of a bipolar transistor is a side effect and not how it primarily works; it is just something which must be taken into account.Ic = Is * e^(q*Vbe/k*t) works over a range of almost 10 decades and tells that the transconductance is 60 millivolts per decade of current at room temperature, and that the emitter resistance in ohms is 26 / (current in mA) at room temperature, while hfe (current gain) varies all over the place and between devices.If you are using bipolar transistors as switches, then hfe becomes essential.

author: davelectronic
author_rank: Frequent Contributor
timestamp: August 29, 2023, 08:16:31 pm
content: Poor grammar fixed, I hadn't noticed. I have a lot to take in before using MOSFETs in a power supply. IGBT's don't seem like a good idea, even if they are made up of a MOSFET and a BJT, it does look like there are a lot of boxes to tick before using a MOSFET in a linear power supply. I will read up more on the mechanisms of how these transitions work.

author: Zero999
author_rank: Super Contributor
timestamp: August 29, 2023, 09:39:37 pm
content: Quote from: davelectronic on August 28, 2023, 06:48:28 pmWhy did you not think it was helpful? Although that post is a little abrupt, it's correct. You obviously did find it helpful, because you corrected the spelling under your avatar. Never allow your ego to get in the way of learning. I admit it's easier said than done sometimes, but is an important lesson.QuoteUnfortunately there are a few nonsense posts on this forum.A common application for linear MOSFETs is op-amps. Many op-amps use MOSFETs for their input transistors because they have an extremely low bias current and it's possible to use the same process to make digital ICs, which reduces the cost.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 29, 2023, 10:17:34 pm
content: Quote from: davelectronic on August 29, 2023, 08:16:31 pmIGBT's are high voltage devices with poor VCEsat, giving a high dropout voltage.One appeal of MOSFETS is the very low dropout that is possible.Other Voltage regulator Controller parts with external MOSFETs are :Microchip MIC5158 & also UC3837 is external N-MOSFET, UC3836 is external PNP.Those are 'old' parts, but they prove it is possible. MIC5158 is still active at Microchip, tho not super cheap.The MIC5158 includes a charge pump for N-FET drive above the rail, has good VCC max, and it has a low 35mV current sense threshold.Newark are showing some at end of line prices

author: MIS42N
author_rank: Frequent Contributor
timestamp: August 29, 2023, 11:58:01 pm
content: Quote from: David Hess on August 29, 2023, 01:24:00 pmNo. They are current amplifiers. Here is a quick grab fromhttp://www.learningaboutelectronics.com/Articles/What-is-hfe-of-a-transistor"hFE of a transistor is the current gain or amplification factor of a transistor.hFE (which is also referred to as β) is the factor by which the base current is amplified to produce the amplified current of the transistor. The unamplified current is the base current, which then undergoes amplification by a factor of hFE to produce an amplified current which flows through the collector and emitter terminals.A transistor works by feeding a current into the base of the transistor. The base current is then amplified by hFE to yield its amplified current. The formula is below:IC= hFEIB=βIBSo if 1mA is fed into the base of a transistor and it has a hFE of 100, the collector current will be 100mA. "The current into/out-of the base encourages formation of more charge carriers. It is an essential part of the operation.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 30, 2023, 12:30:44 am
content: Quote from: MIS42N on August 29, 2023, 11:58:01 pmIt's all a bit academic, as the parts have a transfer curve and you can consider either axis ( I or V )Spice does not care about semantics, it just solves for the transfer equation.

author: moffy
author_rank: Super Contributor
timestamp: August 30, 2023, 12:41:36 am
content: Quote from: PCB.Wiz on August 30, 2023, 12:30:44 amI have to agree with David Hess in that voltage is the motive force or cause and current is the consequence or result, but if I can have it both ways PCB.Wiz is correct in that both approaches work.P.S. A simple way to think about it: What causes a current to flow through a diode? A forward voltage drop, the B-E junction of a BJT is a diode but with the added complexity of the collector but the current out the collector still follows a standard diode equation: Ic = Is*(e^(Vbe/Vt) - 1)

author: David Hess
author_rank: Super Contributor
timestamp: August 30, 2023, 12:42:36 am
content: Quote from: davelectronic on August 29, 2023, 08:16:31 pmIGBTs are advantageous where high voltages must be supported where they have an advantage in current density over MOSFETs for a smaller die size and lower cost, but this also limits their power dissipation capability. For a while Toshiba made complementary IGBTs for linear audio power amplifiers but they were not competitive with linear MOSFETs and bipolars.So again, there is nothing preventing the use of switching power MOSFETs in linear applications, but they have limitations similar to those of power bipolar transistors. Linear power MOSFETs exist, but come with a premium price.Quote from: Zero999 on August 29, 2023, 09:39:37 pmThere is not much cost savings because linear ICs require large areas and special encapsulation. National found this out once when some manager thought all IC packaging was the same, and directed their linear products to use digital IC packages; I mean they look the same on the outside. The result was ruined precision and matching with terrible yields because the cheaper digital encapsulation does not prevent stress from reaching the die. National had to buy ICs from competitors and remark them for months until they could refill their linear IC production pipeline.

author: David Hess
author_rank: Super Contributor
timestamp: August 30, 2023, 12:56:54 am
content: Quote from: MIS42N on August 29, 2023, 11:58:01 pmThat is how bipolar transistors are applied in switching applications, but hfe is not a useful part of the small signal model, if only because it varies so much with operating point.There is sort of an exception however. Transistors with *higher* hfe have a worse Early voltage. Their collector resistance is lower so a change in collector voltage has more of an effect on the collector current. That is why in linear circuits, high hfe transistors must be operated with a cascode transistor to preserve precision.So in a way there are two categories of bipolar transistors. Switching parts have high hfe, and precision linear parts have low hfe.QuoteYet changing the Vbe by 60 millivolts somehow changes the collector current by 10 times over more than 6 or 7 orders of magnitude no matter what the hfe does.

author: TimFox
author_rank: Super Contributor
timestamp: August 30, 2023, 02:28:41 am
content: Quote from: moffy on August 30, 2023, 12:41:36 amAnother component: does voltage cause current to flow in a resistor, or does current flow cause a voltage to appear across it?For the BJT, the Shockley equation is more repeatable for voltage causing a current flow, but just like the resistor the equation is monotonic and can be inverted.

author: MIS42N
author_rank: Frequent Contributor
timestamp: August 30, 2023, 12:42:52 pm
content: Quote from: David Hess on August 30, 2023, 12:56:54 amI am inclined to reword your earlier statement to "The base-emittervoltageof a bipolar transistor is a side effect and not how it primarily works; it is just something which must be taken into account.The base-emitter is a diode. An ideal diode would have no forward voltage drop regardless of current. However, the solid state physics of a diode has a band gap between valence electrons and conduction. Valence electrons require a voltage to raise them to an energy where they can move from atom to atom. Which is why applying 0.2V in a forward direction to a silicon diode results in zero current, it is not enough energy. [although thermal energy can assist and there may be an infinitesimal current].Valence electrons are not all bound by the same energy (a consequence of quantum mechanics), so a forward voltage of 400mV may just take the easy pickings, 460mV takes 10 times as many etc etc. because each increment of voltage allows harvesting exponentially more valence electrons.Consider a base-emitter current of 1A and an hFE of 10. The collector-emitter current will be 10Aregardless of the voltage at the collector[as long as it is above a threshold, below the breakdown voltage of the junction, and within the SOA]To say a BJT is anything other than a current amplifier is to not understand the physics.

author: langwadt
author_rank: Super Contributor
timestamp: August 30, 2023, 01:14:13 pm
content: Quote from: MIS42N on August 30, 2023, 12:42:52 pmso explain the simple two transistor current mirror

author: Njk
author_rank: Frequent Contributor
timestamp: August 30, 2023, 01:20:57 pm
content: Quote from: MIS42N on August 30, 2023, 12:42:52 pmAbsolutely. Go to a construction architect and teach him that Earth is round. It's stupid to assume it's flat

author: TimFox
author_rank: Super Contributor
timestamp: August 30, 2023, 01:47:25 pm
content: Quote from: langwadt on August 30, 2023, 01:14:13 pmExactly: the current mirror does not depend onhfeor matching thereof, but depends on the match betweenVbeat the design collector current.

author: David Hess
author_rank: Super Contributor
timestamp: August 30, 2023, 03:17:46 pm
content: Quote from: MIS42N on August 30, 2023, 12:42:52 pmLogarithmic conformity of a bipolar transistor (but not a diode!) extends significantly below a Vbe of 0.2 volts which could result in a collector current of 10s of picoamps. If this mechanism depended on hfe and the Vbe current, then this could not be the case.This is also why bipolar transistors make much better translinear elements than diodes, and indicates a fundamental difference in operation.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 30, 2023, 07:37:23 pm
content: Quote from: davelectronic on August 29, 2023, 11:54:01 amSome more examples and linear regulator controllers :https://www.ti.com/lit/an/slua072/slua072.pdfUses UC3833 and P-FETTI look to have UC3832 / UC3833 / UC3834 as still active parts.These are 36v rated with a 450mA ? Rated ‘floating’ output NPN, so can drive PNP or P-FET loads.A series power resistor and good PNP’s to keep ib < 100mA will help thermally.Addit: TI have another appnote for NFEThttps://www.ti.com/lit/an/slua476/slua476.pdfthat needs dual supplies, but this example gives information about the opamp FB input, with a 1.2V operating VoutTI also have spice models for this device too.

author: Zero999
author_rank: Super Contributor
timestamp: August 30, 2023, 09:02:35 pm
content: Quote from: MIS42N on August 30, 2023, 12:42:52 pmThen why is it, the collector current is the same, for a given VBE, even if the hFEis varied?Here's a simulation, of transistors, with different hFEs, with the bases connected to the same voltage. The collector currents are identical, despite the fact Q1 has an hFEof 50, Q2's is 100 and Q3's is 200.It's possible to do this in real life too. Select BJTs with the same base-emitter voltages, for a given collector current, yet different hFEs. It will be important to keep the power dissipation to a minimum, so self-heating is insignificant.