BJT collector current equations - Page 1
Forum section: electronics / beginners

author: sy
author_rank: Regular Contributor
timestamp: November 19, 2024, 08:46:47 am
content: Sometimes I see the collector current calculated as Ic=beta*ib, and other times I see it calculated as Ic=Is*exp(Vbe/Vt).I am confused between the use case of the two equations. When is it appropriate the use one or the other?

author: magic
author_rank: Super Contributor
timestamp: November 19, 2024, 09:12:29 am
content: Quote from: sy on November 19, 2024, 08:46:47 amI guess it kinda depends on whether you care about the relationship between Vbe and Ic or between Ib and IcFor example, for the output impedance of an emitter follower (which is never zero) you would care how Vbe increases with load current.But if you want to know how much base current the same transistor will take to drive a 100mA load, suddenly it's β that matters.

author: MrAl
author_rank: Super Contributor
timestamp: November 19, 2024, 10:36:54 am
content: Quote from: sy on November 19, 2024, 08:46:47 amHello,Well, the first is a linear approximation to the collector current with change of base current Ic=Beta*ib.The second is the Shockley Diode Equation.There is a third: Ic=gm*Vbe.There are more but these should be enough for now.The third is also a linear approximation that relates the base emitter voltage to the collector current through the constant 'gm'. This turns the transistor into a voltage controlled current device.The first treats the transistor as a current controlled current device.The second attempts to model the physics of the transistor and would be considered "more accurate" than the other two shown here. It's still not perfect though.The two approximations are used for general analysis where an exact result is not needed.The Shockley Diode Equation can be used to get a deeper understanding of the bipolar transistor. That can be used to illustrate for example how the saturation voltage of the transistor works which shows up as an apparent drop in Beta in the linear model.

author: PGPG
author_rank: Frequent Contributor
timestamp: November 19, 2024, 05:27:30 pm
content: Quote from: sy on November 19, 2024, 08:46:47 amThey are both true and not one against the other. I don't remember details (if your equation is really Ic and not Ib equation) and don't want now spend time searching it.Is is constant parameter. I suggest to replace its name with Isc - the parameter to calculate Ic current.Now let us define the second parameter Isb - the same but to calculate the same way base current.Now you have Ib=Isb*exp(Vbe/Vt) and Ic=Isc*exp(Vbe/Vt).When you divide Ic/Ib=(Isc*exp(Vbe/Vt))/(Isb*exp(Vbe/Vt))=Isc/Isb. So to have Ic/Ib=beta you have Isc/Isb=beta and everything is in one common order and no reason to be confused.From your first equation you can't find Ic and Ib dependencies on temperature while from second you can (Vt depends on temperature).

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 20, 2024, 12:30:19 am
content: https://en.wikipedia.org/wiki/Bipolar_junction_transistor#Theory_and_modelingIt lists at least 6 different ways to model a BJT, and apparently all those methods are currently in use. All models are approximations of the true physics, and they are a compromise between accuracy (for a certain task) and computation intensiveness.

author: TimFox
author_rank: Super Contributor
timestamp: November 20, 2024, 12:45:41 am
content: The relationship between emitter current and base-emitter voltage is very close to the Shockley equation.Very roughly, a small fraction of that emitter current flows through the base-emitter junction: the ratio is called "beta" orhFEand is roughly constant.My approach is to treat the causal chain in this manner, for a transistor in its active region (not saturated), suitable for an amplifier circuit:(1) The base-emitter voltage makes the emitter current flow, and that current is a very strong (exponential) function of the voltage.(2) Since the current is a very strong function of the voltage, one should not rely on it to determine the current.(3) With reasonably high beta, the collector current is slightly lower than the emitter current(4) However, in the vicinity of a quiescent current valueIc, the derivative ("transconductance")gm= dIc/dVbeis predictable, given approximately bygm=Ic/(26 mV) at room temperature. This comes from differentiation of the Shockley equation.(5) Meanwhile, there is a substantial base current caused by the emitter current. However, the manufacturer's specification of beta for a given transistor number (e.g., 2N3904) has a very wide spread (3:1 from min to max); see its datasheet.Therefore, a good design procedure is to use an appropriate circuit to obtain the desired collector current for the application, then check the resulting range of base current over the range of beta. This base current will affect your biasing network, so you need to check that it will work over the range.You can consider this approach as consistent with the transistor being "voltage-operated", with the base current as a "parasitic" effect.If the transistor is operated as a switch, then the ON collector current flowing through the load must drop enough voltage to reduce the collector-emitter voltageVceto a very low value, the "saturation voltage".That collector current, divided by the minimum value of beta, gives the minimum base current requirement to saturate the transistor, and the base drive must supply at least that amount of current. Extra base current (for beta greater than minimum) is acceptable. Details of switching speed and other effects can be added to this consideration.

author: PGPG
author_rank: Frequent Contributor
timestamp: November 20, 2024, 12:16:01 pm
content: Quote from: TimFox on November 20, 2024, 12:45:41 amBase current being beta times lower than collector current gives a border of saturation - Vce being about the same as Vbe so about 0.7V.Quote from: TimFox on November 20, 2024, 12:45:41 amIf you want transistor to operate as a switch extra base current is not only acceptable but really needed.See in datasheet of mentioned by you 2N3904:https://www.mouser.pl/datasheet/2/308/2N3904_D-2309276.pdfEven in table the smallest beta value is 30 at Figure 2 showing saturation they used beta = 10. So base current seriously (at least 3 times) higher than collector current divided by minimum beta.And for BC847C I typically use:https://www.diodes.com/datasheet/download/BC847C.pdfEven for BC847C the minimum beta is 420 (for BC847A is 110) the saturation is shown for Ic/Ib=20 so assuming Ib 21 times (for BC847A 5.5 times) higher than you get from dividing Ic by minimum beta.

author: MathWizard
author_rank: Super Contributor
timestamp: November 20, 2024, 01:24:30 pm
content: I've been doing a bunch of digital MCU stuff lately. I really miss transistors and all those equations, and I can't wait to get back at RF stuff and dig deeper into it all.

author: mawyatt
author_rank: Super Contributor
timestamp: November 20, 2024, 01:52:34 pm
content: Quote from: Doctorandus_P on November 20, 2024, 12:30:19 amAlso there are the VBIC and MEXTRAM Bipolar transistor behavioral models which are more involved than the older EM and GP models.In addition, HICUM is a physics based model rather than behavioral, even more complex and consumes considerable computation resources.VBIC, MEXTRAM and HICUM are often utilized in IC design.Best

author: TimFox
author_rank: Super Contributor
timestamp: November 20, 2024, 02:52:28 pm
content: Quote from: PGPG on November 20, 2024, 12:16:01 pmGood examples of how "beta" is far from constant over the useful range of operation of a BJT.Vce satis actually lower thanVbe.For the 2N3904 at 5 mA, the maxVce satis 0.3 V.

author: mawyatt
author_rank: Super Contributor
timestamp: November 20, 2024, 06:22:05 pm
content: An interesting use and test of a bipolar transistor and model is in an "inverted mode", where there Collector and Emitter are interchanged. Here the "effective Vce saturation" can be a few millivolts and this was used for analog switching over half a century agoBest

author: PGPG
author_rank: Frequent Contributor
timestamp: November 20, 2024, 08:53:03 pm
content: Quote from: mawyatt on November 20, 2024, 06:22:05 pmBut with base current comparable or higher than emitter current

author: mawyatt
author_rank: Super Contributor
timestamp: November 21, 2024, 01:58:59 am
content: Quote from: PGPG on November 20, 2024, 08:53:03 pmFor an NPN, swap the Collector and Emitter so the Emitter sees a + voltage wrt to Collector, and the Base remains the same with a + voltage wrt to the Collector which is now the effective Emitter (Inverted). The Beta (and breakdown) is much lower with Inverted Operation, but the Inverted Vce saturation voltage is very low.If one has a Curve Tracer this Inverted Mode can be shown. Some of the better prepared bipolar transistor Models also show this Inverted operation correctly.The reason for this unique behavior is the heavy doping profiles in the Base to Emitter PN Junction vs the much lighter doping profiles for the Base to Collector PN junction.Best

author: MrAl
author_rank: Super Contributor
timestamp: November 21, 2024, 01:21:28 pm
content: Hello again,Here is a quick basic explanation of how bipolar transistor saturation works. Refer to the attachment for illustrations.In brief, saturation occurs when the base collector diode becomes forward biased due to a decreasing collector emitter voltage, and that forward current 'steals' current from the internal base current that determines the collector emitter current. This feedback mechanism causes less current available to decrease the collector emitter voltage further, and that is where saturation is said to occur.BASIC SATURATION CHARACTERISTICFirst thing to note is the terminals B, C, and E. Everything within the BCE terminals are internal to the transistor. Second thing is the current controlled current source CCCS. Third thing is the current "Ib", that flows into the base terminal B which comes from the external world. R1 and R2 are shown just to bias the transistor. R3 is the collector resistor, R4 is the emitter resistor. Fourth thing is the current "Ibs" and how it only flows through D1 and thus causes the current Ibs*Beta collector to emitter. It's very important to distinguish the external base current Ib from the internal base current Ibs because at some operating point although Ib may increase a lot, Ibs may only increase a little, which at that point means Ic can only increase a little, and thus Vce may only decrease a little.When the CE voltage is high, D2 conducts only a very small reverse current I2 which we can call zero. When the CE voltage becomes low enough, we see D2 start to become forward biased. The current I2 then increases. As I2 increases, more and more current flows through D2, which 'steals' current from the base region. That means for an increase in base current Ib we see less of an increase in the current through D1 (Ibs). Thus the collector emitter current does not increase as much as it did before D2 became forward biased. This also means that the voltage at Vc can not decrease as much as it did before. That means less of an increase in current through R3 which means the CE voltage starts to stabilize even though we increase the base current Ib even more. That's the basic mechanism behind saturation: when D2 becomes forward biased it steals some of that external base current so it makes the Beta look smaller. A common estimate is the Beta is only 10 when in saturation, even though the starting Beta can be as high as 200 or more.The diode equations for I2 when D2 is forward biased is given as well as the equation for D1 but the constants would depend on the actual transistor model values.

author: mawyatt
author_rank: Super Contributor
timestamp: November 22, 2024, 06:27:16 pm
content: Quote from: mawyatt on November 21, 2024, 01:58:59 amFor those folks interested in Vce saturation and inverted device saturation we did a quick test with the 2N3904 as DUT.Using a DSO and AWG we created a simple circuit with two 10K resistors, one to the DUT Base and the other to the DUT Collector, while the DUT Emitter was connected to Ground. We connected the Base resistor to +5VDC from the AWG CH2, or you could use any DC bias source, and the Collector resistor to the AWG CH1. AWG CH1 was was setup with a unipolar ramp of 0 to 4V at 1KHz.The DSO CH1 was connected to the AWG CH1 and the DSO CH2 connected to the DUT Collector. With this arrangement the DSO will display the DUT Current (Yellow) as ~50ua/div on CH1 (4V/10K)/(8 Divisions) and DSO CH2 (Magenta) will display the voltage across the DUT Collector to Emitter, or Vce. Since the DUT will be in saturation, this DSO display will be Vcesat vs Collector current.Here you can see the normal 2N3904 (PNG_94) showing a Vce in saturation of ~10mV at ~200uA Collector current and when Inverted (Exchange DUT Collector and Emitter) of ~4.5mV at 200uA (PNG_92).Anyway, this is an easy setup for most, and fun to see how Vce saturation behaves with Collector and Base current, and when Inverted. Of course one can use a PNP by just changing the polarity of the DC Base Bias and the Collector Ramp.Best

author: David Hess
author_rank: Super Contributor
timestamp: November 22, 2024, 08:59:53 pm
content: Quote from: sy on November 19, 2024, 08:46:47 amIc=beta*ib is for when the transistor is operating as a saturated switch, or to calculate input bias current when the transistor is used as an amplifier.Ic=Is*exp(Vbe/Vt) is used everywhere else where the transistor is operating as a transconductance amplifier, which is any linear application. Here the transistor has a gain in mhos (ohms spelled backwards but now called Siemens) with the units of amps/volt. At room temperature, this comes out to about 60 millivolts of change in Vbe per decade change in collector current. The transconductance is very predictable as the reciprocal of the emitter resistance which is about 26 millivolts/milliamp, so higher collector current means higher transconductance.The above shows why the transistor relies on an input voltage and not an input current. Beta varies all over the place falling at low and high currents, while transconductance is very predictable over more than 9 decades of current.

author: TimFox
author_rank: Super Contributor
timestamp: November 22, 2024, 09:23:17 pm
content: Quote from: David Hess on November 22, 2024, 08:59:53 pm"Beta" is not a "good" parameter: not only does it vary drastically with current level, but also from part to part with the same type number (e.g., 2N3904).In careful use,hFE(with majuscule subscript) is the ratioIc/Ib, and is often used to specify the saturated behavior (although the ratio exists for active operation).The other termhfe= dIc/dIb(with miniscule subscript) is the small-signal parameter (also subject to large variations over current and production) that is mainly useful for estimating the input impedance of a common-emitter stage for small-signal operation around a given operating point. The gain of that stage is better estimated by the transconductance.Since the characteristic curves of a given BJT (family of curves for different base currents) look like the graphs for vacuum tubes and FETs (where the input voltage is the controlling variable), it is popular to consider the BJT as a "current-controlled" device. but that is misleading.Consider a simple PN-junction diode. The current and voltage are monotonic functions of each other, where the current is a very strong function of the voltage and, therefore, the voltage is a weak function of the current. Since both functions are monotonic (see Shockley equation), one could consider either one the "cause" and the other the "effect".With a BJT however, I think it more appropriate to consider the emitter current to be the same function of the base-emitter voltage, and the base current to be a less-well-determined sort-of-linear parasitic function of the emitter current.

author: MrAl
author_rank: Super Contributor
timestamp: November 23, 2024, 12:41:16 pm
content: Quote from: TimFox on November 22, 2024, 09:23:17 pmHi,We do not always use physics in circuit analysis, we use approximations, and one of them is that the transistor is a current controlled current source. The other, as you well know, is that the transistor is a voltage controlled current source.It should be obvious that neither of these is correct. The transistor cannot produce current on its own so it can not generate any current. It can conduct current, but that doesn't make it a true current source. We call it that for simplicity, for the approximations.What this means is that we are free to model it any way we care to. However, the way we care to model it is going to be dependent on the application.The CCCS model is very simple and works pretty well, so we use that sometimes, and other times we use the VCCS model as that works also.In pure physical theory the VCCS is probably closer to how a transistor "works" in real life, but since there is no way we can apply a voltage without also supply current, it can only work if we apply energy to the transistor, and energy requires both current and voltage simultaneously.There is also an old theory that says that there is no horse before the cart; which comes first. Does voltage come before current, or does current come before voltage. The old theory is that they occur simultaneously. The new theory is that there may be a tiny, tiny, delay between the appearance of one of them after the other. It's voltage first and current second if I remember right. The delay is so small though that it's imperceptible in almost every application except maybe something that involves quantum physics. That means for most practical purposes, they come about simultaneously, which brings us back to the energy flow profile.It takes energy to get the transistor to do anything, so if we had to be more concise about the operation, we would have to say that it's an energy-controlled device. We probably won't do that though, so we use whatever model is simplest, yet sill provides us with some useable results: CCCS or VCCS.The reason we see VCCS being pushed sometimes is probably because of FETs which are sort of controlled more by voltage simply because the current is much lower than usual. There's always going to be some current though, although I believe it would be more difficult to explain the operation based on that.

author: mawyatt
author_rank: Super Contributor
timestamp: November 23, 2024, 02:30:15 pm
content: One of the misconceptions of using the classic Ic=Is*Exp(Vbe/Vt) isIsas a constant. When in fact it's obviously linearly dependent on junction area, but also highly dependent on temperature, doping profiles, concentrations, carrier and diffusion characteristics and so on. Thus an extremelyvariable"constant"!!However it also exhibits somewhat uniform behavior across a given processed wafer and with tightly spaced transistors, as on a single chip, becomes quite uniform and allows two transistors operating at the same collector currents, where base to emitter voltages (Vbe) will match within a few microvolts over a reasonable temperature range (assuming outside influences such as temperature, stress or process gradients aren't involved, which Erdi's Cross Coupled Quad addressed). This surprising uniform Vbe behavior from a highlyvariableIsbecomes the basis of the Differential Amplifier common to just about all analog circuits.The Gummel Plot is a good means for one to see how the bipolar transistor behaves as base and collector currents wrt Vbe and also shows compression effects at higher collector currents.Here's a good introductory reference for the bipolar transistor from UC Berkeley.https://www.chu.berkeley.edu/wp-content/uploads/2020/01/Chenming-Hu_ch8-2.pdfBTW MOS FETs are actually not Voltage Controlled but Charge Controlled Devices.Best

author: magic
author_rank: Super Contributor
timestamp: November 23, 2024, 03:06:36 pm
content: Quote from: mawyatt on November 23, 2024, 02:30:15 pmThere is a funny conclusion you can draw from that:Assuming that Ic is constant and Is is constant, if Vt is proportional to absolute temperature then Vbe also is.

author: mawyatt
author_rank: Super Contributor
timestamp: November 23, 2024, 03:15:25 pm
content: Since the discussion involves the bipolar transistor collector, base currents, Beta, and Vbe, here's an interesting Gummel Plot of an early HBT SiGe bipolar transistor.Note the Beta on the plot right, yes that's 1400With HBT SiGe devices one can fashion the base emitter junction profile that produces infinite Beta (Ic/zero base current), might even be possible to produce a device that becomes sort of "Depletion Mode".In that one mustsink currentfrom the base of the HBT SiGe NPN device toreducethe collector current from a high value!! A way to think about this is a device with very high Collector to Base leakage current, thus self biasing and conducting significant collector current with nothing connected to the base. Don't recall this type HBT SiGe device ever being produced tho, and not sure if it would be useful.https://en.wikipedia.org/wiki/Gummel_plot

author: mawyatt
author_rank: Super Contributor
timestamp: November 23, 2024, 03:43:13 pm
content: Quote from: magic on November 23, 2024, 03:06:36 pmExactlyVbe = Vt*ln(Ic/Is), where Vt =kT/qdVbe/dT = k/q*ln (Ic/Is) if Ic/Isis not dependent on temperature.However, we all know dVbe/dT isnegativewrt to temperature (~ -2.2mv/k for typical bipolar transistor at Ic ~ 1ma).So the d(Ic/Is)/dT must override and force the overall dVbe/dT result to be negative. So with a constant Ic thenIsmust be highly temperature dependent which is true and why treating this as aconstantthe classic Ic =Is*exp(Vbe/Vt) isn't correct!!All this begs the question, so how did Widlar's famous Bandgap Reference workOf course the answer isuniformIsacross on chip co-located transistors and area scalingBest,

author: MrAl
author_rank: Super Contributor
timestamp: November 24, 2024, 12:19:59 pm
content: Quote from: mawyatt on November 23, 2024, 02:30:15 pmHello,It does not matter what SINGLE reference you look up it's not going to tell the whole story.There are references right now sitting in the Rutgers library in New Brunswick that talk about the bipolar transistor as being charged controlled. In that book one of the things that is mentioned is how we have to get the charge carriers out of the base region in order to turn the transistor off (what we call the storage time). Then, we've all read about the bipolar transistor being voltage controlled, and also current controlled.So we now have accumulated three different ways the bipolar transistor can be said to be controlled:1. Current controlled.2. Voltage controlled.3. Charge controlled.Why so many ways of describing this phenomenon.It's because each one is saying something different about what happens after the fact. It's like saying that to steer a car you have to get the front wheels to turn. Is that really it. Well, if we were talking about this from the standpoint of the driver of the car, they would say you have to turn the steering wheel. So to steer the car we have to turn the steering wheel. But then someone more into the physics of it would say that it's the friction on the road the steers the car turning the wheels or the steering wheel is only secondary. If it were not for the friction, the car would never turn.It's even possible that someone else might say it's because of the hand and arm movements of the driver. That turns the steering wheel, then that turns the wheels and that makes the car turn.Now which one is the 'correct' one.It depends on what we are looking at, what we want to analyze. If we want to know how much human strength it takes to turn a steering wheel we might look into the way the human arms and hands and even the torso works. If we want to know how a steering wheel makes a car turn, we might look at that object. If we want to know how the tire and road surfaces interact, we could look at that aspect of it.Now getting back to the transistor, if we want to know how the output behaves based on a change in voltage on the 'input', we look at it as a voltage controlled device. If we want to know how the output behaves with a change in input current, we look at it as a current controlled device. If we want to know how it behaves with a change in charge, we look at it as a charge controlled device (which helps to understand the storage time).The bottom line, if we even need to know this, is that it takes ENERGY to do any of these things. That tells me that if we are going to find an all-encompassing terminology for how a transistor is controlled, we'd have to say that it was controlled by energy. However, because of the way we as humans like to analyze things, we like to look at one dimension of a problem at a time, if we can, so we end up choosing one of the simpler methods that do not specify energy but rather one part of that which is voltage or current, and charge if we need to understand other aspects of the transistor.Note also that charge itself is an after the fact specification also, because we can't move charge without energy.This all means that when we read single references to a device like a bipolar transistor, we will see it described by different measurements and there will always be assumptions about some of the other measurements which may be missing from the discussion in order to explain it based on the primary measurements. Different aspects of the transistor will come with different assumptions.Sometimes we pick a certain mode of operation based on the ratio of one quantity as compared to another. For example, if the voltage changes a lot and the current does not change as much, then we might call it voltage controlled. The funny thing here is that voltage is entirely different than current, so it's a subjective description. For current control we might say that the current changes a lot while the voltage changes very little. Still subjective, and subject to the application. We could look at some different applications to see how this plays out.

author: David Hess
author_rank: Super Contributor
timestamp: November 24, 2024, 06:48:52 pm
content: They are charge controlled, but the charge aspects only become relevant at high speeds where you might see a small bypass capacitance at the base or emitter to better control charge inside the transistor.The minority carriers injected into the base-emitter junction have considerable charge associated with them, (1) and this charge is what ultimately controls conduction between the collector and emitter. When saturated, this charge is greater than required to turn the transistor fully on, and then the transistor takes longer to turn off while the charge is removed, leading to the delay time. When a transistor is emitter switched, then this charge gets dumped out of the base terminal.(1) The large amount of charge available is sometimes used to make a capacitor on an IC process. Bob Widlar wrote an application note describing this in the LM110 or LT1010, but I could not find it for this post.

author: MrAl
author_rank: Super Contributor
timestamp: November 25, 2024, 11:39:13 am
content: Quote from: David Hess on November 24, 2024, 06:48:52 pmAs already mentioned, you'll find papers written by knowledgeable people who talk about the transistor being controlled by either voltage, current, or charge."but the charge aspects only become relevant at high speeds"That should tell you something right there

author: mawyatt
author_rank: Super Contributor
timestamp: November 25, 2024, 03:29:20 pm
content: As any PhD Solid State Physics student from MIT, Stanford or Cal Berkeley will tell you the fundamental bipolar transistor operation (read how it actually works!!) is by charge at the atomic level as in minority and majority carriers (read charge carriers as Holes and Electrons) across the Collector-Base and Base-Emitter PN Junctions.If one thinks for a moment and realizes everything about electronics in general boils down toCharge, theChargeof an electron. Current is the time rate of change ofChargeand Voltage is the Potential Difference betweenChargeat two points. Note how both Voltage and Current referenceChargeas part of their fundamental definition.It's only us "users" that want a more simplified Semiconductor model that involves the easier to deal with and measure "Voltage" and "Current" to describe Semiconductor device behavior at theMacro Leveland how such is effectively "Controlled".Best,

author: MrAl
author_rank: Super Contributor
timestamp: November 25, 2024, 04:37:03 pm
content: Quote from: mawyatt on November 25, 2024, 03:29:20 pmI can assure you I read the book on this. There's one in the Rutgers library that talks about the charge controlled transistor.What you seem to be suggesting though is that we can never use voltage controlled or current controlled because you demand that it's charge controlled, which does not make sense. The book at Rutgers also talks about charge control. I have no problem with that, it's just that we can't limit ourselves to that definition. For example, to turn off the transistor we have to get the charge out of the base region. Now, how exactly do we do that. We apply a reverse bias, which is a voltage, and of course a current comes with it. Another way of stating this is that we need ENERGY to control the transistor because we need to move charge, and charge does not move unless we force it to. Another way to say this is that you can't force anything to happen with the transistor because the charge just sits there. That DOES NOT mean that we can never call it charge controlled, it's just that we don't always want to do that, and it's still just a side track for what we really need to do to control the transistor.Also, charge controlled does not sit well with the newer theory about there being a delay, a very small delay, between voltage and current. That would suggest we lean more toward voltage controlled at the micro level. I think this stems from the mass of an electron, but it's been a long while since I read up on that.If you want to stick by the more limited view that's ok with me. I just can't accept it except in situations where we must use it just like anything else we come across (like with the base charge issue, which explains that phenomenon pretty well I think). It's also ok with me if theory on solid state physics wants to explain it that way, no problem here.

author: TimFox
author_rank: Super Contributor
timestamp: November 25, 2024, 04:43:47 pm
content: A classic text emphasizing charge control is E M Cherry and D E HooperAmplifying Devices and Low-Pass Amplifier Design, Wiley 1968.(Don't drop it on your foot.)They find charge control useful for vacuum tubes, BJTs, and FETs as a general concept, especially for frequency and time-dependent circuits.(Note that this includes the space charge which is very important in vacuum tubes.)For elementary BJT audio circuits, ignoring frequency response, I find the voltage-control approach easier for small-signal analysis and DC biasing.Of course, even at sub-MHz frequencies, attention must be paid to capacitance and stored charge to improve the analysis.

author: mawyatt
author_rank: Super Contributor
timestamp: November 25, 2024, 05:06:19 pm
content: QuoteWe wrote this earlier:It's only us "users" that want a more simplified Semiconductor model that involves the easier to deal with and measure "Voltage" and "Current" to describe Semiconductor device behavior at the Macro Level and how such is effectively "Controlled".There's nodemandwrt any of this discussion. Each of us as a "user" can choose our own model, which can be Voltage, Current or Charge Controlled. However as we said this often is a simplification to allow easier understanding and use.The Cherry Hooper book mentioned by Tim above is a superb reference, if reading please pay note to their unique amplifier topology which originally was with tubes, then transistors. We've utilized these with SiGe and InP bipolar HBT devices and even developed a version with a Caprio Cell for improved linearity.Best

author: David Hess
author_rank: Super Contributor
timestamp: November 25, 2024, 09:34:43 pm
content: Quote from: MrAl on November 25, 2024, 11:39:13 am"High speed" includes anything where the delay time becomes significant, which can be microseconds in some cases. Below are a couple of examples of charge control in bipolar transistors from Linear Technology application note 13.In E2, the 12 picofarad capacitor moves charge in and out of the 2N2369 to turn it on and off faster. During turn-off, the base-emitter voltage of the 2N2369 is reversed and goes negative to sweep charge out of the base.In E3, Q1, a slow 2N2907, is emitter switched for highest possible speed of this slow transistor. Q1's load prevents its collector voltage from decreasing, so it never saturates. Drive to Q2 is a current from Q1 allowing high compliance, however Q2 is Baker clamped by the schottky diode to hold it out of saturation minimizing charge storage. The 1000 picofarad capacitor quickly moves charge in and out of the base of Q2.

author: jonpaul
author_rank: Super Contributor
timestamp: November 25, 2024, 09:46:23 pm
content: Ebers-Moll was original BTL/MIT model.https://courses.cit.cornell.edu/ece315/Lectures/handout18b.pdfBy 1970s see Gummel-Poon.https://en.wikipedia.org/wiki/Gummel%E2%80%93Poon_modelJon

author: TimFox
author_rank: Super Contributor
timestamp: November 25, 2024, 09:59:45 pm
content: Quote from: mawyatt on November 25, 2024, 05:06:19 pmUnique amplifier topology: are you referring to cathode-degenerated triode followed by pentode or triode with shunt (plate-grid) feedback?I have used that successfully with Nuvistor triodes.

author: mawyatt
author_rank: Super Contributor
timestamp: November 26, 2024, 01:40:23 am
content: Quote from: TimFox on November 25, 2024, 09:59:45 pmThe secondFound some old notes from back in 2005 where we implemented the CH Amp with AGC in SiGe, later we did such in a faster SiGe and even faster InP.Best,

author: MrAl
author_rank: Super Contributor
timestamp: November 27, 2024, 11:48:47 am
content: Quote from: mawyatt on November 25, 2024, 05:06:19 pmHello again and thanks for the reply.The reason I suggested there might be some kind of "demand" was because if someone mentions a list of things, then someone else comes back with one of those things on the list, then someone mentions the same list of things again then that other someone comes back yet again with just one of those things on the list, it begins to sound like that other someone is pushing that one thing on the list.For example, a list:apples, oranges, grapeswith an explanation about the characteristics about each one, then someone else comes back and explains only what grapes are, that's not a big deal I guess. But when those three are listed again and that same person comes back an explains what grapes are again, then it sounds like they are pushing grapes as the better food.Maybe it's just because I've seen this happen so many times. Especially with some people I've dealt with in the past. I'll mention a few things, then they come back with just one of those things as important. I recognize their point, and that they are right with that one thing although there are other things to consider, then they acknowledge that there are other things or ways of doing things, but still come back again with their one individual belief. This would go on for as long as I mention the group of items or ways of doing things. They say that they recognize that all three are important, yet they keep coming back to reemphasize their one thing. So even though I include their view within my view, they can't resist repeatedly mentioning their one thing.You would think that a viewpoint that includes all other possible viewpoints would suffice, but if often doesn't maybe because they think if you mention all viewpoints you somehow diminish their viewpoint as limited as it has to be.Not a really big deal i guess, and I'm not pointing any fingersSill an interesting discussion too BTW, and I always appreciate other members inputs.