The Very Best Amplifier I Have Ever Heard!!!!

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#2,541
overall feedback said:
You just see it from different point of view.
Think like this: BJTs have very small base emitter resistance when voltage exceed cca 0.6V barrier (approx. 0.026/Ic ohm) and can not be driven by voltage source directly because of overcurrent.
FETs have very high resistance even threshold is exceeded (cca 10^9 ohm). So it can not be driven by current source directly because of overvoltage.
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Yes - you got it - you need a voltage source capable of driving current into the control inputs of the device. For a BJT this current is well known and the cause of the confusion described here. For a FET this current charges the gate capacitor. For a triode this current addresses grid current (unless you operate the valve over a narrower range of operating conditions). All of these devices and their relatives have two control inputs and require a voltage difference to be applied across these two control inputs - and the voltage source must have enough current drive capability to get the job done according to the circuit design.
 
#2,542
Bonsai said:
. . . and not helped by countless introductory text books over the decades stating that BJT's are current controlled devices, or graphs with titles that included " . . . . showing the dependency of Ic on Ib . . ." or the tap analogy (the base is the tap handle, as it is turned more water - i.e. Ic - flows)
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I guess I missed those. I posted a link to the late Marshal Leach's very nice class notes which echo all the texts that I have ever seen.
 
#2,543
And for an easy to digest, hand waving summary you can't go past the good book, "The Art of Electronics", 🙂 ...


2.01 First transistor model: current
amplifier

Let's begin. ...

...

Clearly our transistor model is incomplete and needs to be modified in order to handle this circuit situation, as well as others we will talk about shortly. Our fixed-up model, which we will call the transconductance model, will be accurate enough for the remainder of the book.


EBERS-MOLL MODEL APPLIED TO BASIC TRANSISTOR CIRCUITS

2.10 Improved transistor model:
transconductance amplifier

The important change is in property 4 (Section 2.01), where we said earlier that Ic = hFE*IB. We thought of the transistor as a current amplifier whose input circuit behaved like a diode. That's roughly correct, and for some applications it's good enough. But to understand differential amplifiers, logarithmic converters, temperature compensation, and other important applications, you must think of the transistor as a transconductance device - collector current is determined by base-to-emitter voltage.
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#2,546
fas42 said:
And for an easy to digest, hand waving summary you can't go past the good book, "The Art of Electronics", 🙂 ...
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"collector current is determined by base-to-emitter voltage"

For same applications you can "think" of it as "transconductance device".
That is different from "it is transconductance device". As there is Ic/Vbe relation it is not proof that it is driven by voltage source. BJT must be current driven to be functional. Pure voltage can not drive BJT.
 
#2,547
No, look at the text more carefully ... it's "roughly correct" to think of "the transistor as a current amplifier" but to understand all behaviours "you must think of the transistor as a transconductance device".

Edit: Part of the confusion may be that base current results from voltage drive - which almost doesn't exist for FET's. But for BJT's that base current is a by-product, so to speak, of the voltage drive, and just happens to mostly fit the Ic = hFE*Ib relationship.
 
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#2,548
fas42 said:
Part of the confusion may be that base current results from voltage drive - which almost doesn't exist for FET's. But for BJT's that base current is a by-product, so to speak, of the voltage drive, and just happens to mostly fit the Ic = hFE*Ib relationship.
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Yes, right. Base current makes confusion. You can think of it as by-product or essential load. As it is always there, we must consider in precise calculation, as well as Vbe, that changes with Ib. Ib is exponentially related to Vbe (Ib proportional to exp(VBE/VT) ), that said, for small signal amplitudes it is more transconductance than for larger amplitudes.

Suppose we all agree now. 🙂
 
#2,552
FAS 42

Overall feedack may agree But I will not agree.😀😀😀

Ib a by product. 😕
You can write Ic as a function of Ib or Vbe,because both ib and Vbe are related to each other.you can write Ib as a function Vbe.You can also model a BJT as a transconductance amplifier.I agree
But that does not mean BJT is a voltage controlled amplifier.To get better idea about this try to understand operation and the current flow mechanism in junction transistor.how the electron/hole movement between P and N material contribute to Ic,Ib and Ie.Compare it with the operation of FET.Then you will understand why BJT is called current controlled device and FET is voltage controlled device.

Mathematical modelling will not help us,because ib can write as a function of Vbe and viceversa.


Bigun you need lot of energy to convince me.🙂🙂🙂
 
#2,553
You do realise, joshvi, that you're missing out on the best ever, diyAudio group hug ... 😉 ?

The current and voltage across the base are the two sides of "diode action" across the base-emitter - you can't have one without the other. And it's this diode conduction, talking about npn, that injects electrons into the base region, where they are "strongly attracted to the collector" - there's a nice, somewhat linear correlation between the base and collector current, because the more electrons enter the base region, the more can exit via the base and, mostly, the collector terminals. But what precisely controls the collector current is the rate of injection of electrons "into the base region, which is an exponentional function of the BE potential difference".

This again is straight from Horowitz and Hill ... 🙂
 
#2,554
fas42 said:
You do realise, joshvi, that you're missing out on the best ever, diyAudio group hug ... 😉 ?

The current and voltage across the base are the two sides of "diode action" across the base-emitter - you can't have one without the other. And it's this diode conduction, talking about npn, that injects electrons into the base region, where they are "strongly attracted to the collector" - there's a nice, somewhat linear correlation between the base and collector current, because the more electrons enter the base region, the more can exit via the base and, mostly, the collector terminals. But what precisely controls the collector current is the rate of injection of electrons "into the base region, which is an exponentional function of the BE potential difference".

This again is straight from Horowitz and Hill ... 🙂
Click to expand...

One good practical example that BJT is voltage controlled device is optimum bias for B class EF OPS defined by Vbe and not by bias current.
 
#2,555
fas42 said:
You do realise, joshvi, that you're missing out on the best ever, diyAudio group hug ... 😉 ?

The current and voltage across the base are the two sides of "diode action" across the base-emitter - you can't have one without the other. And it's this diode conduction, talking about npn, that injects electrons into the base region, where they are "strongly attracted to the collector" - there's a nice, somewhat linear correlation between the base and collector current, because the more electrons enter the base region, the more can exit via the base and, mostly, the collector terminals. But what precisely controls the collector current is the rate of injection of electrons "into the base region, which is an exponentional function of the BE potential difference".

This again is straight from Horowitz and Hill ... 🙂
Click to expand...


Fas 42,

Now you said it.😀😀.

Last sentence you can rewrite" rate of injection of electrons into base region" = base current (Ib).

I dont want to miss your hug😀😀😀😀
 
#2,557
You carefully read the lastsentence.If it is from emitter you call it as emitter current.Is it an exponential function Vbe?It is base current and is an exponential function of BE potential difference ie.Vbe.

last sentence you can rewrite like this."But what precisely controls the collector current is Ib, which is an exponentional function of the Vbe"

hope you will agree.
 
#2,558
Nope, 🙂. It is the voltage potential between the base and emitter regions of the semiconductor material which causes the movement of the charge carriers; the emitter, strangely enough 😀, "emits" electrons (for npn) from its heavily doped region into the base region and most of these are "collected" by the collector, under the influence of the voltage potential applied there, to form the collector current. The "leftover" base current that occurs is a function of the geometry and variations in the manufacture of the device - which is why the values of hFE are all over the place, for a particular device number.
 
#2,560
fas42 said:
You don't get off that easy!! ... those electrons are injected from the emitter region, the n doped material - not from the base lead!
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Don't wanna spoil the hug, just wanna say that this is never ending story.
"
Voltage, current, and charge control

The collector–emitter current can be viewed as being controlled by the base–emitter current (current control), or by the base–emitter voltage (voltage control). These views are related by the current–voltage relation of the base–emitter junction, which is just the usual exponential current–voltage curve of a p-n junction (diode).[1]"

Bipolar junction transistor - Wikipedia, the free encyclopedia

If someone look at it that way it is ok. But, keep in mind that BJT needs base current, specially high power devices.
 
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