Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Understanding Transistor

Status
Not open for further replies.
No it's not. A transistor is a base current controlled vs collector current device. How you deliver that current to the base maybe voltage controlled but it's the base current that affects the collector current.
.

WTP Pepper,

Have you ever tried to explain the current mirror principle or Barry Gilbert's translinear loops based on a current controlled BJT?
 
Last edited:
WTP Pepper,



In the link below, notice the crucial sentence in the second paragraph, "The root cause of the this misconception is the inability to distinguish a functional relation from a causal relation." After you read the link, we can discuss it more.

Ratch

https://www.kevinaylward.co.uk/ee/voltagecontrolledbipolar/voltagecontrolledbipolar.html

I think when you learn the physics then we can talk. I don't need to quote someone elses opinion.

"This equation dictates that however Vd is achieved, Id through the junction will be related by the above equation". From your own quoted ref
 
Last edited:
WTP Pepper,

I think when you learn the physics then we can talk.

OK, I have been learning physics all my life, so I guess we can talk.

I don't need to quote someone elses opinion.

Sure you do. Even if you you quote from a reputable source, you are quoting someone's opinion. Unless you discover the principle, law, point, or conjecture yourself, you are quoting someone's opinion. Everything depends on how reputable the source, and thereby the opinion is.

I quoted the opinion of someone else for its description, but he also gave a reason for what he said. You either have to accept that or show how it is wrong.

This equation dictates that however Vd is achieved, Id through the junction will be related by the above equation". From your own quoted ref

That is true. But a relationship does not establish a cause. The example he quoted was that the rising sun coinciding with the hands of a watch does not prove that the watch controls the sunrise.

OK, let's talk physics. Semiconductor physics in particular. I can show you proof without a shadow of a doubt according the diode equation, that the current existing in the base terminal of a BJT is exponentially proportional to the voltage between the base terminal and emitter terminal (Vbe). I think you know that. That is a quantitative statement. The qualitative reason is that the Vbe lowers the barrier voltage caused by the uncovered charges that are left behind from the diffusion of the electrons from the N-type material into the P-type material and vice versa. All this happens without any causative input from the collector current. According to the diode equation, Ic is just not a causative factor for Ib. It is also true that the collector is exponentially proportional to Vbe. Again, the current in the base does not influence the collector current from a causative standpoint. Ic and Ib are mathematically related in a somewhat linear manner because they are both exponentially related through Vbe, but it is Vbe that is actually controlling everything. Because Vbe is causing everything to happen, that makes a BJT a VCCS causally. The ratio of Ic to Ib makes a BJT a CCCS functionally. That was what the person I quoted was trying to say.

Ratch
 
Last edited:
I think when you learn the physics then we can talk. I don't need to quote someone elses opinion.

What do you think I learnt when I was doing my degree in electronic engineering?

We studied Physics, Quantum Mechanics & how the BJT works in fine detail.
 
What do you think I learnt when I was doing my degree in electronic engineering?

We studied Physics, Quantum Mechanics & how the BJT works in fine detail.

Thanks for that, but it doesn't mean you understood anything. I studied semi-conductor physics at degree level.

I re-quote:
"A transistor is a base current controlled vs collector current device. How you deliver that current to the base maybe voltage controlled but it's the base current that affects the collector current"

Winstone: Go on..explain the current mirror principle without references to currents or using the approx Vbe on voltage to cause current to flow.
 
Last edited:
Thanks for that, but it doesn't mean you understood anything. Thank you for that comment.I studied semi-conductor physics at degree level.

That means that my Electronics 1 lecturer did not understand either. In his first lecture on the BJT, he stressed that the BJT is voltage controlled.

We are not claiming that there is no Ib. Of course there is, but the Ic is controlled by the Vbe.
 
WTP Pepper,

I re-quote:
"A transistor is a base current controlled vs collector current device. How you deliver that current to the base maybe voltage controlled but it's the base current that affects the collector current"

Still don't "get it"? Both the collector current and the base current are controlled by Vbe. That cannot be denied. That means that there is a relationship between collector current and base current called "beta". That does not mean that the base current controls the collector current. So a BJT mimics a CCCS functionally, but at its base level, it is a CVCS causally. Vbe is the voltage controlling the collector current and base current. There is a lot of confusion about this.

Ratch
 
WTP Pepper
I have attached the simulation of a transistor storage time.

Would you please tell us how this can be explained using the “current control” proposition?

Questions:-

1. Why is it called “Storage time”?
2. What is stored?
3. Why does the base current fall immediately from 430 uA to -69 uA?
4. Why is the base current negative during the storage time period?
5. Why does the collector current only decline slightly during the storage time period?
6. Why is the collector current positive when Ib is negative?
7. Why does the base emitter voltage remain at approximately 644mV?
8. Why is the wave shape of Ic very similar to that of Vbe?
 
Last edited:
1. Base charge due to parasitics.
2. Charge Q=it
3. I would build it for the exact values.
4. Excess minority carriers drawing -ve base current.
5. Uneven distribution of these minority cuurrent.
6. Because it's not acting as a true current amplifiying transisitor during this period
7. Early effect.
8. As it is similar to to Ib....

Not sure what your point is, but hope you didn't get penalised for handing it in late.

End of my contributions
 
WTP Pepper,

1. Why is it called “Storage time”?
1. Base charge due to parasitics

Parasitics have nothing to do with storage time. Storage time occurs when a transistor in saturation is not able to turn off immediately because of the excess minority-carrier charge in the base region. Until this charge is removed, the transistor will not be able to respond to input signals. The time it takes from the time of the turn off signal to the time for the collector current to drop to 90% of its saturation value is the storage time.

2. What is stored?
2. Charge Q=it

Correct. Excess charge in the base region.

3. Why does the base current fall immediately from 430 uA to -69 uA?
3. I would build it for the exact values.

What does that mean? In reality, the base current is draining away the excess charge from the base region.

4. Why is the base current negative during the storage time period?
4. Excess minority carriers drawing -ve base current.

I hope you are saying the excess charge carriers are draining away when the BJT is trying to turn off.

5. Why does the collector current only decline slightly during the storage time period?
5. Uneven distribution of these minority cuurrent.

Nope, the Vbe controls the collector current, so as long as the Vbe is at the value it was when the transistor was in saturation, the collector current will stay at its same level, too.

6. Why is the collector current positive when Ib is negative?
6. Because it's not acting as a true current amplifiying transisitor during this period

The correct answer is that Ib is not controlling Ic. It never was. Vbe is really what is in control of Ic.

7. Why does the base emitter voltage remain at approximately 644mV?
[7. Early effect.

The Early effect for a BJT in the saturaion region does not exist and has no effect. Since Vbe is controlling Ic, Ic will not drop until Vbe does.

8. Why is the wave shape of Ic very similar to that of Vbe? .
8. As it is similar to to Ib....

Look again. Ic is not following Ib. It is following Vbe.

In conclusion, Vbe controls Ic, not Ib controls Ic.

Ratch
 
The answers to most of the questions can be found in my post #39.

As I said in that post, Ic is controlled by the charge in the base/emitter region, and since q = CV, then you can say that it is voltage controlled.

The Ebers-Moll equations show clearly that Ic is an exponential function of Vbe.

The attachments are scans from two textbooks that I used as a student.

If you don’t believe Ratch & and me, perhaps you’ll believe Millman, Taub & Halikis.
 
Good heavens not "H1" again. This heresy is so often repeated, I refer to it as "heresy no. 1", or simply "H1" for short. This issue was thoroughly dissected in a thread from 2010:

https://www.electro-tech-online.com/threads/how-does-a-transistor-amplify-current-or-voltage.109630/

Vbe does not "cause" everything to happen. I pointed out, & you conceded in 2010, that Ie/Ib both lead Vbe in time. A cause can never occur after its effect, so Vbe cannot be the *cause* of Ib/Ie. A b-e junction has a diffusion capacitance, Cdiff. In order for Vbe to change, current must transit through b-e junction to charge/discharge Cdiff. Ib & Ie changes both chronologically take place ahead of Vbe changes. What I'm saying is that your theory of Vbe being the "cause" of Ib/Ie is a 0 on a scale of 10.

Interested parties should review the 43 page thread I gave in the link. I will clarify if needed. BR.
 
Last edited:
Claude,

Goodness, haven't heard from you in a long time. You are very late in responding to this thread. Anyway, nice to hear from you again.

Vbe does not "cause" everything to happen. I pointed out, & you conceded in 2010, that Ie/Ib both lead Vbe in time. A cause can never occur after its effect, so Vbe cannot be the *cause* of Ib/Ie. A b-e junction has a diffusion capacitance, Cdiff. In order for Vbe to change, current must transit through b-e junction to charge/discharge Cdiff. Ib & Ie changes both chronologically take place ahead of Vbe changes. What I'm saying is that your theory of Vbe being the "cause" of Ib/Ie is a 0 on a scale of 10.

If you bring diffusion capacitance into this discussion, then we have transients to discuss. I am only concerned about slow or steady state conditions where the diffusion capacitance is small or has negligible effect. Also the BJT is in the active region.

You aver that Vbe does not control Ic. According to a respected text by Sedra and Smith, Ic = Is*e^(Vbe/Vt), where Is is a constant called the saturation current, and Vt is the thermal voltage. Likewise, Ib = (Is*e^(Vbe/Vt))/β . That appears to be quite definitive to me that Vbe is causing both Ic and Ib to happen.



Interested parties should review the 43 page thread I gave in the link. I will clarify if needed.

We talked about a lot of things in that thread, including things that did not apply to the question of BJT being a voltage controlled current source. Things like saturation and cutoff. I don't see or recall conceding that Vbe does not control both Ic and Ib on page 43 or anywhere else. Perhaps a post number from that page is in order. Anyway, I believe this current thread is a good discussion on "H1".

So let the discussion begin.

Ratch
 
Ah, we're back to the chicken and egg question again. ;) So here's another varient.

If a BJT is strictly a voltage controlled device then why can't a BJT be built with zero base current when the collector is conducting (at least for low collector currents)? It's particularly a puzzle since the typical modern transistor has a very low beta at low currents, indicating that significant base current must flow before any collector current (above leakage) starts to flow.

Inquiring minds want to know.
 
crutschow,

Ah, we're back to the chicken and egg question again. So here's another varient.

The chicken and egg allegory does not apply to what controls the current in a BJT operating in the active region. The Sedra and Smith reference and equations I posted, as well as other transistor physics books, specifically show that Ic and Ib are dependent on Vbe. The fact that there is a relationship between Ic and Ib does not mean that Ib causes Ic. Both Ic and Ib are causually tied to Vbe, but only functionally to each other.

If a BJT is strictly a voltage controlled device then why can't a BJT be built with zero base current when the collector is conducting (at least for low collector currents)? It's particularly a puzzle since the typical modern transistor has a very low beta at low currents, indicating that significant base current must flow before any collector current (above leakage) starts to flow.

Why are you assuming that a voltage controlled device has to have an infinite input impedance, which it would for zero base current? That is not a requirement for a voltage controlled current source. A voltage controlled current source means that the voltage controls the current source (as stated by Sedra and Smith) regardless of how much current the the voltage source takes.

Inquiring minds want to know.

So now they should know.

Ratch
 
Last edited:
...
What are you assuming that a voltage controlled device has to have an infinite input impedance, which it would for zero base current? That is not a requirement for a voltage controlled current source.
...

Nor is it a requirement for a current controlled device to start operating at zero volts. ;)

Isn't the defining factor whether a change in input current best controls output current, or a change in input voltage best controls output current?

The way i see it, some voltage Vbe is needed for the diode junctions in the silicon to start operating, then once operating the transistor is a current controlled device. No great physics argument needed.
 
Mr RB,

Nor is it a requirement for a current controlled device to start operating at zero volts.

How does the above statement tie into what we are discussing?

Isn't the defining factor whether a change in input current best controls output current, or a change in input voltage best controls output current?

It depends on whether you are talking about it functionally or causally. The Ic in a BJT operated in the active region appears to be dependent on the Ib. You can even make calculations based on this behavior. But the cause is really the Vbe affecting both the Ic and Ib. So although the BJT mimics a current controlled current source, it really is a voltage controlled current source.

The way i see it, some voltage Vbe is needed for the diode junctions in the silicon to start operating, then once operating the transistor is a current controlled device. No great physics argument needed.

As long as the BJT is operating in the active region, its Ic and Ib are dependent on Vbe, according to Sedra and Smith, and others. That is the physics of a BJT in the active region. No other viewpoint or presentation can abrogate that fact.

Ratch
 
..................

It depends on whether you are talking about it functionally or causally. The Ic in a BJT operated in the active region appears to be dependent on the Ib. You can even make calculations based on this behavior. But the cause is really the Vbe affecting both the Ic and Ib. So although the BJT mimics a current controlled current source, it really is a voltage controlled current source.

...............
So although it looks like a chicken and sounds like a chicken and walks like a chicken, it's really an egg. :rolleyes:
 
crutschow

So although it looks like a chicken and sounds like a chicken and walks like a chicken, it's really an egg.

You have to delve into the relationship between Ic, Ib, and Vbe to determine what is cause and effect.

Ratch
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top