How does a transistor amplify current or voltage?

[BrownOut]

Vbi is the barrier voltage and is constant, whether a source is connected or not. The source will not reduce the barrier voltage, it will reduce the junction voltage Vj = Vbi -Vbe. Vj = Vbi only when Vbe = 0. I don't have the same Sedra book you have.

vbi is not the barrier voltage. The source reduces the barrier voltage, just as Sedra and Smith says. It's in the book, you only have to read it. Also see Gray and Mayer.

Barrier or built-in voltage is fine. The junction voltage is the result of subtracting Vbe from Vbi. We appear to agree on that.

Also, the reduction in barrier voltage is equal to vbe, This might seem trival, but the ramification is if you control the barrier voltage, then you control vbe. And Sedra and Smith shows that current controls barrier voltage by neutralizing charge in the depletion region. If you need an equation, vbe = vt*ln(Ie/Is), also given in S&S

 

[MrAl]

MrAl,



Hot flash! Increasing the collector current too much is what brings a transistor into saturation. Saturation occurs when Ib no longer controls Ic. A zero Ic is the cutoff point. How come no one but I pointed that out to you?

Were you trying to infer something about causal control of a BJT when it is not in the active region?

Ratch

Hello again,


We are definitely not on the same page here. I made it clear that i was talking about increasing the collector current to force the transistor OUT of saturation. I made it clear with the example of the oscillators that come out of sat due to increased collector current when the magnetics saturate and draw a much greater current. The transistor pulls abruptly out of sat and the collector voltage rises sharply. That is coming OUT OF SATURATION, certainly not going into saturation, which you for some reason want to talk about even though i made it clear how the operation was happening. No one else point this out because it doesnt happen that way. I cant imagine what the heck you are talking about there. Perhaps a typo or something?

Well now we come to another interface: Active or not active? Perhaps you would like to define that too and want to restrict our discussion further to only part of the transistor operating area where it's only considered in the 'active' region. Let me guess, you want to define this area at the place where the transistor looks like it is Vbe controlled?

Heck, from now on i say that we no longer allow the transistor to operate over the full range of possibilities. From now on we have to restrict the base emitter operation to 10ma to 10.1ma and the collector current from 100ma to 101ma. So you see everyone here and everyone in the past is totally wrong about the transistor...it's really just a big fat resistor! That's it
The point is, if we restrict the operation to what we want it to be we can make it look like whatever we want it to look like. Ii think it's a big zener diode, because i can force it to operate with a fixed collector voltage over a reasonable temperature range Yeah that's it, the transistor is really a big fat zener diode in disguise And to think we all thought that the transistor could amplify, ha.

BTW, if i remember right when we increase Ic the Vbe does increase, in the active mode. Can we really say that Vbe is controlling anything then? If i remember right, this component is kT/q*ln(Ic/Ic0) where Ic0 is collector current at T0 and Vbe0, and k is the Boltzmann constant, q is the charge on one electron, T is the temperature in K, Ic is collector current.

One more little question. What would happen if we used the photoelectric effect to generate a current to control the transistor. That would mean pushing electrons into a higher energy band using photons. Thus, a current would flow without the need for a potential voltage difference.


Still Ratchet, i will certainly consider what you are pushing if only you could provide some experiment that will conclusively show that what you say is true. Relax and have a cup of coffee, take a step back, sit down, design a little experiment...that's all it takes. When a scientist wants to prove something he says is true he designs an experiment that proves it one way or the other. We cant do this with everything in the universe, but we should certainly be able to do this with something we can put our hands on like a transistor.

 

[Ratchit]

MrAl,

We are definitely not on the same page here. I made it clear that i was talking about increasing the collector current to force the transistor OUT of saturation. I made it clear with the example of the oscillators that come out of sat due to increased collector current when the magnetics saturate and draw a much greater current. The transistor pulls abruptly out of sat and the collector voltage rises sharply. That is coming OUT OF SATURATION, certainly not going into saturation, which you for some reason want to talk about even though i made it clear how the operation was happening. No one else point this out because it doesnt happen that way. I cant imagine what the heck you are talking about there. Perhaps a typo or something?

The fact that a transistor comes closer to saturation when its collector current increases is not in doubt. The only way to bring it out of saturation is to lower its current, or increase the reverse bias of the base-collector. This link directly contradicts what you say. **broken link removed**

Well now we come to another interface: Active or not active? Perhaps you would like to define that too and want to restrict our discussion further to only part of the transistor operating area where it's only considered in the 'active' region. Let me guess, you want to define this area at the place where the transistor looks like it is Vbe controlled?

From your comment, I can tell that you are not in possession of a good basic transistor textbook. This is basic stuff. The following link lists the 5 regions of BJT operation. Look in the "Regions of Operation" section of the link. Notice that they say that saturation facilitates high current conduction. You can expand your knowledge from there. Yes, I do want to confine this discussion to the active region. Can you think of why? Ans: Because you cannot control the Ic with Ib in the saturation region of operation.

Heck, from now on i say that we no longer allow the transistor to operate over the full range of possibilities. From now on we have to restrict the base emitter operation to 10ma to 10.1ma and the collector current from 100ma to 101ma. So you see everyone here and everyone in the past is totally wrong about the transistor...it's really just a big fat resistor! That's it
The point is, if we restrict the operation to what we want it to be we can make it look like whatever we want it to look like. Ii think it's a big zener diode, because i can force it to operate with a fixed collector voltage over a reasonable temperature range Yeah that's it, the transistor is really a big fat zener diode in disguise And to think we all thought that the transistor could amplify, ha.

No one can tell anyone how to use or apply a transistor. And a transistor is not a resistor or a zener diode. Yes, a transistor can amplify.

BTW, if i remember right when we increase Ic the Vbe does increase, in the active mode. Can we really say that Vbe is controlling anything then? If i remember right, this component is kT/q*ln(Ic/Ic0) where Ic0 is collector current at T0 and Vbe0, and k is the Boltzmann constant, q is the charge on one electron, T is the temperature in K, Ic is collector current.

The current Ic and Vbe have a monotonic relationshiip with each other. That means that if either one increases, the other never decreases, at least not in the active region. No one tries to control Vbe by changing Ic. In practice, they try to control Ic by changing Ib, which in turn changes Vbe, which in turn controls Ic.

One more little question. What would happen if we used the photoelectric effect to generate a current to control the transistor. That would mean pushing electrons into a higher energy band using photons. Thus, a current would flow without the need for a potential voltage difference.

Since voltage is the energy density of the charge, increasing the energy will increase the voltage. If you study a phototransistor, you will see that it can operate without its base lead connected to anything. **broken link removed** In practice, the base lead is usually used for biasing. By the way, you should say charge will flow or current exists, not "current will flow".

Still Ratchet, i will certainly consider what you are pushing if only you could provide some experiment that will conclusively show that what you say is true. Relax and have a cup of coffee, take a step back, sit down, design a little experiment...that's all it takes. When a scientist wants to prove something he says is true he designs an experiment that proves it one way or the other. We cant do this with everything in the universe, but we should certainly be able to do this with something we can put our hands on like a transistor.

I have no idea what you are talking about. Just about every experiment that can be done has been down with BJT's and written about. BJT's are a very mature technology. Have you been following this thread closely where I explained my thinking? What don't you understand about what I said, even if you don't agree with it?

Ratch

 

[MrAl]

MrAl,



The fact that a transistor comes closer to saturation when its collector current increases is not in doubt. The only way to bring it out of saturation is to lower its current, or increase the reverse bias of the base-collector. This link directly contradicts what you say. **broken link removed**



From your comment, I can tell that you are not in possession of a good basic transistor textbook. This is basic stuff. The following link lists the 5 regions of BJT operation. Look in the "Regions of Operation" section of the link. Notice that they say that saturation facilitates high current conduction. You can expand your knowledge from there. Yes, I do want to confine this discussion to the active region. Can you think of why? Ans: Because you cannot control the Ic with Ib in the saturation region of operation.

<snip>

Ratch

Hello again,

Quoted from your link:
START QUOTE
If you increase the BE junction voltage by just a small amount in the forward biased direction, it increases BE emitter current and also collector current. Collector current increases, voltage drop across the collector resistor increases and, collector voltage with respect to ground, decreases toward zero or some small fraction of a volt.
END QUOTE


I'm sorry but that link you provided does not talk about the conditions i had set up for that experiment where the collector current was doing all the controlling. That link is out of context for that discussion because it talks about changing Vbe again and that is NOT what the experiment is made to do. For that experiment we DO NOT CHANGE Vbe.
It appears that your hands on experience may be a little limited here since you didnt understand how that oscillator works. If you understood you would have known immediately what i was talking about as others here knew right away. Actually there are several different topologies for these kinds of oscillators often used in boost converters.

See drawing attached...
Using that circuit as an illustration, perhaps you can tell me how Q2 turns off, that is, how does Q2 begin to turn off, what makes it start to turn off.

 

[Claude Abraham]

Ratchit, nothing in the references you cite even remotely state that Vbe is "causal". Repeating it an infinitum does not make it true. Ic is simply Ie - Ib, but Vbe has a functional relation w/ all 3. You've been presented w/ the fact that Ib/Ie as well as Ic change before Vbe does. Your response is that it does not matter that Vbe lags the currents, it is still the cause variable. In all scientitfic as well as non-scientific debates, as soon a variable is shown to lag behind another, it is automatically ruled out as "causal". If you refuse to acknowledge that a cause must always precede its effect, then what is the point of continuing? Obviously if the timing of events does not dissuade you, then nothing well.

Equations give good info regarding the quantitative relation between the variables. The references you cite are valid sources, but you spin them to suit your pre-conceived "voltage is causal" foregone conclusion.

I stated earlier that your assertion of Vbe being causal is "dogma". Your response was to invoke the "Galileo persecution complex". I hate to inform you, butI, as well as Brownout & Mr. Al are the "Galileo's" in this issue, & you are not accepting logic because it weakens your strongly held convictions. Every person on both sides of any issue can claim the galileo complex. Because Galileo was opposed by the majority, & you are opposed by the majority, does it logically follow that since Galileo was right, you must be as well? Galileo? You take yourself way too seriously.

In Galileo's day, the majority was farmers & tradesmen. Little industry existed & lay people were not schooled in science as they are today. Galileo's concepts were counter-intuitive to farmers, coachmen, milkmaid, blacksmmiths, actors, musicians, doctors, midwives, etc. The industrial revolution has taken place since then. The people whom you are at odds with are degreed, & working scientists/engineers. I have but 3 patents, w/ 2 pending. I have 2 published papers. That in & of itself does not make me right by any stretch. It is just being mentioned to remind you, that unlike Galileo's critics, I am not a milkmaid, coachmen, actor, etc.

You are opposing highly skilled people from National Semiconductor, Texas Instruments, Fairchild, On Semiconductor, etc. THese people are much more qualified than Galileo's critics. Equating yourself w/ Galileo is absurd & ridiculous to say the least. If I asserted that the moon is made of green cheese, & you call me crazy, then I respond with "Galileo was called crazy as well", does that vindicate me?! Invoking Galileo is a joke.

Regarding what voltage is, it is work per unit charge transporting said charge from point a to b. It is impossible to move charges through a p-n jcn w/o affecting both current & voltage. As I've stated before, both are involved & important to know. But both cannot be the control quantity. One is forced or controlled, the other is consequential, albeit equally important.

The Baker clamp illustrates that a clamped & unclamped bjt are driven to saturation. Then when the base bias is forced to zero, there is a delay in turn off. The unclamped bjt takes longer to cut off due to stored charge. Charge control details this property, but VC does not, nor does CC. Since voltage & current are both related to charge, you can force a relationship based on I or V. But the quantity of Q determines the recovery time. This is equally true w/ diodes. QC is the best model for this internal action. It hasn't changes in more than half a century. My most recent product design involves a 10 hp 3-phase brushless dc motor controller, using bjt, FET, & IGBT as well as diodes. The design of the hardware cannot be done w/o the QC info from the OEM data sheet. Neither CC nor VC is adequate to develop the hardware.

 

[tvtech]

Hopefully no soon to be qualified Electrical Engineers are following this thread.

Ratchit is simply posting to try and confuse the known facts about Semi Conductor junctions which have worked for yonks for qualified Electrical Engineers who have gone on to design literally Millions of successful products.

Ignore Ratchit. He is simply baiting. Let him be.

He has his own world to live in afterall.

 

[MrAl]

Hello tvtech,

Well, if he can explain how that circuit i posted (two posts back) can operate with only Vbe control, i'll take more interest in his point of view. I know his point is not null and void in the right context, but he seems to be pushing it as the ONLY point possible and this is what i dont understand too well. Some people want to say that the field comes first, others say the charge comes first, i wont condemn either point.

Maybe we should all chalk this up to a simple difference in opinions and leave it at that. I know that theoretically it's not about opinion, but some opinions are more deeply rooted than others, perhaps because of the methods the teachers of the subject matter use, im not sure.

In any case, i'll wait to see how Ratchit wants to explain that circuit operation and then i guess i'll move on. It has been interesting though, and i wont condemn any person here for their point of view either, no matter what it is, but do suggest that they keep an open mind; i try to do so also...and i think i did in this thread.

 

[tvtech]

Thanks MrAl.

You guys all have amazing knowledge. But here you guys sit with someone who wants to upset the whole book of known facts......

I keep asking myself. Why???. Nothing constructive can come out of this thread. Except confusing everybody.

Cheers

 

[Ratchit]

MrAl,

I'm sorry but that link you provided does not talk about the conditions i had set up for that experiment where the collector current was doing all the controlling. That link is out of context for that discussion because it talks about changing Vbe again and that is NOT what the experiment is made to do. For that experiment we DO NOT CHANGE Vbe.

Yes, the link does talk about the conditions from the transistors point of view. Your last answer to me was the first time you published the circuit. Vbe does if fact change and direct the Ic before it goes into saturation. It also cuts off the collector current when in saturation. You are changing Vbe indirectly.

It appears that your hands on experience may be a little limited here since you didnt understand how that oscillator works. If you understood you would have known immediately what i was talking about as others here knew right away.

How would you know what my experience is, and whether I understand how it works?

If you understood you would have known immediately what i was talking about as others here knew right away.

And if you published the circuit the first time, I would have known what you were talking about. Just because many did not comment on it does not mean they understood it.

Using that circuit as an illustration, perhaps you can tell me how Q2 turns off, that is, how does Q2 begin to turn off, what makes it start to turn off.

Sure, easily. Q2 switches back and forth from saturation to cutoff. When in saturation, it shunts the current from the LED, turning it off. When Q2 is cutoff, the current passes through the LED and it turns on. Q2 is biased to saturate by the collector current from Q1 passing through Q2's base circuit. While the current in Q2 is building up toward saturation, the current in the coil L2 is also increasing and building up a magnetic field. When the saturation current stabilizes, the coil's magnetic field collapes and sends a positive voltage through C1 to the base of Q1. This turns off Q1 and thereby Q2, because Q2 doesn't have any more base current. This continues until the coil's magnetic field finishes collapsing, and cannot sustain any more positive voltage to the base of Q1, at which time the cycle repeats itself. So you see, the transistor Q2 comes out of saturation because its collector current is cut off by its base current being cut off. Not because its collector current is increasing.

Ratch

 

[Ratchit]

Claude,

Ratchit, nothing in the references you cite even remotely state that Vbe is "causal". Repeating it an infinitum does not make it true. Ic is simply Ie - Ib, but Vbe has a functional relation w/ all 3. You've been presented w/ the fact that Ib/Ie as well as Ic change before Vbe does. Your response is that it does not matter that Vbe lags the currents, it is still the cause variable. In all scientitfic as well as non-scientific debates, as soon a variable is shown to lag behind another, it is automatically ruled out as "causal". If you refuse to acknowledge that a cause must always precede its effect, then what is the point of continuing? Obviously if the timing of events does not dissuade you, then nothing well.

Correct. Causal is a term Kevin A. used to describe the relationship as opposed to functional. I thought it was appropriate so I use it. That is right, timing or event sequence has nothing to do with it. For instance, a adjustment screw can have a "backlash" or "play" due to loose fitting parts, but still be in control. As I explained in my former post, casual is defined by how something does it, not when it does it.

I stated earlier that your assertion of Vbe being causal is "dogma". Your response was to invoke the "Galileo persecution complex". I hate to inform you, butI, as well as Brownout & Mr. Al are the "Galileo's" in this issue, & you are not accepting logic because it weakens your strongly held convictions. Every person on both sides of any issue can claim the galileo complex. Because Galileo was opposed by the majority, & you are opposed by the majority, does it logically follow that since Galileo was right, you must be as well? Galileo? You take yourself way too seriously.

In Galileo's day, the majority was farmers & tradesmen. Little industry existed & lay people were not schooled in science as they are today. Galileo's concepts were counter-intuitive to farmers, coachmen, milkmaid, blacksmmiths, actors, musicians, doctors, midwives, etc. The industrial revolution has taken place since then. The people whom you are at odds with are degreed, & working scientists/engineers. I have but 3 patents, w/ 2 pending. I have 2 published papers. That in & of itself does not make me right by any stretch. It is just being mentioned to remind you, that unlike Galileo's critics, I am not a milkmaid, coachmen, actor, etc.

I did not invoke Galileo as a description of the conditions during the Renaissance or today. I used it to describe the state of mind of one man, the Pope, who must have thought Galileo was dogmatic, and a threat to the Church's status quo. As a result, he set back science in Italy 50 years.

Yes, you are or were a staff engineer at Bendix Commercial Systems, and that has to be respected. But I will take responsibility for being wrong if I disagree with you.

You are opposing highly skilled people from National Semiconductor, Texas Instruments, Fairchild, On Semiconductor, etc. THese people are much more qualified than Galileo's critics. Equating yourself w/ Galileo is absurd & ridiculous to say the least. If I asserted that the moon is made of green cheese, & you call me crazy, then I respond with "Galileo was called crazy as well", does that vindicate me?! Invoking Galileo is a joke.

I think we were talking about being dogmatic, not crazy, right, or wrong.

Regarding what voltage is, it is work per unit charge transporting said charge from point a to b. It is impossible to move charges through a p-n jcn w/o affecting both current & voltage. As I've stated before, both are involved & important to know. But both cannot be the control quantity. One is forced or controlled, the other is consequential, albeit equally important.

Yes, I like to call it the energy density of the charge. I never said one was more important than another. You inferred that. I only said that Vbe directly controls Ic.

The Baker clamp illustrates that a clamped & unclamped bjt are driven to saturation. Then when the base bias is forced to zero, there is a delay in turn off. The unclamped bjt takes longer to cut off due to stored charge. Charge control details this property, but VC does not, nor does CC. Since voltage & current are both related to charge, you can force a relationship based on I or V. But the quantity of Q determines the recovery time. This is equally true w/ diodes. QC is the best model for this internal action. It hasn't changes in more than half a century. My most recent product design involves a 10 hp 3-phase brushless dc motor controller, using bjt, FET, & IGBT as well as diodes. The design of the hardware cannot be done w/o the QC info from the OEM data sheet. Neither CC nor VC is adequate to develop the hardware.

That may be true, but I always said that Vbe should not be used for design and calculation of Ic. That makes the viewpoint of Vbe controlling Ic trival, except perhaps in temperature stability calculations. I don't know why so much angst is generated about this proposition.

Ratch

 

[Ratchit]

tvtech,

Hopefully no soon to be qualified Electrical Engineers are following this thread.

Ratchit is simply posting to try and confuse the known facts about Semi Conductor junctions which have worked for yonks for qualified Electrical Engineers who have gone on to design literally Millions of successful products.

Ignore Ratchit. He is simply baiting. Let him be.

He has his own world to live in afterall.
​

If you have been following the thread, you would know that I do not use or advocate using the voltage control viewpoint to design with BJT's. How could you have missed that after I repeated it so many time during this discussion? Whether you consider BJT's to be CC or CV is trivial, I have said many times that a BJT should be functionally thought of as CC device.

Ratch

 

[MrAl]

MrAl,



Yes, the link does talk about the conditions from the transistors point of view. Your last answer to me was the first time you published the circuit. Vbe does if fact change and direct the Ic before it goes into saturation. It also cuts off the collector current when in saturation. You are changing Vbe indirectly.



How would you know what my experience is, and whether I understand how it works?



And if you published the circuit the first time, I would have known what you were talking about. Just because many did not comment on it does not mean they understood it.



Sure, easily. Q2 switches back and forth from saturation to cutoff. When in saturation, it shunts the current from the LED, turning it off. When Q2 is cutoff, the current passes through the LED and it turns on. Q2 is biased to saturate by the collector current from Q1 passing through Q2's base circuit. While the current in Q2 is building up toward saturation, the current in the coil L2 is also increasing and building up a magnetic field. When the saturation current stabilizes, the coil's magnetic field collapes and sends a positive voltage through C1 to the base of Q1. This turns off Q1 and thereby Q2, because Q2 doesn't have any more base current. This continues until the coil's magnetic field finishes collapsing, and cannot sustain any more positive voltage to the base of Q1, at which time the cycle repeats itself. So you see, the transistor Q2 comes out of saturation because its collector current is cut off by its base current being cut off. Not because its collector current is increasing.

Ratch

Hello again,


You're right, i had no right to comment on your experience not knowing you that well yet. It's just after your other comments i thought you had no idea about circuits like these, but that was partly my fault because i did not post the schematic right off. I always like a drawing too, which is why i asked you for one. Now you see why i wanted a drawing so bad, because it sometimes conveys information that is very hard to get across by simply chatting. Still, it may not help that much anyway but if you like you can do a drawing at some point.

I also have to comment on your patience again, because i can see you have quite a bit there, and i am happy about that so we can continue our discussion without getting all upset about some theory or another. So i have to thank you again for your patience, and you have to realize that im a bit older now and not as sharp as when i was back in school so it may take more to show me what you mean then it did say when i was back then.

In the case of this very circuit however, which i posted as an example where Vbe control would be VERY hard to claim, i think you need to take another look at the action of the transistor when it starts to turn off. This action is paramount to my illustration, to this experiment and what exactly it was meant to convey.
Quoting you from above if you dont mind, just to get to the heart of the matter:

"When the saturation current stabilizes, the coil's magnetic field collapes and sends a positive voltage through C1 to the base of Q1."

Well, that would be nice i guess if it happened that way, but i am sorry to say that that's not the way it happens. If the saturation current 'stabilized' the magnetic field would never change, as in an electromagnet. So what would make you think the field would collapse all by itself, ie what is the mechanism that forces the field to collapse especially if the current somehow stabilized? The coil is not AC coupled BTW, but is solidly DC coupled as the schematic shows.

 

[BrownOut]

How would you know what my experience is, and whether I understand how it works?

It becomes apparent after reading that lame circuit description. You got almost nothing right.

 

[Jony130]

I always thought that in this type of oscillators its thanks to C1 and his charge Q1 is start to cut-off.
When Ic current of a Q2 is rising despite Vbe2 increases the Q2 "switch" from saturation to the active region.
And this causes Vce2 to increase. And this change in Vce go trough C1 to cut-off Q1/Q2.

 

[Ratchit]

MrAl,

I always like a drawing too, which is why i asked you for one. Now you see why i wanted a drawing so bad, because it sometimes conveys information that is very hard to get across by simply chatting. Still, it may not help that much anyway but if you like you can do a drawing at some point.

A circuit diagram is a specialized drawing that would not help in describing my viewpoint.

I also have to comment on your patience again, because i can see you have quite a bit there, and i am happy about that so we can continue our discussion without getting all upset about some theory or another. So i have to thank you again for your patience, and you have to realize that im a bit older now and not as sharp as when i was back in school so it may take more to show me what you mean then it did say when i was back then.

I think you get rusty on things you have not looked at for quite a while, but your cognitive skills should not deteriorate until you get very old. Also you now have more experience.

In the case of this very circuit however, which i posted as an example where Vbe control would be VERY hard to claim, i think you need to take another look at the action of the transistor when it starts to turn off. This action is paramount to my illustration, to this experiment and what exactly it was meant to convey.
Quoting you from above if you dont mind, just to get to the heart of the matter:

"When the saturation current stabilizes, the coil's magnetic field collapes and sends a positive voltage through C1 to the base of Q1."

Well, that would be nice i guess if it happened that way, but i am sorry to say that that's not the way it happens. If the saturation current 'stabilized' the magnetic field would never change, as in an electromagnet. So what would make you think the field would collapse all by itself, ie what is the mechanism that forces the field to collapse especially if the current somehow stabilized? The coil is not AC coupled BTW, but is solidly DC coupled as the schematic shows.

You are right, I got ahead of myself, so let me clarify. The coil is not DC coupled to Q1 due to the capacitor linking the two.

During the increase of current in Q2, the magnetic field builds up, and the coil opposes the B+ voltage. After saturation is reached, the current increase stops, and the coil stops opposing the B+ voltage. So the saturation current is constant, the voltage across the coil is zero, the magnetic field is present, and the base of Q1 receives a positive voltage, which cuts off Q1 and Q2. This stops the existence of the current in Q2 and L1, and that will cause the magnetic field to collapse and sustain a positive voltage on Q1 for a time.

Ratch

 

[Jony130]

After saturation is reached, the current increase stops, and the coil stops opposing the B+ voltage. So the saturation current is constant, the voltage across the coil is zero

Well I think that it is another way around. When Q2 is first "ON" is in saturation and with time during the increase of current in Q2. Q2 "switch" from saturation to active region. And thanks to C1 an this "switch" to active region Q1 and Q2 start to cut-off.
So it is the increases of a collector current that cause BJT to cut-off.

 

[BrownOut]

Jony130 -- You're on the right track. You'll probably be told you're wrong, but you're much closer than what's been previously posted.

 

[MrAl]

MrAl,



A circuit diagram is a specialized drawing that would not help in describing my viewpoint.



I think you get rusty on things you have not looked at for quite a while, but your cognitive skills should not deteriorate until you get very old. Also you now have more experience.



You are right, I got ahead of myself, so let me clarify. The coil is not DC coupled to Q1 due to the capacitor linking the two.

During the increase of current in Q2, the magnetic field builds up, and the coil opposes the B+ voltage. After saturation is reached, the current increase stops, and the coil stops opposing the B+ voltage. So the saturation current is constant, the voltage across the coil is zero, the magnetic field is present, and the base of Q1 receives a positive voltage, which cuts off Q1 and Q2. This stops the existence of the current in Q2 and L1, and that will cause the magnetic field to collapse and sustain a positive voltage on Q1 for a time.

Ratch

Hello again Ratchit,


Well, I dont know what makes you think that the saturation current has to stop rising. With the transistor CE shorted out and Resr of the inductor zero, the current would rise to an infinite level. Obviously this cant happen, so something has to give. Now if the transistor CE was a fuse instead of a transistor junction that fuse would blow and that would cause a halt to the current. Incidentally, the 'collector' voltage would also rise sharply once the fuse blew out, and that would couple through the cap and cut off Q1 (once, but it would ha ha).

Now this might seem somehow coincidental, so lets take Q1 out of the picture by shorting that out collector to emitter. That puts a constant bias on Q2, and it's collector current rises as the circuit is switched on. Now the current reaches the saturation point of the coil, and it starts to draw a ton of current, and that current does not increase gradually but very sharply. The transistor CE is now conducting heavily, but the current still increases. Eventually it reaches a point where the coil is conducting so heavily that the current gets to a point where the transistor saturation voltage starts to rise a little. It's still in sat, but it's Vsat starts to rise. With more time, the current goes even higher, until the Vsat reaches the max it can be and still be called "in saturation". Next the current continues to rise, but we've exceeded that breakpoint and the transistor starts to come out of saturation. All the while the current is increasing sharply too, and the transistor CE voltage increase sharply. Eventually the coil looks shorted and the transistor is conducting heavy current but it's Vce is almost as high as B+ itself. The transistor is now dropping all of the voltage so Vce=B+ and it is definitely out of saturation. Nothing else happens because we shorted out Q1 collector to emitter. If we hadn't done that, the sharp rise in collector voltage would have been felt through the capacitor and into the base of Q1 which would cut it off, which would in turn cut off Q2, and collector current in Q2 would die down fairly quick. That action of course causes a high back emf from the coil.

A second experiment is where we connect the transistor Vbe to a voltage source of about 0.8 volts, and connect a 5 ohm resistor from the collector to B+, and emitter to ground. As we increase B+, we see the transistor Vce rise following the rise in B+ until it reaches saturation, and then the Vce levels off, but as we increase B+ even more at some point the CE breaks out of saturation and the Vce starts to rise also, following the rise in B+ almost exactly (with a drop).
In other words, we can still force the transistor out of saturation even though we hold the input of the transistor Vce at a fixed level, in this case 0.8 volts.
That's an experiment you can easily perform in a circuit simulator with say a 2N2222A.

A third experiment is where we connect the same circuit, but instead of a 5 ohm resistor we use instead an inductor with low esr like 0.1 ohms. Now the behavior is time dependent (like the oscillator) where the collector rises more and more and more until the transistor breaks out of sat.
This cane also be done very easily in a circuit simulator with say the same transistor and say a 100uH inductor.

Both of these are very interesting to do and dont take much time.

BTW the circuits that Jony posted illustrate the same basic principle of more or less Vbe control vs Ic control.

OH yeah, another little experiment with this circuit is to increase the inductors Resr higher and higher. What happens is proof that it works according to the above. When the Resr reaches the level where the current can not get high enough to pull Q2 out of saturation (it limits it by Imax=V/Resr), the transistor Q2 'sticks' in saturation and the oscillations halt. This actually occurs at a value of Resr that would seem to allow everything else to work properly had it worked on some other principle, but it doesnt.

 

[Ratchit]

MrAl,

Well, I dont know what makes you think that the saturation current has to stop rising. With the transistor CE shorted out and Resr of the inductor zero, the current would rise to an infinite level.

That is an easy one to answer. When a transistor goes into saturation, it does not short. It has a small saturation resistance. According to the data sheet for the 2N4401, its saturation resistance is around 1.5 ohms at Ib = 500 ma. With a 3 volt supply, you are just not going to get an "infinite" increase in saturation current. So when the saturation does level off, as certainly will, a positive voltage will be applied to the base of Q1, and turn off both Q1 and Q2. By the way, what is Resr?

Next you propose three "experiments" for whatever reason. In order to keep disciplined and stay focused on the original question of how your oscillator works, I am going to ignore them, and reply only to your answer about what I said in regard to saturation resistance, or any mistakes I made. Since I was the first to explain in detail how your oscillator works, we should address my explanation first before going off on any other divergent tangents. So I expect your next posting to point out any mistakes in my analysis.

BTW the circuits that Jony posted illustrate the same basic principle of more or less Vbe control vs Ic control.

It is a comparative illustration with no meaning. The two equations are simply inverses of each other. No one tries to control Vbe with Ic.

Ratch

 

[Ratchit]

Brownout,

vbi is not the barrier voltage. The source reduces the barrier voltage, just as Sedra and Smith says. It's in the book, you only have to read it. Also see Gray and Mayer.

Also, the reduction in barrier voltage is equal to vbe, This might seem trival, but the ramification is if you control the barrier voltage, then you control vbe. And Sedra and Smith shows that current controls barrier voltage by neutralizing charge in the depletion region. If you need an equation, vbe = vt*ln(Ie/Is), also given in S&S

We seem to have a definition problem here. What you call the barrier voltage I would call the junction voltage. You don't think the Vbi is the barrier voltage, and you wish the term "junction voltage" would just go away. No one will ever convince you otherwise, but I think you do have the empirical relationship down correctly.

It becomes apparent after reading that lame circuit description. You got almost nothing right.

Well, that is easy to say, but you did not step up to the plate with a explanation. And you did not point out how my explanation was insufficient or wrong. So why not shower us with your wisdom?

Ratch