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Transistor as a switch

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crutschow

crutschow

danadak said:
his is what caught my eye.....
Click to expand...
There you are going all academic on me again. :rolleyes:
I'm not talking about saturation current or drift velocity or any transistor physics.
I referring to when the transistor is considered to be saturated when used as a switch.
That saturation point of a BJT transistor is not determined by it's geometry within it's operating limits.
Of course the geometry will determine those limits, but that's a side issue here.
The switch saturation occurs when the collector-emitter voltage becomes lower than the base-emitter voltage.
 
HANKMARS

HANKMARS

crutschow said:
That's not what "saturation" means for a BJT.
It just means the transistor is fully on where the Vce is less than Vbe, and can occur at any collector current.
It has nothing to do with the transistor limits.

Interestingly, saturation has a different meaning for FETs (just to confuse us).
A FET is saturated when, for a given Vgs, the increase in drain current does not increase significantly with an increase in drain voltage.
That also has nothing to do with limits.
Click to expand...
Footnote: I have no passionate desire that industry abide by my "Definition of Terms." If I were to lead a group on a development project, I would most certainly demand that definition of terms be adhered to. If a majority of the team preferred that the "restroom" be labeled the "water closet" then water closet is now the gospel and intentional use of terms not water closet, is blasphemy. If it is proven that using water closet is detrimental to our efforts of development, then the matter will be reviewed ASAP. My following argument is sincere rather than being generated by some force that would cause me to fabricate a frivolous opposition to an other's statement. I have a problem with your term "fully on." I may well be out of line with my argument as to terminology defined as conventional by industry standards. I honestly do not know if I am breaking convention or not. My argument stems from personal observations made during an effort to use beta lines from specs on the 2N3055 power transistor, and regulate the Iout of the 2n3055 with that current out being the speed (or force) controller of a DC motor shaft. Considering the high current needed to make these DC motors perform a desired function, my notion of a varying constant current, ( as I envisioned the circuit layout and principles ) was undeniably an ill-conceived plan. But, that is not the point. The foundation of my argument is built from personal observation of the circuits designed for, and built, for myself. I, by no means, wish to infer that my conclusions drawn from my observations are of empirical status. Also I just realized that I do not know how to save a draft or call up a draft of my text, and I have a task to perform, so I will post what I have so far written and will compose an explanation, in my next post, as to why I believe that your use of "fully on" is invalid in the context you have used it. I'll be back soon.
 
D

danadak

crutschow said:
No.
Just your pedantic attitude.
Sorry, I ruffled your academic feathers.
Click to expand...

No worry of the feathers, they are clean and flight ready.

Pedantic, challenge for you is accuracy, then you can join us PEDs in
knowledge land, ready to accept all who try and care about accuracy.
Who adapt to new knowledge evolution from new evolving scientific findings.

Regards, PEDEE here.
 
HANKMARS

HANKMARS

hevans1944 said:
What you have done is jump-start a veeery old thread that attempted to discuss how a transistor works. The thread became a tug-of-war between those who said the "transistor is a (base) current-controlled device" and the "transistor is a (base-to-emitter) voltage-controlled device. Nothing was "solved" to the satisfaction of everyone who participated in the thread. Eventually the thread died, and now I cannot even find it in the archival records. The new owners of these forums are probably not anxious to open the discussion again.

There are many states and levels of conduction that a transistor can be in, between "Cut off" and "Saturation". There is no "Threshold" state that I am aware of. The BC457 NPN transistor will "turn on" whenever the base-emitter junction is forward biased with a voltage that makes the base positive with respect to the emitter. Note that some collector current occurs even with small amounts of forward bias. There is no magic "threshold" that must be crossed before the transistor will turn on.

The transistor will "turn off" whenever the base-emitter junction voltage is zero or slightly negative. Meanwhile, the collector-base junction is reverse biased as the transistor begins to conduct in its collector circuit. With a series-resistance (or reactance) load in the collector circuit, the collector-to-emitter voltage will decrease with increased conduction until the transistor saturates: further increases in the base-emitter forward-bias voltage does not cause any significant increase in collector current when the transistor is in saturation.

When using the BJT as a switch, it is desirable that the saturation voltage be very small (usually in the neighborhood of 0.1 volts) and the cut off (collector) current also be very small (usually in the neighborhood of a few dozen nano-amperes.) To accomplish saturation will require an input signal voltage well above the 0.6 to 0.7 volt "knee" in the transfer function between base-emitter bias voltage and collector current. Apply the base-emitter bias voltage through a fixed-value resistor that limits the base current to a value of roughly icc/10, where icc is the saturation current of the transistor. It is a good idea to insert another, larger-valued, fixed resistor between the base and the emitter to guarantee cut off when no signal is applied.

One final comment on the use of a transistor as a switch: in general, a MOSFET switch will accomplish the switch function faster, better, and cheaper than the BJT implementation. You should look into that.
Click to expand...
It has been my understanding that a transistor turns off if Vbe is less than forward breakdown voltage of base/emitter junction. A transistor does not fire (turn on) then need to be brought to 0Vbe to turn off as I think maybe an SCR needs to be. I did say maybe an SCR. I'll have to review SCR TofO to know for sure. In your second paragraph, you state that conduction from emitter to collector will occur if Vb is positive in respect to Ve. Is that correct? I would suspect leakage current may increase but my understanding is that Vbe must meet or exceed Vfb,eb,j. to conduct e to c in any useful manner.
 
HANKMARS

HANKMARS

crutschow said:
That's not what "saturation" means for a BJT.
It just means the transistor is fully on where the Vce is less than Vbe, and can occur at any collector current.
It has nothing to do with the transistor limits.

Interestingly, saturation has a different meaning for FETs (just to confuse us).
A FET is saturated when, for a given Vgs, the increase in drain current does not increase significantly with an increase in drain voltage.
That also has nothing to do with limits.
Click to expand...
Back to observations of a dimly lit design of a motor speed control circuit and their relevance to my disagreement of your definitions or use of terms saturation and fully on. Using my device's data sheets, I did some quick calcs to determine base current limiting resistor values that would deliver an Imax = 10A from the 2N3055 output transistor in my doomed design. I breadboarded the control circuit using the Rbase values that resulted from my calcs. As I recall, a small signal Xtor received a variable voltage from an adjust pot. That Xtor's emitter drove the base of another small signal Xtor who's emitter drove the base of a general purpose Xtor in a 220 package whose output was driving the base of the 3055 power Xtor in a TO-3 package. A cascaded arrangement of Xtors taking a logic level signal to a 12v 10A output. Using my base resistor values I had calculated, the circuit was conducting (on) but nowhere close to delivering the desired 10A at its output. I changed base resistor values a few times, eventually getting into single digit values and still did not obtain my desired Iout. The next step was to eliminate the Ib limiting resistors, except the resistor in the first small signal Xtor base circuit. Its value had decreased considerably since the first layout. From that point on, the transistors were cascaded, emitter to base with no base resistors. Hence, in the earlier designs, the transistors were on ( fully on as in your reference ) but to get the high current flow through the 3055 power out tranny, I had to eliminate base resistors to drive the trannies into "saturation." So if the case is that once a transistor starts to conduct, it is "fully on" then I had to take steps to turn the transistors "more fully on." More fully on is not a phrase that I am comfortable with. That and the differing descriptions of transistor saturation is why I wrote this novel to express my view of these concepts. I may have left out other circuit details but I hope I conveyed my perspective on definition of terms.
 
Arouse1973

Arouse1973

Adam
Interesting play with words isn't it. What does fully on mean and is it anymore important than half on or quarter on. What do we benefit in general from using the words saturation, fully on, fully off etc.

For example a supplier might consider their transistor is off at 0.5 volts. But you could still use this transistor with a base voltage of 0.45 so it's off but it's on.

My personal preference is to ask myself is the transistor working within it's maximum limits and is it performing how it should for that circuit. If not then I need to change something. I might myself use the words it's not conducting enough or drawing enough current, I may need to adjust the base voltage or something like that.

Thanks Adam
 
HANKMARS

HANKMARS

Arouse1973 said:
Interesting play with words isn't it. What does fully on mean and is it anymore important than half on or quarter on. What do we benefit in general from using the words saturation, fully on, fully off etc.

For example a supplier might consider their transistor is off at 0.5 volts. But you could still use this transistor with a base voltage of 0.45 so it's off but it's on.

My personal preference is to ask myself is the transistor working within it's maximum limits and is it performing how it should for that circuit. If not then I need to change something. I might myself use the words it's not conducting enough or drawing enough current, I may need to adjust the base voltage or something like that.

Thanks Adam
Click to expand...
RE: Term saturation. If I had a device which utilized a heating element that was difficult to 'hands on' access, and was not delivering the required temperature in the heating chamber and inhibiting proper function of the device, it would be a logical troubleshooting step to take the machine to the point in the control program where the heating element power transistor (drive transistor) is "on," assuming I can readily access drive transistor leads or test points. Now when I call my boss to gain permission to begin a $600 repair procedure of replacing John Q Customer's heating element in his malfunctioning device, my boss may very well inquire why I think that the heating element is faulty. I need to explain the situation of the device, when powered up, is performing properly until the "heater on" point, in its control program, is reached. I could verbalize that the 'heater on' annunciator lamp illuminates and sufficient base current is being supplied to element drive transistor and by taking a reading of the drive transistor's collector to +Vs, the collector is delivering a current flow very near its absolute maximum current rating. Or, I can sez, "Da heedah light come on when idz pose to and Q2 turns 'on' an go inda 'satch-e-ation.' To me, it is the same difference as calling customer service and speaking with Benjiman Ma, located in New Delhi, India, or speaking with Bubba Kilz, sitting in Dry Hole, Texas. Just a matter of personal preference really.
 
HANKMARS

HANKMARS

Arouse1973 said:
Interesting play with words isn't it. What does fully on mean and is it anymore important than half on or quarter on. What do we benefit in general from using the words saturation, fully on, fully off etc.

For example a supplier might consider their transistor is off at 0.5 volts. But you could still use this transistor with a base voltage of 0.45 so it's off but it's on.

My personal preference is to ask myself is the transistor working within it's maximum limits and is it performing how it should for that circuit. If not then I need to change something. I might myself use the words it's not conducting enough or drawing enough current, I may need to adjust the base voltage or something like that.

Thanks Adam
Click to expand...
If you have a 2N2222 transistor that begins to conduct at 0.45 V on the base in reference to emitter V, I would conclude that you purchase your electronic devices at Radio Shack. Radio Shack, "Utilizing the floor sweepings of the world."
 
crutschow

crutschow

HANKMARS said:
I did some quick calcs to determine base current limiting resistor values that would deliver an Imax = 10A from the 2N3055 output transistor in my doomed design.
Click to expand...
And what transistor parameter values did you use for that?

I don't see why there's a quibble about when a BJT is saturated.
What is the definition that it's when the Vce is less than Vbe not correct?
 
D

danadak

Then there is this, eg. all bipolar, or so I think (vert axis is Ic)-

iu


Ebers-Moll has no total shutoff (due to leakage) -

iu


Gummel-Poon -

https://www.iue.tuwien.ac.at/phd/rottinger/node61.html

By the way Ebers-Moll & Gummel-Poon are right about this for the deniers.


Regards, Dana.
 
Last edited:
Arouse1973

Arouse1973

Adam
Can I ask everyone a question please? When a transistor is in saturation do you think if you increase the base emitter voltage would you expect an increase in collector current? Thanks Adam
 
CircutScoper

CircutScoper

Arouse1973 said:
Can I ask everyone a question please? When a transistor is in saturation do you think if you increase the base emitter voltage would you expect an increase in collector current? Thanks Adam
Click to expand...

No. In fact, that might be the best practical definition of saturation -- when AC beta (d_Ic/d_Ib) drops to zero.
 
D

danadak

CircutScoper said:
No. In fact, that might be the best practical definition of saturation -- when AC beta (d_Ic/d_Ib) drops to zero.
Click to expand...

I always thought of this as an DC effect, but it does show up in switching characterizes
when a base is saturated with current and effects incurred due to storage time.

Wiki def of saturation -

Saturation
With both junctions forward biased, a BJT is in saturation mode and facilitates high current conduction from the emitter to the collector (or the other direction in the case of NPN, with negatively charged carriers flowing from emitter to collector). This mode corresponds to a logical "on", or a closed switch.
Click to expand...

https://en.wikipedia.org/wiki/Bipolar_junction_transistor


Regards, Dana.
 
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