BJT Transistor operation on simple transistor-zener voltage regulator circuit

[Willen]

I thought that NPN (Negative+Positive+Negative) passes Negative supply (like N-Channel FET) and PNP (Positive+Negative+Positive) passes Positive supply (like P-channel FET) when Base of it is powered. But seeing these two voltage regulator which are opposite than my understanding.

In the circuit NPN is passing positive supply and PNP is passing negative supply.

My questions are-
- how NPN passes positive and PNP passes negative supply?
- how transistors pass regulated voltage through CE if base of it is regulated? (I know emitter have 0.6V less than base voltage though).

Please tell me the operations (maybe path of current/voltage flow and working principle) of these regulators in simple words (circuit analysis). Simple words because I am just a hobbyist. Thank you!

 

[Mosaic]

The transistors are configured as emitter followers, effectively creating a low impedance zener sometimes called an amplified zener. The zener holds the base voltage steady and the emitter follower then delivers the same voltage shifted by 0.6Vbe at its emitter.

 

[Diver300]

In a BJT, when a small current flow from the base to the emitter, that allows a large current to flow from the collector to the emitter.

In the common-emitter circuits you are probably more familiar with, the base current flows to ground, and the collector current the useful output of the circuit.

In the emitter follower circuit shown, the emitter current is the useful output of the circuit. The zener hold the emitter at a constant voltage. The base-emitter drop of about 0.6 V means the emitter will be around 0.6 V lower than the zener voltage. When current is taken by the load, some comes from the base, but this base current is amplified by the transistor, so nearly all of the load current comes from the collector.

This has the advantage over a zener regulator that the resistor and zener can take far less current than the output current, so with no load the regulator takes little current.

https://en.wikipedia.org/wiki/Common_collector may help but it is a bit mathematical.

 

[crutschow]

Don't think about polarity, think about current direction.
Just remember that the normal current direction in a BJT goes in the direction of the emitter arrow.
Thus it flows from base to emitter (also collector to emitter) in an NPN, and flows from emitter to base (also emitter to collector) in a PNP.
Similarly the current direction is from source to drain in an N-MOSFET, and from drain to source in a P-MOSFET.
(The arrow direction on a MOSFET can be a little confusing since it shows the substrate diode direction, which is normally reverse biased. Thus the drain-source current direction in opposite to the arrow direction).
If you follow those rules then you will always be able to determine the relative voltage polarities across the transistor connections when operating normally.

 

[Ratchit]

I thought that NPN (Negative+Positive+Negative) passes Negative supply (like N-Channel FET) and PNP (Positive+Negative+Positive) passes Positive supply (like P-channel FET) when Base of it is powered. But seeing these two voltage regulator which are opposite than my understanding.

Your thinking about what constitutes a NPN or PNP transistor is misguided. N and P refer to the doping added to the pure silicon during manufacturing, not the voltage applied to the leads when used in an application. Don't try to infer how a circuit operates by comparing a BJT to a FET. A BJT is a bipolar (two charge carriers) device that works by diffusion, whereas a FET is a unipolar device that works by constricting the current. Both are voltage controlled devices.

In the circuit NPN is passing positive supply and PNP is passing negative supply.

What does passing a certain polarity voltage mean and what difference does it make?

My questions are-
- how NPN passes positive and PNP passes negative supply?
- how transistors pass regulated voltage through CE if base of it is regulated? (I know emitter have 0.6V less than base voltage though).

The first question does not make sense.
For the second question, the transistor controls its CE current according to the voltage applied to its base.

Please tell me the operations (maybe path of current/voltage flow and working principle) of these regulators in simple words (circuit analysis). Simple words because I am just a hobbyist. Thank you!

OK, as stated before, a transistor operates by changing its CE or EC current according to the voltage applied to its base, provided the transistor is properly biased. The current existing in the base is waste current that is lost from the CE path. It is proportional to the CE current (beta), but it does not control the CE current. As others have told you, the circuit is an emitter follower, i.e., the emitter voltage follows the base voltage, and differs by an offset voltage of around 0.6 volts. The zener diode keeps the base voltage constant, and thereby the load attached to the emitter will also be at a constant voltage. The transistor will automatically adjust its CE current to assure the emitter load is at a constant voltage. That's about all you have to know. Ask if you have any specific questions.

Ratch

 

[Ratchit]

In a BJT, when a small current flow from the base to the emitter, that allows a large current to flow from the collector to the emitter.

Wrong, wrong and wrong again. The emitter-base current does not control the emitter-collector current. The base current is waste current that happens to be proportional to the current entering/leaving the collector terminal (beta). A BJT is a voltage controlled current source device, also known as a transconductance amplifier. Please ask about this or for clarification.

Ratch

 

[Mosaic]

I found this to be a practical demo of how to use bipolar transistors.

 

[crutschow]

Wrong, wrong and wrong again. The emitter-base current does not control the emitter-collector current. The base current is waste current that happens to be proportional to the current entering/leaving the collector terminal (beta). A BJT is a voltage controlled current source device, also known as a transconductance amplifier. Please ask about this or for clarification.

It may be "wrong, wrong wrong and wrong again" to you but that's not the whole story to most practicing engineers.

I know you enjoy educating the unwashed masses as to the physics of BJT operation but that can be overkill for the unwashed.
Engineers and hobbyists are mainly interested in the black box characteristics of a transistor that are useful in circuit design using the device. Thus a BJT appears to be a current operated device with the base-emitter input looking like a forward biased diode (even if that's not true on a quantum level) when using them in large signal applications such as switches, DC bias design for AC amps, and power applications, and that's the easiest way to do a large signal design with them. That's why the current gain (Beta or hfe) is given in the data sheets of all BJTs (gasp) and that's why current gain is used in the design of large signal designs.
Using gm for the design of large-signal BJT circuits is difficult and awkward (how would you even use it to determine the base drive needed to properly saturate a BJT switch?) and I doubt that any practicing designers use if for those types of designs.

The transconductance characteristics of a BJT are mainly of interest to a designer in small signal design, such as low level AC amplifiers (audio to RF) and are properly used in the design of those. But those designs are a small subset of electronic circuit design with BJTs.

So you are certainly more than welcome to continue on your pedantic, Don Quixote quest to convert us to the truth of BJT operation but, in the meantime, the rest of us will continue to view BJTs as current-operated devices in large-signal applications.

Cheers.

 

[Diver300]

It may be "wrong, wrong wrong and wrong again" to you but that's not the whole story to most practicing engineers.
I know you enjoy educating the unwashed masses as to the physics of BJT operation but that can be overkill for the unwashed.

Thank you for that. Your post arrived while I was writing something to defend my simple transistor model.

It is interesting to note that the current gain of a transistor is quoted more often than other gains, and that the Wikipedia page that mentions the more complicated models has a warning that it is too technical for many readers.

 

[Mosaic]

This convo is beginning to look like EE6002 @ MIT.
Edit; Where is @KISS when u need him

 

[Ratchit]

It may be "wrong, wrong wrong and wrong again" to you but that's not the whole story to most practicing engineers.

I know you enjoy educating the unwashed masses as to the physics of BJT operation but that can be overkill for the unwashed.
Engineers and hobbyists are mainly interested in the black box characteristics of a transistor that are useful in circuit design using the device. Thus a BJT appears to be a current operated device with the base-emitter input looking like a forward biased diode (even if that's not true on a quantum level) when using them in large signal applications such as switches, DC bias design for AC amps, and power applications, and that's the easiest way to do a large signal design with them. That's why the current gain (Beta or hfe) is given in the data sheets of all BJTs (gasp) and that's why current gain is used in the design of large signal designs.
Using gm for the design of large-signal BJT circuits is difficult and awkward (how would you even use it to determine the base drive needed to properly saturate a BJT switch?) and I doubt that any practicing designers use if for those types of designs.

The transconductance characteristics of a BJT are mainly of interest to a designer in small signal design, such as low level AC amplifiers (audio to RF) and are properly used in the design of those. But those designs are a small subset of electronic circuit design with BJTs.

So you are certainly more than welcome to continue on your pedantic, Don Quixote quest to convert us to the truth of BJT operation but, in the meantime, the rest of us will continue to view BJTs as current-operated devices in large-signal applications.

Cheers.

Well, every time I see a transistor wired up and demonstrated as a current amplifier, it is in a circuit, usually with lots of resistance in the base and emitter. So the black box you refer to is really a current amplifier circuit. Show me a "naked' transistor that acts like a current amplifier. I observe a dichotomy here. For instance, everyone agrees that a ordinary operational amplifier is a dual high gain opposite polarity voltage amplifier with a combined output. From that one device building block, one can make just about any electrical function including a current amplifier. Yet, I don't see folks insisting that it is better to think of an op amp by itself as a current amp, differentiator, integrator, etc. So, why does a true transconductance device like a BJT get referred as a current amplifier when a op amp does not? So when I say that a BJT is not a current amplifier, I mean just that. I mean the device itself without being in a circuit.

Ratch

 

[Ratchit]

Thank you for that. Your post arrived while I was writing something to defend my simple transistor model.

It is interesting to note that the current gain of a transistor is quoted more often than other gains, and that the Wikipedia page that mentions the more complicated models has a warning that it is too technical for many readers.

A transistor by itself responds to its Vbe, not the base current. The physics of the BJT transistor proves that. It is true that the wasted base current has an exponential relationship to the collector useful as an indicator, but this base current does not control the collector current.

Ratch

 

[Tony Stewart]

NPN and N-Ch Enhanced are used as low side switches. PNP and P-ch enhanced are used as high side switches. Switches are always inverting from Base to collector or Gate to Drain

Emitter Followers are non inverting Current buffers.

Switches are characterized by their saturated resistance Rce or Rdson

 

[Ratchit]

NPN and N-Ch Enhanced are used as low side switches. PNP and P-ch enhanced are used as high side switches. Switches are always inverting from Base to collector or Gate to Drain

Emitter Followers are non inverting Current buffers.

Switches are characterized by their saturated resistance Rce or Rdson

Interesting, but what do those nonlinear applications have to do with whether a BJT is current or voltage responding?

Ratch

 

[crutschow]

Well, every time I see a transistor wired up and demonstrated as a current amplifier, it is in a circuit, usually with lots of resistance in the base and emitter. So the black box you refer to is really a current amplifier circuit. Show me a "naked' transistor that acts like a current amplifier. I observe a dichotomy here. For instance, everyone agrees that a ordinary operational amplifier is a dual high gain opposite polarity voltage amplifier with a combined output. From that one device building block, one can make just about any electrical function including a current amplifier. Yet, I don't see folks insisting that it is better to think of an op amp by itself as a current amp, differentiator, integrator, etc. So, why does a true transconductance device like a BJT get referred as a current amplifier when a op amp does not? So when I say that a BJT is not a current amplifier, I mean just that. I mean the device itself without being in a circuit.

Okay. If the only BJT you've seen as a current amplifier is with "lots of resistance in the base and emitter" then you've led a sheltered life.

You put a current from a current-source into the base-emitter junction of a "naked" BJT and a voltage from a voltage source between the collector and emitter (no resistors in sight).
The collector-emitter current is then proportional (as related by the Beta) to the base-emitter current.
This device has a relatively low input impedance and a high output impedance.
How is this black box device then not a current amplifier as per the common definition of such?
(And it's not in a "circuit").

A naked op amp is a voltage amplifier, no argument there.

The only "dichotomy" observed here is one that you may see but I doubt many others do.

Your tilt.

 

[Tony Stewart]

Interesting, but what do those nonlinear applications have to do with whether a BJT is current or voltage responding?

Ratch

Fundamentals of the switch are what was asked "Please tell me the operations (maybe path of current/voltage flow and working principle)"
It has everything to do with any application be the load linear or not. The same principles apply even for RF CLass J.

It is fallacious to say BJT responds to voltage drive or current, since VI characteristics are intrinsic. You can determine or drive either way from a voltage source or current source or anything in between. THe best rule of thumb is BJT switches when saturated at rated Vce(sat) is they are current amplifers with a gain much less than hFE. Every datasheet will specify in the standard Ic:Ib = 10, 20 or 50 depending on the junction quality for saturation. Due to special doping processes)

 

[Ratchit]

Okay. If the only BJT you've seen as a current amplifier is with "lots of resistance in the base and emitter" then you've led a sheltered life.

You put a current from a current-source into the base-emitter junction of a "naked" BJT and a voltage from a voltage source between the collector and emitter (no resistors in sight).

Oh yes there is. The current source is a voltage source with a relatively large resistor. Therefore, anytime you hook up a current source to the base of a BJT you are making a transistor circuit by adding the external resistor of the current source.

The collector-emitter current is then proportional (as related by the Beta) to the base-emitter current.
This device has a relatively low input impedance and a high output impedance.
How is this black box device then not a current amplifier as per the common definition of such?
(And it's not a "circuit").

Yes it is a circuit due to the added resistance in the base from the current source.

A naked op amp is a voltage amplifier, no argument there.

The only "dichotomy" observed here here is one that you may see but I doubt many others do.

So, where did I go wrong in my reasoning?

Your tilt.

Your turn.

Ratch

 

[Tony Stewart]

Oh yes there is. The current source is a voltage source with a relatively large resistor. Therefore, anytime you hook up a current source to the base of a BJT you are making a transistor circuit by adding the external resistor of the current source.



Yes it is a circuit due to the added resistance in the base from the current source.



So, where did I go wrong in my reasoning?



Your turn.

Ratch

YOur reasoning lacks the intrinsic VI characteristics are reversible. One is obtain from the other, so either may be applied to their respective models using gm or hfe. But most people use the current amplifier model.

 

[Ratchit]

Fundamentals of the switch are what was asked "Please tell me the operations (maybe path of current/voltage flow and working principle)"
It has everything to do with any application be the load linear or not. The same principles apply even for RF CLass J.)

No, the OP asked about the operation of a voltage regulator, not a switch or RF operation. Read his question again. He does not mention the word "switch" in his post.

It is fallacious to say BJT responds to voltage drive or current, since VI characteristics are intrinsic.

No so. The physics of a transistor in the active region show it to be voltage responding. If you drive it with a current, then you are using a transistor circuit as explained to my answer to Crutschow.

You can determine or drive either way from a voltage source or current source or anything in between.

So you can, but then don't call a transistor a current amplifier when operating in the active region with a current source. Then it is a current amplifier circuit.

THe best rule of thumb is BJT switches when saturated at rated Vce(sat) is they are current amplifers with a gain much less than hFE. Every datasheet will specify in the standard Ic:Ib = 10, 20 or 50 depending on the junction quality for saturation. Due to special doping processes)

Fine, but the OP was not asking about switches

Ratch

 

[Ratchit]

YOur reasoning lacks the intrinsic VI characteristics are reversible. One is obtain from the other, so either may be applied to their respective models using gm or hfe. But most people use the current amplifier model.

Empirical models show what a device does, but not how a device works, unless the model is based on the actual physical parameters of the device. I don't understand what you mean by reversible. For instance, if I apply 0.59 volts to the base-emitter and read 10 ma collector current, will I get 0.59 volts on the base-emitter if I somehow force 10 ma through the CE?

Ratch