Basic "dynamization" idea

If we look around, we can see that most things in this world are changeable, adaptable, dynamic... According to this observation, we have to make our current source react somehow to the load "intervention", to resist to its attemption to lower the current. That means something inside the current source has to change, in order to compensate the disturbing voltage drop across the load. What can change in this simple electric circuit?

Varying (dynamic) internal voltage

Vladimir: We might change the voltage... Yes, it is the one possible solution. Vladimir suggests, if the voltage drop V_L across the load begins rising, the internal voltage V_VAR of the current source to begin rising as well in the same direction with the same rate so that the difference V_R between them to stay constant. This is the first idea: dynamic current source with following internal voltage. Till when will this magic continue? Of course, until V_VAR finally reaches the positive supply rail. After that point, our wonderful "voltage-dynamic" current source becomes an ordinary one.

Have you heard the legend about Baron Munchhausen that was using his own boot straps to pull himself out of the sea? Our circuit is just one possible implementation of Munchhausen's bootstrapping idea in electronics.

Is there a negative feedback here? No, it isn't. The varying voltage source follows blindly the load voltage; it does not "interested" in the magnitude of the current.

I am not sure if a coil can be treated as a way to "produce" dynamic voltages for keeping the current constant (at least temporarily). Or is there practical way of doing so in real circuits?

Thanks,

/bruin

Yes, an inductor behaves exactly as a dynamic voltage source that opposes the changes of the input (exciting) voltage source so that to keep the current constant. Both the time-dependent elements (inductors and capacitors) behave as opposing voltage sources; only, inductors oppose the input voltage in the beginning while capacitors - in the end of the transitional process. Circuit-fantasist (talk) 17:36, 4 June 2009 (UTC)

A transistor acting as a current-stable resistor

What electronic component has such a current-stable behavior? A student: The operational amplifier... No, no, it has to be a simpler element. Remember in this lab cicle we consider diode and transistor circuits; so, it has to be a diode or a transistor. A hint: the one has a behavior of a voltage source (more precisely, of a voltage-stable element); the other has a behavior of a current source (current-stable element). Well, which of the two behaves as a current-stable element? Of course, the transistor...

Well, let's then see why the transistor has such a behavior. You have already studied a subject about semiconductor elements; so, you have to know how it behaves. Do you have any reminiscence of it? What can a transistor do? A student: ...to amplify... No, it might sound paradoxical but the transistor does not amplify; it attenuates!?! The only thing that a transistor can do is to change its present resistance thus dissipating a power and, as a result, regulating the current through or the voltage across the load.

Here is an n-p-n transistor connected in series with a load R_L and a power supply V_CC. It is a three-terminal element (a triode) while a diode is a two-terminal element. A triode is too complicated for our reasoning:) so, we break down it into two two-terminal parts: an input base-emitter part and an output collector-emitter part. Each of them has an IV characteristic; now we are interested in the output characteristic of the collector-emitter part. Why they name it "IV characteristic"? Because we can do two things with it: to apply a voltage across it and to measure the current through it or to pass a current through it and to measure the voltage drop across it. Also, they name this curve output characteristic because it represents the output transistor's part. How does it look? Let's present the circuit operation graphically.

For this purpose, we have to superimpose two characteristics - one of the source and one of the load. First, let's draw the IV characteristic of the voltage source acting here as a power supply. What is it? It is a vertical line that is shifted with the magnitude of the voltage source V_CC. Then, we have to show the presence of the load by inclining the line to the left according to the load resistance R_L. The resulting curve represents an IV characteristic of a real voltage source having a voltage V_CC and an internal resistance R_L (they name it "load line"). After, we have to plot the transistor's output characteristic on the same coordinate system. The crossing point is known as an operating point; it represents the present magnitudes of the voltage and the current.

When we vary the load, the load line rotates and the operating point moves horizontally along the transistor's output characteristic. But why? How does it do this magic? What does the transistor actually do to achieve such an effect? Can someone explain? Let's try.

When we lower the load resistance the load line stands up; the transistor does the opposite - it increases its present collector-emitter resistance R_CE (or just R_T) so that to keep up a constant total circuit resistance. We may show R_T by another line that begins from the coordinate system origin (the trick is to think of the present resistance R_T as an ohmic one). So, this line will rotate around the origin and incline as well. As a result, the crossing point moves along a new almost horizontal line - the output transistor characteristic. This line is imaginary, unreal, artificial but we can see only this line; it represents the behavior of a current-stable resistor.

The text below is written by a student from this group:

The BJT transistor as a three-legged creater (??? Circuit-fantasist (talk) 12:28, 27 May 2008 (UTC)) looks much like a light switch (2 legs for IN and OUT, and one for the ON/OFF button), and basicly that's what it is - a switch. But this one here is like those fancy light switches with a rotating knob or even the more fancy ones - with a metal "touch-plate", which can be used to set a different light intensity in a continuous range by rotating the knob, or keeping your finger on the metal plate for a short time. But unlike these mechanically controlled switches, the transistor is an electrically controled one - on this scheme the output collector-emitter(C-E) current is controlled by the input base current or the base voltage, whichever suits us more.

So here comes the simple idea - if we apply a constant base voltage, we will set a constant C-E current. When the C-E voltage increases (e.g. due to decrease in voltage on the load resistor R_l) the C-E resistance will increase too (remember we set the C-E current to constant using a constant base voltage). So what do we get - a dynamic varying resistor which keeps a constant current flow. In short - a current-stable resistor. Which is exactly what we need in order to make a constant current source.

This behaviour is shown on the IV diagrams below. They can be explained as a mechanism with the I line acting as a "fixed rail", on which the crossing of the two "pivots" - R_l and R_t lines - point A slides. As we decrease the load (R_l that is) point A slides down the R_l line, up the R_t line, and to the right on the I line. This means that the transistor resistance increases, so the whole resistance of the circuit remains the same, and therefore the current flow remains the same. As we do the opposite - guess what - exactly the opposite things happen ;)

--V.konushliev (talk) 08:42, 19 May 2008 (UTC)

Building a simplest transistor current source on the whiteboard

Please, read carefully this story. IMO, this is the fanciest and the most incredible story about a transistor circuit that I have ever seen on the web! What do you think about this assertion? Circuit-fantasist (talk) 05:55, 20 May 2008 (UTC)

Looks scary at first sight :) But none of the elements involved bites, and everyone has it's role.

Maybe, it looks scary because we have placed here the final, complete and perfect circuit solution? Maybe, we would build it step-by-step snapping every drawing on the whiteboard to show the circuit evolution? Maybe... Circuit-fantasist (talk) 05:37, 20 May 2008 (UTC)

In the middle, surrounded by a blue line is the "boss" of the circuit - the varying load resistor R_l. It's he who leads the concert, and the whole curcuit is built for him - to satisfy his needs and desires for constant current. The nameless resistor just beneath the boss is his "bodyguard" - just in case someone curious moves the slider of the "boss resistor" to its very end and try to short-curcuit the power source through the transistor.

Everything outside the blue line is the BJT current source itself - a whole team of elements working for the "boss". There's the "hero" of the curciut - the workhorse - the mighty bipolar junction transistor (BJT), who does all the dirty job (donkey work:) of keeping the constant current. Connected to its base is his "manager" (modern heros need to have managers), the varying resistor (potentiometer) P, who is basicly a varying voltage divider, and by the means of base voltage V_b tells the transistor what to do. In this case - using a constant base voltage the manager tells him to keep the current flow at a constant rate. The "manager" also has a "bodyguard" - R_b keeping the power source safe from short-curcuit's caused by someone playing with the potentiometer P.

Then there are our watchfull eyes - the voltmeter V₂, used to keep an eye on the voltage drop on the load resistor (the boss), another voltmeter V₁, watching over the voltage drop on the transistor's collector-emitter junction, and an ammeter, which gives us a sneak-peak on the current flow.

Last, but not least - the power source V_CC, who brings the whole curcuit to life :) -- V.konushliev (talk) 13:22, 19 May 2008 (UTC)

...by varying the load resistance RL...

... we observe a change in V1 and V2, but no change in the ammeter. We're happy: the current flow is constant :) How did that happen?

When we increase the load resistance R_l so does its voltage drop, which can be seen on voltmeter V2 (logically simple - more resistance generates more pressure). As V2 shows an increment, V1 respectively shows a decrement (their sum represents the voltage of the power sourse Vcc). Since we made the transistor act as a current-stable resistor, it "senses" the decrement of the C-E voltage drop and obediently reduces its resistance, so the sum of resistances in the circuit branch remains the same. Therefore the current flow remains the same too.

When we decrease the load resistance, exactly the opposite happens (V2 shows a decrement, V1 shows an increment, the transistor rises its resistance, the sum of resistance remains the same, and so does the current flow).

...by varying the supply voltage VCC

...we observe the same pleasing facts: no change in the ammeter, although V1 and V2 show changes. We're happy again: the current flow is still constant :)

This time both the voltmeters V1 and V2 showed increments of voltage drops, with the increment of the supply voltage Vcc. The transistor again "senses" the change in voltage drop and rises its resistance. But if the transistor reacts the same way as in the case above, how does the current remain the same, if there is no change in R_l? The difference comes with the fact, that the "manager" of the transistor tells him to rise it's resistance even more, by applying a larger base-emitter voltage. Of course this is due to the fact, that the "manager", as a simple voltage divider, divides a larger voltage (Vcc), and therefore gives a larger result (Vbe). At the end, the sum resistance of the curcuit branch rises enough to compensate the increment in supply voltage Vcc and keep the current flow the same.

And again, with the opposite input (decrement of Vcc) the opposite phenomena is observed, but the result stays the same - constant current flow. A job well done by the BJT and it's "team" :) --V.konushliev (talk) 15:11, 19 May 2008 (UTC)

Vladimir, now I am realizing that I have misled you to some extent in this experiment. The circuit is not as correct as it seems. It would be more consistent, if we have made all the possible to make the input "manager" keep a constant voltage. Here we rely on the constant power supply; so, we shouldn't change its voltage. We should supply the potentiometer by another voltage source or replace the input voltage-divider circuit by a voltage-stabilizing one (e.g., by connecting a diode into the lower leg). This would be a prelude to the famous current mirror circuit (see group 66b and group 67b). Please, edit your explanation according to these circumstances.

Vladimir, I have a suggestion to. Would you contribute the Circuit idea story about the simplest transistor constant current source (and not only it)? Your writing style is unique; so, you might tell your lively story in a separate page as a different viewpoint at this famous circuit. Circuit-fantasist (talk) 06:46, 20 May 2008 (UTC)