How does the voltage reference work here?

[hayowazzup]

Hi,
I don't quite understand how the circuits below work, hope anyone there could help me out.

For the current source diagram anyone know how the diodes and R1 work as a voltage reference? As I don't understand whats the purpose of havnig R1 and thought that the current source cannot provide a constant current since if the Vcc varies Base Voltage will also be varied.

for me the current sink circuit looks easier to understand as the Base Voltage here will remain constant (2*Vd) as Vcc changes.

 

[rho-bot]

circuit for CurrentSource and CurrentSink work in the same manner:
R1 limits the current through the diodes. The voltage drop above the 2 diodes is considered to be (almost) independent of the current through them, therefore the two diodes act as a constant voltage source. R2 acts as a current feedback in this circuit. Transistor works in analog working point.
Therefore the current through the LEDs will be ((2*Vd)-Ube)/R2 ==> (1V4-0V7)/30R=23,3mA

If the LED current increases, the voltage drop at R2 will also increase and (as the voltage at the two diodes is constant) it will reduce Ube, which reduces the Collector current.

 

[Syncopator]

The right-hand circuit works similarly.

The voltage between the base and earth is set at 12 by the zener. The emitter voltage is 0.7 less than this, at 11.3.

The current through each of the two l.e.d. branches is 11.3 minus 3 x Vf of the l.e.ds, divided by 30.

e.g., if Vf of each l.e.d. is 3.4, total Vf is 10.2. Current through the l.e.ds is therefore 36.7mA.

 

[hayowazzup]

Hi, thanks for the replies.

Questions:
Does R1 draw the current flowing from collector to base terminal? so for the most suitable current for the diode, should I look for the MAX value of If(continous forward current) in its datasheet?

I don't understand how the voltage drop across R2 increase as the LED current increases, since to increase the LED current, the resistance of R2 should be decreased, and voltage across R2 = Ic * R2

---------- Post added at 03:14 ---------- Previous post was at 03:13 ----------

The right-hand circuit works similarly.

The voltage between the base and earth is set at 12 by the zener. The emitter voltage is 0.7 less than this, at 11.3.

I tried simulating the current sink circuit on spice, the base voltage is the same as the voltage across the diodes, which is approx. 1.4 volts. , but in the current source circuit, base voltage isn't the same as the voltage across the diodes. So by voltage referencing, does it mean the Base emitter voltage is the voltage that should be regulated, not the base voltage?

 

[Syncopator]

Questions:
Does R1 draw the current flowing from collector to base terminal?

No.

I don't understand how the voltage drop across R2 increase as the LED current increases

I suspect that nobody else does either. That's not how it works.

The base current is so small compared with the collector current that the emitter and collector currents are assumed to be equal.

The collector current, in all three cases, is fixed by the value of the emitter resistor and the voltage across it.


In the source and the sink cicuits, the voltage across the emitter resistor is equal to two diode drops minus one diode drop (Vbe).

i.e. 2 x 0.7 - 0.7.

The emitter/collector current is then 0.7/30 = 23.3mA. That's it. Increase Vcc, remove a l.e.d.; collector current is still 23.3mA.




---------- Post added at 03:14 ---------- Previous post was at 03:13 ----------

I tried simulating the current sink circuit on spice, the base voltage is the same as the voltage across the diodes, which is approx. 1.4 volts. , but in the current source circuit, base voltage isn't the same as the voltage across the diodes.

What's important is the voltage across the emitter resistors, as explained above.

So by voltage referencing, does it mean the Base emitter voltage is the voltage that should be regulated, not the base voltage?

The base-emitter voltage is not regulated. It is a result of, and depends on, the base-emitter current and, although the figure can vary a little depending on the magnitude of the current, it is essentiually constant at any given current.



Do this.

Draw the currrent source circuit upside-down. Label what was Vcc as earth (that's what is drawn in the diagram), or "ground"or 0V or whatever else keeps Spice happy, and label what was earth as -V, or Vdd or whatever satisfies Spice if it's fussy.

Now Vb is the same as the voltage across the two diodes.

 

[hayowazzup]

So in reality, will I still get the base voltage equal to the voltage drop across the diodes if I put the positive Vcc at the top and ground at the bottom?

Questions:
Does R1 draw the current flowing from collector to base terminal?

Sorry, my mistake. I meant the current flowing from emitter to base in the current source.

 

[Syncopator]

So in reality, will I still get the base voltage equal to the voltage drop across the diodes if I put the positive Vcc at the top and ground at the bottom?.

In the case of the Current Source, as drawn, no.

It's a matter of what you use as a reference when making voltage measurements.

For instance, it is usual practice to connect one lead of the voltmeter to "earth/ground/0V/system common" or whatever it might be called on a particular diagram, for all voltage measurements, placing the other lead at whatever point you want to measure.

"Earth/ground/0V/system common" is the reference. It is the point to which all other voltage measurements relate, unless stated otherwise. e.g. Vbe is the voltage between base and emitter.

In all three circuits of your diagram, Vb, a base voltage, is measured between a base and earth. Ve, an emitter voltage is measured between an emitter and earth. Vcc is measured between the supply rail and earth.

You can use any point you wish as a reference, if you think it serves any useful purpose. In the Current Source circuit you can use Vcc as reference and measure everything with respect to it.
So, by putting one voltmeter lead on Vcc, the base voltage is equal to the voltage across the two 1N4454 diodes..

Measuring with respect to earth, the two voltages are not equal. But nothing has changed.

Using the same logic, In the Current sink circuit, using earth as the reference, the base voltage and the voltage across the two diodes are, obviously, equal.
However, using Vcc as a reference, the two voltages are not equal.

Sorry, my mistake. I meant the current flowing from emitter to base in the current source.

Yes, base bias current is from Vcc, emitter-base, R1, earth.

And there is the diode current from Vcc, the diodes, R1, earth.

 

[biff44]

www.doe.carleton.ca/~rmason/Teaching/283-b.pdf

 

[hayowazzup]

I really appreciate your help, thanks alot!

One thing I still couldn't get is why the Vcc should be taken as the reference in the current source whereas ground should be the reference in current sink. Is it related to how the current flow? or it is always better to take the reference from Vcc for PNP and ground for NPN?

 

[rho-bot]

I don't understand how the voltage drop across R2 increase as the LED current increases, since to increase the LED current, the resistance of R2 should be decreased, and voltage across R2 = Ic * R2

Maybe my explanation was too bad. What i wanted to figure out is that the current source is a closed loop regulation, where the emitter resistor acts as the current feedback to the transistor working in analog mode.
But maybe thats too much detail here.

One thing I still couldn't get is why the Vcc should be taken as the reference in the current source whereas ground should be the reference in current sink. Is it related to how the current flow? or it is always better to take the reference from Vcc for PNP and ground for NPN?

Both circuits work in the same manner: Transistor in Emitter circuit working in active area with current feedback.
The names "Source" and "Sink" are only defined by the direction of the current flow. Currents flowing out of a circuit are often named ISource and flowing into a circuit are named Isink.