# constant current sources

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#### walters

How does a Constant current source works?

Does the Transitor have to Re-Bias itself it self adjusts with different LOADS?

How does it self adjust because of the different emitter voltage drops it puts out the same current how?

#### yjkwon57

##### Full Member level 4
Hi.

If you have two transistors, which is assumed to be the same. If same bias is applied to the two transistors, then, their electrical responses will be the same. The current source is working based on this principle. By the way, many people have worked for improving, i.e., increasing, the output resistances of the current sources.

#### walters

But how does it keep the same current at different loads?

#### VVV

This is not an absolutely accurate answer, but think of the output characteristics of the transistor. The collector current is almost constant regardless of the output voltage, if the base current is constant.
I mean, when you say Ic=β*Ib, there is nothing in this equation about the voltage.

Now β changes with collector current and so other circuitry is generally used to keep the current more constant. Generally that involves measuring the voltage across a resistor in series with the output current and driving the transistor accordingly. The resistor is most often in series with the transistor's emitter.
As an example, a very simple and unpretentious current source consists of a transistor with a resistor in series with its emotter and having the base held at a constant voltage, Vb.
Then the current is nearly constant, equal to: Ic≈Ie=(Vb-Vbe)/Re.
As you can see, for Vb»Vbe, the current varies very little with changes in Vbe.

#### walters

Thanks for the information VVV

its emotter and having the base held at a constant voltage, Vb.
Then the current is nearly constant

The base is held at a constant voltage from the bias voltage

So the collect current is going to be constant also the transistor and biasing self adjust to keep it constant but how does it do this?

Its like a comparator ?

How does it self adjust to keep a constant current?

#### VVV

Well, the current is constant, because the base current is constant and so Ic is constant.

Think of the circuit I described. The base is held at a constant voltage. The emitter has a resistance to ground. Then the voltage at the emitter is Vb-Vbe. But this voltage occurs across the emitter resistor, Re. So the emitter current is Ie=(Vb-Vbe)/Re and it is almost constant, since Vb is constant, Vbe«Vb, so small changes in Vbe make little difference. Since Ie=(β+1)*Ib, then Ib is almost constant, if β is constant. With Ib constant, β constant, then Ic=Ib*β is also constant.

Think about it another way: let's assume the collector current decreases. Then the voltage across the emitter resistor decreases. Since Vb=Vbe+Re*Ie, if Re*Ie decreases because Ie decreases, then Vbe tends to increase. But an increase in Vbe will cause the collector current to increase back to where it was. Similarly, if the collector current were to increase, the Vbe would decrease, which would in turn decrease the collector current back to the initial value.
This is, if you will, the way the transistor "adjusts" itself in such a circuit.

#### walters

Thanks VVV for the information

the current is constant, because the base current is constant and so Ic is constant.

Yes the base current is constant but the LOAD on the Ic or collector is Variable
it changes the resistance but the Ic is constant even with different resistance loads
it will have the same current flow with different resistance loads on the collect because the base current is constant how does it do this its like magic how the transitor can still have different resistance loads on the collector have put out the same current