# Effect of load voltage variation in this circuit

1. Apr 23, 2015

### brainbaby

If load voltage vary..will Vce_Q1 changes in opposite direction in order to compensate change in Vce_Q2 in order to eliminate early effect in Q2...??

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• ###### Vbe reference current source.png
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2. Apr 23, 2015

### Staff: Mentor

Hey, that circuit is from Horowitz and Hill

The collector current of Q2 is mostly set by the current through R2, which is set by its resistance and the voltage across it (which is being held at about a diode drop by the E-B junction of Q1. As long as the load voltage doesn't get big enough to take Q2 out of the active region, the load current should be pretty independent of the load voltage.

3. Apr 23, 2015

### Jony130

Forget about early effect in this circuit. this circuit has a negative feedback mechanism which reduce the early effect.
As load current tries to increase his value, the voltage drop across R2 and VbeQ1 also increases. This turns-ON Q1 harder (Ic1 current increase ), larger Ic1 current increase voltage drop across R1 resistor this reduces the Vb2 voltage and Vbe2 in the same time, thereby reducing the collector current of Q2 and load current.
As you can see negative feedback restore the previous value of a load current.
The same things happens when load current tries to decreases. In this case Vbe1 is dropping which reduces Ic1 collector current, this make less voltage drop across R1.
So Vbe2 must increase his value and in result bring back the previous value of a load current.

4. Apr 23, 2015

### brainbaby

thanks buddy i got it.......great job