- #1
Effect of load voltage variation in this circuit
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Discussion Overview
The discussion centers on the effects of load voltage variation in a specific circuit configuration, particularly focusing on the behavior of transistors Q1 and Q2 and the implications of negative feedback and the Early effect. The scope includes theoretical analysis and technical explanations related to circuit behavior.
Discussion Character
- Technical explanation
- Debate/contested
Main Points Raised
- Some participants propose that if the load voltage varies, Vce_Q1 may change in the opposite direction to compensate for changes in Vce_Q2 to mitigate the Early effect in Q2.
- Others argue that the collector current of Q2 is primarily determined by the current through R2, which is influenced by its resistance and the voltage across it, suggesting that as long as Q2 remains in the active region, the load current will be relatively independent of load voltage.
- Several participants emphasize that the circuit features a negative feedback mechanism that reduces the Early effect. They describe how increases in load current lead to changes in voltage drops across resistors, which in turn affect the operation of Q1 and Q2 to stabilize the load current.
- There is a reiteration of the negative feedback mechanism, detailing how it functions to restore load current when it attempts to increase or decrease.
Areas of Agreement / Disagreement
Participants express differing views on the significance of the Early effect and the role of negative feedback in the circuit. While some acknowledge the feedback mechanism's stabilizing role, others question the extent to which it mitigates the Early effect, indicating that the discussion remains unresolved.
Contextual Notes
The discussion does not resolve the assumptions regarding the active region of Q2 or the specific parameters affecting the feedback mechanism. There are also unresolved mathematical details related to the circuit's behavior under varying load conditions.
- #2
berkeman
Admin
- 69,531
- 25,110
brainbaby said:
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
Jony130
- 626
- 137
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.
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
brainbaby
- 232
- 5
thanks buddy i got it...great jobJony130 said:
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