How a current buffer transistor cancels the early effect (Cascode)

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Discussion Overview

The discussion centers on the operation of a current buffer transistor (Q2) in a circuit designed to mitigate the early effect in another transistor (Q1). Participants explore how Q2 maintains a constant voltage to Q1, thereby preventing variations in collector current due to load changes. The conversation includes technical details about voltage levels and current calculations.

Discussion Character

  • Technical explanation
  • Mathematical reasoning
  • Debate/contested

Main Points Raised

  • Some participants propose that Q2 keeps the voltage at collector of Q1 (Vc_Q1) constant at 1.75V, which prevents the early effect from occurring.
  • Others argue that without Q2, load variations would cause Vce_Q1 to change, leading to variations in collector current due to changes in transistor β.
  • It is noted that with different load resistances (RL), the voltage levels (Vc and Vce) change significantly, affecting the collector current.
  • A participant suggests that to maintain a constant Vce for Q2, an additional transistor may be required to isolate Q2 from the load.
  • Mathematical expressions for load current (I_load) are provided, showing how it is calculated based on the collector current of Q1 and the β values of Q2.

Areas of Agreement / Disagreement

Participants generally agree on the role of Q2 in stabilizing the voltage for Q1, but there is a suggestion about the need for an additional transistor to further isolate Q2, indicating that the discussion remains unresolved regarding the best approach to achieve constant Vce for Q2.

Contextual Notes

There are assumptions regarding the behavior of transistors under varying load conditions and the impact of β on collector current that are not fully explored. The discussion does not resolve the implications of adding another transistor for isolating Q2.

brainbaby
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In the given circuit Q2 is a current buffer tranistor which maintains constant current voltage for Q1 just to cancel the harms of early effect...how does it do so??
 

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Simply Q2 keeps Vc_Q1 fairly constant at 1.75V. And since Vce_Q1 is not changing, no early effect (base width modulation) occurs.
Without Q2 load variations causes Vce_Q1 to change with load. For example for RL=1kΩ we have Vc = 9V and Vce = 8V; But for RL = 5kΩ we have Vc = 5V and Vce = 4V.
And this change in Vce will also change collector current due to transistor β changes (early effect).
So by adding Q1 we make Vce_Q1 fairly constant. And therefore Ic_Q1 ≈ I_load is also constant. Vce_Q2 will vary with the load but this variations will have almost no effect on load current. Because Q2 work here as a current buffer (common base), and this is why changes in Vce_Q2 due to load variations will have no effect on load current.
I_load = IcQ1* β_Q2/(β_Q2 + 1)
I_load = 1mA * 50/51 = 0.9803mA
I_koad = 1mA * 100/101 = 0.9900mA
 
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Jony130 said:
Simply Q2 keeps Vc_Q1 fairly constant at 1.75V. And since Vce_Q1 is not changing, no early effect (base width modulation) occurs.
Without Q2 load variations causes Vce_Q1 to change with load. For example for RL=1kΩ we have Vc = 9V and Vce = 8V; But for RL = 5kΩ we have Vc = 5V and Vce = 4V.
And this change in Vce will also change collector current due to transistor β changes (early effect).
So by adding Q1 we make Vce_Q1 fairly constant. And therefore Ic_Q1 ≈ I_load is also constant. Vce_Q2 will vary with the load but this variations will have almost no effect on load current. Because Q2 work here as a current buffer (common base), and this is why changes in Vce_Q2 due to load variations will have no effect on load current.
I_load = IcQ1* β_Q2/(β_Q2 + 1)
I_load = 1mA * 50/51 = 0.9803mA
I_koad = 1mA * 100/101 = 0.9900mA
If i for any reason want to make Vce_2 constant then do i have to add another transistor which would isolate Q2 from load and acting as a buffer for Q2..?
 
Yes, you need to add another transistor on top off Q2.
 

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