Transistor darlington pair question

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Discussion Overview

The discussion revolves around the operation of a Darlington pair transistor configuration, particularly focusing on how it functions in terms of current flow, saturation mode, and the connection of the collector to a load (lamp) instead of Vcc. Participants explore theoretical aspects, practical implications, and specific questions regarding the circuit behavior.

Discussion Character

  • Technical explanation
  • Conceptual clarification
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant expresses confusion about the operation of the Darlington pair, questioning how TR1's base-emitter conduction affects TR2 and the brightness of the lamp.
  • Another participant explains that the Darlington pair operates in saturation mode, emphasizing that the gain is a fixed value (hfe^2) and that TR1 must be driven high to turn on TR2 and the lamp.
  • A participant questions the design choice of connecting TR1's collector to the lamp instead of Vcc, suggesting that this limits TR2's collector potential and affects saturation.
  • There is a repeated emphasis on the importance of saturation for the Darlington pair to function correctly, with a focus on current flow through the lamp as the load.
  • Concerns are raised about the power dissipation in TR1 when connected to a higher voltage, indicating that it could lead to unnecessary heat generation.
  • Another participant reiterates the power dissipation concern, agreeing with the previous point.
  • A participant questions the operation of a transistor switch, particularly how collector current can flow without a potential at the collector terminal, seeking clarification on the need for base current to drive TR1 into saturation.
  • One participant suggests a resource for further understanding of transistor operation, indicating a potential gap in knowledge among participants.

Areas of Agreement / Disagreement

Participants express various viewpoints regarding the operation of the Darlington pair, with some agreeing on the importance of saturation and the implications of collector connections, while others raise questions and uncertainties about specific operational mechanics. The discussion remains unresolved with multiple competing views presented.

Contextual Notes

Participants highlight limitations in understanding the transistor operation, particularly regarding the conditions for saturation and the implications of collector connections. There are also unresolved questions about the necessary conditions for current flow in the circuit.

likephysics
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A bit confused about how it works.
Link - http://www.technologystudent.com/elec1/transis2.htm (3rd fig from top)

The way I understand, transistor TR1's Base-emitter conducts, which fwd biases B-E junction of TR2. This in turn will turn on the lamp, but it won't be really bright.
Once its on, the collector of TR1 is at +Vcc minus the drop across the lamp. This will increase emitter current of TR1, which in turn increases the base current going to TR2, which is amplified and makes the lamp glow even more.
Correct?

Also, why is TR1's collector connected to lamp instead of Vcc?
 
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please observe that the darlington pair is being operated in saturation mode/cutoff mode in quiescent point. The gain of the transistor pair is a fixed value : hfe^2.
when base of TR1 is driven high VBE is > 0.7V hence TR1 enters sat mode, gushing out emitter current. which inturn drives TR2.
The lamp is the load here, it is turned on when current flows through it.
this is possible only if the Darllington pair in saturation to provide a path to ground.
Hope this explanation helps.
 
likephysics said:
Also, why is TR1's collector connected to lamp instead of Vcc?

I'm puzzled by this too. If TR1's collector were connected to Vcc, then TR2's collector could be brought to about a diode's drop away from ground. As it is, TR1's collector can't go closer than about two diode's drops from ground without taking the transistors out of saturation.
 
Chandra214 said:
please observe that the darlington pair is being operated in saturation mode/cutoff mode in quiescent point. The gain of the transistor pair is a fixed value : hfe^2.
when base of TR1 is driven high VBE is > 0.7V hence TR1 enters sat mode, gushing out emitter current. which inturn drives TR2.
The lamp is the load here, it is turned on when current flows through it.
this is possible only if the Darllington pair in saturation to provide a path to ground.
Hope this explanation helps.

Initially when TR1 has no base current, its collector is open. Just base current is enough to drive it into saturation? No need for BC junction to be reverse biased?
Now, I can't figure out how a transistor switch works. How can collector current flow when there is no potential at Collector terminal.
 
Figure the wattage dissipated by TR1 when its collector is tied to +9V. It will unnecessarily be dissipating well over a watt.
 
Averagesupernova said:
Figure the wattage dissipated by TR1 when its collector is tied to +9V. It will unnecessarily be dissipating well over a watt.

Good point.
 
likephysics said:
Initially when TR1 has no base current, its collector is open. Just base current is enough to drive it into saturation? No need for BC junction to be reverse biased?
Now, I can't figure out how a transistor switch works. How can collector current flow when there is no potential at Collector terminal.

likephysics , I would suggest you to go through transistor operation @ tpub.com
I have found the explanations simple and accurate.
 

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