Understanding BJT Operating Regions: Saturation and Cutoff | Lab Homework

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SUMMARY

The discussion focuses on the operation of a Bipolar Junction Transistor (BJT) in saturation and cutoff regions during a lab experiment involving switching. The user successfully configured the circuit to operate in saturation with appropriate values for RB and RC. When the input is toggled to low, the transistor enters the cutoff region, resulting in zero emitter current (IE = 0A). This switching behavior between saturation and cutoff explains the observed 180-degree phase shift in the output waveform on the oscilloscope.

PREREQUISITES
  • Understanding of BJT operation principles
  • Knowledge of DC analysis in transistor circuits
  • Familiarity with oscilloscope usage and waveform interpretation
  • Basic circuit design involving resistors RB and RC
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  • Investigate the effects of AC signals on transistor behavior
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Electronics students, hobbyists working with BJTs, and engineers involved in circuit design and analysis will benefit from this discussion.

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Homework Statement


Hey all, I'm midway through a lab on 'BJT for switching', we are suppose to choose values for RB and RC between a certain range so that it operates in saturation region. I have this circuit:
https://dl.dropboxusercontent.com/u/53341615/simulated%20circuit%20step%202.png

DC analysis shows that it is operating in the saturation region (meets the criteria, all good :) ).

When I toggle the switch to the AC signal and view the waveform on the oscilloscope, the transistor is in the saturation region when input is high 5v, however I am unsure what region it is in when the input is low.

Oscilloscope: https://dl.dropboxusercontent.com/u/53341615/Waveform%20view%20step%203.png

Is this now operating in the cutoff region, IE = 0A . I've done DC analyses at 0V and it does show IE to be 0A.

So effectively its switching between saturation and cutoff, every cycle of the AC, which would explain the 180 degree phase shift on the oscilloscope.

Am I understanding this correct ? Any feedback would be great!

Thanks for your time.
 
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It sounds like you have it right. :smile:

When there is no base current, the transistor is in cut-off, and this means there is no voltage dropped across Rc.
 

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