Understanding BJT Operating Regions: Saturation and Cutoff | Lab Homework

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In summary: When the base voltage is high enough to produce a base current and the transistor is in saturation, the voltage drop across Rc increases. In summary, the conversation discusses choosing values for RB and RC to operate a BJT in the saturation region, and the observation that the transistor switches between saturation and cut-off when the input is high and low, respectively.
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TheTopGun
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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.
 

1. What are the different operating regions of a BJT?

The three operating regions of a BJT (Bipolar Junction Transistor) are the active, saturation, and cutoff regions. The active region is where the BJT is functioning as an amplifier, while the saturation region is where it is acting as a switch. The cutoff region is when the BJT is not conducting any current.

2. How is saturation achieved in a BJT?

Saturation in a BJT is achieved when the base-emitter junction is forward biased and the collector-base junction is reverse biased. This allows for a large amount of current to flow from the collector to the emitter, resulting in the BJT acting as a closed switch.

3. What is the significance of the cutoff region in a BJT?

The cutoff region is important because it allows for the BJT to be fully turned off, acting as an open switch. This is necessary for controlling the flow of current in electronic circuits. It also helps prevent the BJT from overheating and getting damaged.

4. How does the base current affect the operating regions of a BJT?

The base current plays a crucial role in determining the operating regions of a BJT. In the active region, a small change in the base current can result in a large change in the collector current, making the BJT an efficient amplifier. In the saturation region, a high base current is needed to fully turn on the BJT as a switch. In the cutoff region, no base current is present, resulting in no collector current.

5. What is the purpose of studying the operating regions of a BJT?

Understanding the operating regions of a BJT is important for designing and analyzing electronic circuits. It allows for proper utilization of the BJT as either an amplifier or a switch, depending on the desired application. It also helps in troubleshooting and diagnosing issues in circuits involving BJTs.

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