What is the Quiescent Operating Point for FET's?

  • Thread starter Thread starter Physicslearner500039
  • Start date Start date
  • Tags Tags
    Point
Click For Summary

Discussion Overview

The discussion revolves around the concept of the Quiescent Operating Point (Q-point) for Field Effect Transistors (FETs) and Bipolar Junction Transistors (BJTs), particularly in the context of their operation as switches versus amplifiers. Participants explore the necessity of calculating the Q-point under different operating conditions, including the implications of applying AC signals.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants propose that when operating a transistor as a switch, calculating the Q-point may not be necessary, suggesting that it is primarily relevant when avoiding distortion in AC applications.
  • Others argue that ensuring the transistor operates in the saturation region is sufficient for switch design, and specific conditions for biasing BJTs are provided.
  • A participant questions the understanding of DC sweep calculations, seeking clarification on the relationship between switching speed and the need for Q-point calculations.
  • There is mention of using DC sweep analysis for simulating switching circuits, indicating that varying input voltage can help analyze circuit behavior.

Areas of Agreement / Disagreement

Participants express differing views on the necessity of calculating the Q-point when using transistors as switches. While some agree that it may not be needed, others provide conditions under which it is relevant, indicating that the discussion remains unresolved.

Contextual Notes

Some participants express uncertainty regarding the definitions and implications of DC sweep analysis, as well as the criteria for determining switching speed, highlighting potential gaps in understanding.

Physicslearner500039
Messages
124
Reaction score
6
Hi All,

I was learning a lot of how to calculate the Quiescent operating point for FET's. But suddenly I got this doubt that if i am operating any transistor (BJT, FET) as a switch then I need not calculate the Q - point. Am I correct in this? Only when some AC signal is applied to avoid distortion I need to calculate the Q point?

Thanks in advance,
Regards,
Satya
 
Engineering news on Phys.org
Calculate dc sweep if switching is not very fast.
 
satya77 said:
Hi All,

I was learning a lot of how to calculate the Quiescent operating point for FET's. But suddenly I got this doubt that if i am operating any transistor (BJT, FET) as a switch then I need not calculate the Q - point. Am I correct in this? Only when some AC signal is applied to avoid distortion I need to calculate the Q point?

Thanks in advance,
Regards,
Satya

Yes, you are right. The FET or BJT used as a switch would normally be biased OFF so the device can be turned ON by the pulses that drive the switch.

If you used the device as a linear amplfier, you arrange the biasing to avoid distortion in the output, but this is not done when the device is used as a switch.
 
Kholdstare said:
Calculate dc sweep if switching is not very fast.

I am sorry I did not understand what you said, for me when browsed for dc sweep is the calculation of currents and voltages, am i correct in this? how to know if the switching is fast or not ? Could you please explain me?

Thanks in advance,
Regards,
Satya
 
If you design the switch you don't need to calculate any Q point.
When BJT work as a switch this means that hi work on the saturation region.
So all you need to do is to ensure that BJT is in saturation region.
And simply if we ensure that
Ic = β * Ib > Vcc/Rc the the BJT is in saturation region.
So for a given Rc we need Rb smaller then

Rb ≤ (Vin - Vbe)/(Vcc - Vce(sat)) *βmin/K * Rc

Where
K is a overdrive factor, typical from 2 to 5.

Or even simpler rules
Ensure that
Ic/Ib = 50...10
 
satya77 said:
I am sorry I did not understand what you said, for me when browsed for dc sweep is the calculation of currents and voltages, am i correct in this? how to know if the switching is fast or not ? Could you please explain me?

Thanks in advance,
Regards,
Satya

Do you have any idea of dc sweep of CMOS inverters? You BJT/FET switching circuit can be simulated like that. FYI, dc sweep is varying the dc value of input voltage and looking at the voltage and currents in the circuit.

There are capacitance values provided in the datasheet of the component. From there you can have an idea of range of operating frequency.
 

Similar threads

  • · Replies 3 ·
Replies
3
Views
3K
Replies
4
Views
3K
Replies
4
Views
2K
  • · Replies 7 ·
Replies
7
Views
7K
  • · Replies 17 ·
Replies
17
Views
3K
Replies
3
Views
15K
  • · Replies 41 ·
2
Replies
41
Views
6K
Replies
2
Views
3K
Replies
37
Views
7K
  • · Replies 4 ·
Replies
4
Views
2K