# Power Dissipation Across A Transistor In Cutoff

Discussion in 'Electronics Homework Help' started by Akshatha Venkatesh, Mar 22, 2017.

1. ### Akshatha Venkatesh

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Jan 14, 2017
what's the Vce across a pnp common base transistor operating in cutoff , denoted as in the datasheet ?

2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Do you want Vce or dissipation?

By definition dissipation will be at or very close to zero and Vce will be at or very near maximum.

3. ### Akshatha Venkatesh

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Jan 14, 2017
power dissipation of transistor = Vce* Ic right ?

4. ### Akshatha Venkatesh

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Jan 14, 2017
Icbo , I know from the datasheet . but how do i calculate Vce ?

5. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Ic is zero (or very close to zero) so Pd is zero (or very close to zero).

So Vce must be the village across the to notes when no current flows between them.

Is this homework? Show us the question in full.

6. ### Akshatha Venkatesh

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Jan 14, 2017
Below is the circuit I'm trying to analyze. If TM+ is 6v and Vdd5 is 5v fixed. I'm trying to guess the output and also calculate all the transitors power dissipation.

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7. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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None of the transistors will be turned on. The will be some dissipation due to the the approx 0.4V across the bias resistors in T201. All of the other transistorthan are off, so assuming the voltages you mention are stiff and the loads on the output are high impedance, 10E will be Vdd5, and 11 will be TM+.

This will not apply at some higher TM+ or lower Vdd5, but not knowing the ratio of the bias resistors, I can't say with any certainty how much they need to change.

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8. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Oh, and where did you get the idea that any of those transistors was operating in common base mode?

9. ### Akshatha Venkatesh

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Jan 14, 2017
Isn't the Transistor T201 (Pnp) in common base configuration ? because base is connected to +5v

10. ### Akshatha Venkatesh

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Jan 14, 2017
the T201 bias resistors are 22k each and T200 resistors are 10k

11. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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No, it's in common emitter config. It's pretty much impossible for these biased transistors to be anything else.

T201 will start turning on when TM+ exceeds Vdd5 by more than about 1.8V.

12. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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When I refer to T201 I refer to the one on the left. At about 7V on TM+ both outputs will fall to zero.

13. ### Akshatha Venkatesh

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Jan 14, 2017
Can you please explain why 1.8V ?
Thank You

14. ### Akshatha Venkatesh

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Jan 14, 2017
T201 is a dual transistor. It has both PNP and NPN transistors. I was asking you abt the PNP transistor config .

15. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
To enable Vbe of the left T201 to be at about 0.6V.

The transistors would be better named as T201a and T201b then.

16. ### Akshatha Venkatesh

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Jan 14, 2017
Now I'm confused , the PNP trans of T201 is in common base configuration, right ?
I know that the other two transistors on the right hand part are in common emitter config(since their emitters are connected to ground)

17. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Jan 21, 2010
Well, if you wanted to describe T201a as common base you could, but all that would do is confuse you.

These are transistors designed for switching. Common base configuration is used as a current buffer or voltage amplifier. Here it is used as a switch when one voltage rises significantly above another. It is easier to consider Vdd5 as the input voltage, especially since the output is ground referenced, not referenced to Vdd5.

It might be different if there was a pull down between the collector and ground and the load was between Vdd5 and the collector -- that would look a lot more like common base.

When you see these biassed transistors you should think digital switching, so emitter followers and common base are not the places you should be quickly drawn to.

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18. ### Akshatha Venkatesh

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Jan 14, 2017
Thank You , I think that put a lot into perspective.

19. ### Akshatha Venkatesh

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Jan 14, 2017
And can you please tell me what the Ic is of T200 when TM+ is 13.5 ? So that I can calculate the power dissipation of it.

20. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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I'm not going to do all your homework for you.

You know how the circuit works now, you should be able to do it yourself.

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