Continue to Site

Welcome to EDAboard.com

Welcome to our site! EDAboard.com is an international Electronics Discussion Forum focused on EDA software, circuits, schematics, books, theory, papers, asic, pld, 8051, DSP, Network, RF, Analog Design, PCB, Service Manuals... and a whole lot more! To participate you need to register. Registration is free. Click here to register now.

Register Log in

BJT with really tight tolerance on hfe?

Status
Not open for further replies.
From what an expert told me, a manufacturer installs resistors in the legs to the transistor, so as to produce a certain gain, or limit gain to a certain level. In this way they get predictable performance, by conservative utilitization of components.
 

Hi,

What hfe value? At what frequency?

Klaus
 

BradtheRad said:
From what an expert told me, a manufacturer installs resistors in the legs to the transistor, so as to produce a certain gain, or limit gain to a certain level. In this way they get predictable performance, by conservative utilitization of components.
Click to expand...
Your "expert" is wrong.
The range of current gain is determined by the manufacturing process variability.
There is no adding of resistors to control this gain.
 

What hfe value? At what frequency?
Click to expand...
hfe can be anything from 10 to 150, but as long as it is not widely toleranced.
Frequency?...well, it is to amplify the feedback current signal of a switch mode power supply, that signal is at about 10khz maximum.
 

Hi,

why don´t you use local feedback to set gain?

Klaus
 

we want to amplify the feedback current so as to reduce the current drawn through the opto-diode.
The only way to do this is to amplify the current signal as it goes into the topswitch-jx controllers.

Sorry what do you mean by "local feedback"?
 

Why do you think you need a tight tolerance for Q2's gain in the referenced diagram?
That circuit is quite tolerant of any gain variation.
 

Wishes for tight transistor gain tolerances pit against opto coupler CTR tolerance which already spans a 1:2 range. Using selected transistors like BC847xx multiply gain uncertainty by another 1:2 factor. Apparently the circuit designer had no problem to put in a loosely tolerated MMBT3904. But whatever you choose, you get a large temperature dependent gain variation on top.

Any current amplifier with tightly tolerated gain I can think of would use considerably more components.
 

Why do you think you need a tight tolerance for Q2's gain in the referenced diagram?
That circuit is quite tolerant of any gain variation.
Click to expand...
The gain of the Q2 is a feedback loop parameter as you know. Therefore, if we are to accurately compute gain and phase margins for the SMPS, we need to know the gain of Q2, ie the hfe of Q2.

- - - Updated - - -

But whatever you choose, you get a large temperature dependent gain variation on top.
Click to expand...
I thought R25 provided emitter degeneration which got rid of most of the temperature variation of the hfe of Q2? (I am referring again to fig31, page 25 of the AN-47 document referred to in the top post)
 

I thought R25 provided emitter degeneration which got rid of most of the temperature variation of the hfe of Q2?
Click to expand...
How? The amplifier stage is current in/current out. If R25 helps against temperature variations, it also would against initial tolerances.

R25 is apparently just providing current limiting as a protection means.
 

crutschow said:
Your "expert" is wrong.
The range of current gain is determined by the manufacturing process variability.
There is no adding of resistors to control this gain.
Click to expand...

As you state, transistors all have different hfe. Then that is the gain when no resistor is in the emitter leg. However by putting in an emitter resistor, it reduces gain. Say we want gain =20. From what I understand there is a certain combination of resistors that gives us that. Then we can manufacture a thousand radios, all with gain=20, regardless of transistor hfe, whether 100 or 200 or 300.

To do that, a formula probably exists which can be applied to this schematic.

9288690800_1457375410.gif
 

R25 is apparently just providing current limiting as a protection means.
Click to expand...

Fig24, page 19, of the AN-47 document (below) shows again the Darlington configuration used to amplify the optocoupler feedback current. {the explanation for the series resistor of 20 ohms is given in AN-47 on page 21 (bottom right of page) to 22 (top of page)}

AN-47 document on topswitch-jx:
https://ac-dc.power.com/sites/default/files/product-docs/an47.pdf

…page 22 says that the series 20 ohm resistor is added for “stability” reasons. –But it does not elaborate on this. I do not understand why the 20 ohm resistor is needed for "stability" reasons, because compensating for the extra gain of the darlington pair can be done by increasing the value of the R(LED) resistor , ie, the resistor in series with the opto-diode. So what they mean by “stability” I do not know. Do you know?
 

we are interested in current gain, as you see from the schematic of the top post, a current is going into the base , and we are fetching out the current (amplified) from the collector of the bjt.
We would best like a bjt with a tight tolerance on hfe. Do you know of one? hfe doesn't have to be high either, just tightly tolerance.
 

BradtheRad said:
As you state, transistors all have different hfe. Then that is the gain when no resistor is in the emitter leg. However by putting in an emitter resistor, it reduces gain. Say we want gain =20. From what I understand there is a certain combination of resistors that gives us that. Then we can manufacture a thousand radios, all with gain=20, regardless of transistor hfe, whether 100 or 200 or 300.
..............
Click to expand...
You are mixing apples and oranges.
The gain of 20 is the voltage gain, not the current gain.
The emitter resistor has no effect on the transistor current gain.
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating
hFE is DC current gain. hfe is AC current gain.
They show a surface-mount 2N3904 that has an hFE from 100 to 300 when it is at room temperature and has a collector current of 10mA.
A BC547A has an hFE from 110 to 220 when it is at room temperature and has a collector current of 2mA.
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating
2N3904 that has an hFE from 100 to 300
Click to expand...
300 is typical hFE, not maximal. Maximal value isn't specified in the datasheet. I would expect a maximal value of 600 or 800 as with similar transistors.
 
  • Like
Reactions: treez

    T

    Points: 2
    Helpful Answer Positive Rating
FvM said:
300 is typical hFE, not maximal. Maximal value isn't specified in the datasheet. I would expect a maximal value of 600 or 800 as with similar transistors.
Click to expand...
Whose datasheet are you looking at?
 

Attachments

  • 2N3904 datasheet.png
    2N3904 datasheet.png
    41.7 KB · Views: 139
  • Like
Reactions: FvM

    FvM

    FvM

    Points: 2
    Helpful Answer Positive Rating
Status
Not open for further replies.

Similar threads

Part and Inventory Search

Welcome to EDABoard.com

Sponsor

Back
Top