# transistor

Discussion in 'General Electronics Chat' started by electronics1, Sep 3, 2009.

1. ### electronics1 Thread Starter Active Member

Aug 21, 2009
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I used transistor as switch In the basis I put a resistor
How I calculate the resistor so that don't burn the transistor

2. ### mik3 Senior Member

Feb 4, 2008
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What is the required current into the collector?

3. ### alitex Active Member

Mar 5, 2007
122
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if u want to use it as switch,put 4.7 K ohm to collector and 1K to base
put 5V to collector through 4.7K resistor take output between collector and 4.7K ohm,that if you want take (not switch),
if you want opposite put 4.7k to emitter

4. ### hgmjr Moderator

Jan 28, 2005
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Assuming you are using the transistor in the common-emitter configuration then I would suggest you read this AAC ebook material on common-emitters to get familiar with this configuration.

hgmjr

5. ### mik3 Senior Member

Feb 4, 2008
4,846
63
The current through the collector is limited by the 1K resistor.

To find R2 you need to know Ic.

Assuming Ic is 10mA

then

Ib=Ic/β

β depends on the transistor but for your application R2=2K is ok.

To find R2:

R2=(Vapplied-0.7)/Ib

6. ### ELECTRONERD Senior Member

May 26, 2009
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β depends upon the transistor. In the transistor you provided in the specs, they say that it can go from a β of 120 to 700. You can choose the β within those values, and it represents the gain of the transistor.

7. ### alphacat Active Member

Jun 6, 2009
186
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But he wishes to operate the transistor in the saturation region.
In BJT's saturation region, IC doesnt equal to β*IB (Since in saturation region, both junctions - BC and BE - are forward biased).
So how can β be helpful in the saturation region?

Feb 17, 2009
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9. ### alphacat Active Member

Jun 6, 2009
186
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But cant β be used only when the transistor is in the forward biased region, and not in the saturation region?

In order to reach ibmin=83uA, you used the formula ib = ic/β, which is valid only in the forward biased region.
In the case of using the BJT as a switch, you force it to be in the saturation region.

10. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,962
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You suggest that the bjt will not be saturated for Rc=1K and RB=47K, Vcc=12V.
Maybe you build this circuit.
What then, cause bjt to saturted? Can BJT be saturate when Rc=0

11. ### alphacat Active Member

Jun 6, 2009
186
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Well,
I'm talking in general.

Which of the two following general notes isnt true?
- When using a BJT as a switch, you want to have the BJT saturated.
- When the BJT saturated, IB doesnt equal IC/β.

If they're both true, then how could you use β when using the BJT as a switch?

12. ### hobbyist Distinguished Member

Aug 10, 2008
764
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If the transistor needs to be saturated, then according to the data sheet IB=10mA. @ IC = 100mA.

There using a forced B=10.

So your schem shows Ic=10mA. min. so a starting point for R2 could be choose IB=1mA.

(5V.-0.7V) / 1mA = 4.3K ohms.

Then work from there checking Vce, as well as IC, then adjust the R2 value as needed.

13. ### Jony130 AAC Fanatic!

Feb 17, 2009
3,962
1,097
I simply assume that β will not drop below 120. And then for Rb<Rc/β_min BJT must be saturated.

Try the other way. Determine in which state the BJT (BC548) is for:
Rb=100K; Rc=1K; Vcc=10V

Last edited: Sep 5, 2009
14. ### Audioguru New Member

Dec 20, 2007
9,411
896
The beta (hFE) is used when the transistor is not saturated and is a linear amplifier with 6V or more between its collector and its emitter.
The datasheet shows that the transistor saturates pretty well as a switch when its base current is 1/10th its collector current.

15. ### alphacat Active Member

Jun 6, 2009
186
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Hey Audiogure,

Could you please give an example of a datasheet which shows that when IB = IC/10, the BJT is saturated?

Feb 17, 2009
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17. ### alphacat Active Member

Jun 6, 2009
186
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Thank you very much for this knowledge!

18. ### The Electrician AAC Fanatic!

Oct 9, 2007
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According to Fig. 4, that's quite the transistor. The collector current ranges from 1 amp to 1000 amps!

19. ### Audioguru New Member

Dec 20, 2007
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Almost every little American transistor like the 2N3904 and 2N4401 has a spec'd max Collector-Emitter Saturation Voltage when the base current is 1/10th the collector current. Its spec'd beta is much higher than only 10.

The European BC548 has a spec'd max Collector-Emitter Saturation Voltage when the base current is 1/20th the collector current which is also far from its spec'd beta.

Sep 16, 2009
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