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The Projects Forum Working on an electronics project and would like some suggestions, help or critiques? If you would like to comment or assist others with their projects, this is the place to do it.

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  #1  
Old 09-02-2012, 07:10 PM
rougie rougie is offline
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Default Collector feedback bias...

hello,

wow...wow...wow

I did my first collector feedback bias circuit and I was impressed as to how close the measured and calculated values were. See attached file.

BUT THEN.... I swapped the transistor with another one with the same part # and wooops!

The Ie was now 1.36ma and Vc was 3.74!!!

I can't believe how volatile these things really are!!!!

But I guess this circuit is alot more stable that the simple base bias one!

Okay... I will now learn the other 2 types!!!!

thanks guys!
r
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Old 09-03-2012, 02:27 AM
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I think a 2 resistor base bias is more stable, as in, not depending on the gain of the transistor to decide the operating point. Eagerly awaiting you next post when you tell us what you have learned.
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Old 09-03-2012, 04:19 AM
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The hfe or beta of transistors varies widely, in general, if your design requires a specific value of hfe to work correctly, it's not a very good design.

It's often kind of horrifying to see the data sheet spread of hfe and realize what this means for your design- for it to work correctly with such a huge spread of gain. For example, the 2N4123 data sheet says it goes from 50 to 200!
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Old 09-03-2012, 07:56 AM
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Except for Voltage Divider Biasing all other biasing with negative feedback circuit for a BJT in common emitter configuration will be dependent.

Note in Voltage Divider Biasing, the effect of is less but its not obsolete

I will prefer you to use some simulator software to perform these experiments,before doing them in real.

LTSpice will be good.
http://www.linear.com/designtools/software

Good Luck
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  #5  
Old 09-03-2012, 10:58 AM
rougie rougie is offline
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Default

</
I think a 2 resistor base bias is more stable, as in, not depending on the gain of the transistor to decide the operating point. Eagerly awaiting you next post when you tell us what you have learned.
/>

One thing though, asides from showing better stability than
a feedbackless base bias common emitter circuit, there isn't
much use for this circuit .... Right?

As you said, I can't even control it !!!

</
The hfe or beta of transistors varies widely, in general, if your design requires a specific value of hfe to work correctly, it's not a very good design.
\>
Yes Indeed!

Thanks
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Old 09-03-2012, 11:21 AM
rougie rougie is offline
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I will eventually look into LTSpice...

Thanks for the heads up
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Old 09-03-2012, 01:05 PM
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Default How to display a quote

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The hfe or beta of transistors varies widely, in general, if your design requires a specific value of hfe to work correctly, it's not a very good design.
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Old 09-03-2012, 02:05 PM
DickCappels DickCappels is offline
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If you reduce the value of the (470k) feedback resistor to something like 47k, the collector voltage and current will be much less dependent on the current gain of the transistor.

If taken to the limit -that is shorting the base to the collector, it will be even more stable, but less useful :-)
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Old 09-03-2012, 03:31 PM
ramancini8 ramancini8 is offline
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The calculated emitter current for Hfe=200 is 1.35mA, and for Hfe=50 is .671mA. The Hfe varies by a factor of 4 while the emitter current only varies by a factor of 2. Drop Rb to 47K and the numbers are Hfe=200, Ie=1.97mA and Hfe=50, Ie=1.69. Hfe varies by a factor of 4 while emitter current varies by a factor of 1.16.

This shows the dramatic effect that feedback can have on DC stability, but the circuit gain is reduced proportionately. If the gain required is AC it can be restored by bypassing Rb with a capacitor. A popular configuration splits Rb with a bypass capacitor to control DC and AC gain.

The two resistor base bias is useless unless there is an emitter resistor involved. Take the Thevinin looking from the base to the junction of the two base resistors and you get one resistor in series with a source; no feedback.
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Old 09-03-2012, 03:35 PM
ramancini8 ramancini8 is offline
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Quote:
Originally Posted by #12 View Post
I think a 2 resistor base bias is more stable, as in, not depending on the gain of the transistor to decide the operating point. Eagerly awaiting you next post when you tell us what you have learned.
Not true unless you are using emitter degeneration (an emitter resistor). What don't I see????
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