All About Circuits Forum Collector feedback bias...
 Register Blogs FAQ Members List Today's Posts Search Today's Posts Mark Forums Read

 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.

#1
09-02-2012, 07:10 PM
 rougie Senior Member Join Date: Dec 2006 Posts: 410
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
Attached Images
 T-CFB1.jpg (208.1 KB, 44 views)
#2
09-03-2012, 02:27 AM
 #12 Senior Member Join Date: Nov 2010 Location: 15 miles west of Tampa, Florida Posts: 9,048 Blog Entries: 9

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.
__________________
It's only my opinion, and sometimes I'm wrong.
(I learn something almost every day on this site.)
#3
09-03-2012, 04:19 AM
 Sensacell Senior Member Join Date: Jun 2012 Location: Bangkok, Thailand Posts: 373

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!
#4
09-03-2012, 07:56 AM
 debjit625 Senior Member Join Date: Apr 2010 Location: Blue planet... Posts: 790

Except for Voltage Divider Biasing all other biasing with negative feedback circuit for a BJT in common emitter configuration will be $\beta$ dependent.

Note in Voltage Divider Biasing, the effect of $\beta$ 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
__________________
Debjit Roy
__________________
A man would do nothing,if he waited until he could do it so well,that no one would find fault with what he has done.
- CARDINAL NEWMAN
#5
09-03-2012, 10:58 AM
 rougie Senior Member Join Date: Dec 2006 Posts: 410

</
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
#6
09-03-2012, 11:21 AM
 rougie Senior Member Join Date: Dec 2006 Posts: 410

I will eventually look into LTSpice...

#7
09-03-2012, 01:05 PM
 MrChips Super Moderator Join Date: Oct 2009 Posts: 8,989 Blog Entries: 23
How to display a quote

If you wish to insert other people's comments into your post, use the QUOTE tag.
Instead of the </ and /> that you used, end the quote with [/quote].
Begin the quote with [quote].

For example:

Quote:
 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.
__________________
Most computer problems can be attributed to a simple problem - a loosewire behind the keyboard.

#8
09-03-2012, 02:05 PM
 DickCappels Senior Member Join Date: Aug 2008 Location: Thailand Posts: 847

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 :-)
 The Following User Says Thank You to DickCappels For This Useful Post: #12 (09-03-2012)
#9
09-03-2012, 03:31 PM
 ramancini8 Senior Member Join Date: Jul 2012 Location: Central Florida Posts: 330

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.
#10
09-03-2012, 03:35 PM
 ramancini8 Senior Member Join Date: Jul 2012 Location: Central Florida Posts: 330

Quote:
 Originally Posted by #12 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????

 Tags bias, collector, feedback

 Related Site Pages Section Title Worksheet Class A BJT amplifiers Worksheet Bipolar junction transistors in active mode Textbook Radio circuits -- INCOMPLETE : Practical Analog Semiconductor Circuits Video Lecture Transistor Biasing - Transistors and Transistor Circuits Textbook Transistor ratings and packages : Bipolar Junction Transistors Textbook Current mirrors : Bipolar Junction Transistors Textbook Biasing calculations : Bipolar Junction Transistors Textbook Biasing techniques : Bipolar Junction Transistors Textbook Meter check of a transistor : Bipolar Junction Transistors Textbook Bipolar junction transistors : Solid-state Device Theory

 Similar Threads Thread Thread Starter Forum Replies Last Post leoss General Electronics Chat 5 02-12-2012 09:44 PM dcd528 General Electronics Chat 4 07-29-2011 02:58 PM tel_amp lady Homework Help 0 08-22-2009 11:07 PM clam General Electronics Chat 12 11-23-2007 03:35 PM electronics_shamim The Projects Forum 1 05-08-2007 10:49 AM

 Thread Tools Display Modes Linear Mode

 Posting Rules You may not post new threads You may not post replies You may not post attachments You may not edit your posts BB code is On Smilies are On [IMG] code is On HTML code is Off Forum Rules
 Forum Jump User Control Panel Private Messages Subscriptions Who's Online Search Forums Forums Home Electronics Forums     General Electronics Chat     The Projects Forum     Homework Help     Electronics Resources Software, Microcomputing, and Communications Forums     Programmer's Corner     Embedded Systems and Microcontrollers     Computing and Networks     Radio and Communications Circuits and Projects     The Completed Projects Collection Abstract Forums     Math     Physics     General Science All About Circuits Commmunity Forums     Off-Topic     The Flea Market     Feedback and Suggestions

All times are GMT. The time now is 03:42 PM.