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#1
11-10-2009, 11:45 PM
 Ronscott1 Junior Member Join Date: Nov 2009 Posts: 15
Collector-emitter feedback bias Dc calculations help

I was given the problem for homework to design the values of Rc, Rb, and Re given the following specifications:

Vcc=9V
Vce at midpoint
Ic=1 mA
βdc=300

Using the guidelines for VDB from Albert Malvinos "Electronic Principles" I came up with:
Ve=0.1*Vcc
Ve=(0.1)(9V)
Ve=0.9V
Re=Ve/Ie Since Ie≈Ic
Re=0.9V/1mA
Re=900Ω
Rc=4Re
Rc=4(900Ω)
Rc=3.6kΩ
Rb= βdc(((Vcc-Vbe)/Ie)-Rc-Re)
Rb=300(((9V-0.7V)/1mA)-3.6kΩ-900Ω)
Rb=1.14MΩ

But when I plugged these numbers back into the equation
Ie=(Vcc-Vbe)/Rc+Re+Rb/βdc, I got Ie=1.8mA.

Is there something that I am doing wrong? How should I go about calculating these resistor values? Also are there any specific guidelines that I should follow?

Best,
Ron
Attached Images
 Collector Emmiter Feedback Bias.JPG (8.1 KB, 26 views)
#2
11-11-2009, 12:07 AM
 hobbyist Senior Member Join Date: Aug 2008 Posts: 716 Blog Entries: 21

Are they the guidelines your suppose to use, or are they your choice to use?
#3
11-11-2009, 12:18 AM
 ELECTRONERD Senior Member Join Date: May 2009 Location: Outer Space Posts: 1,145

Your schematic diagram is rather strange, since Rb usually connects above Rc to the Vcc source. But, as you may have realized, they form a voltage divider when connected to the collector. A useful equation is that $I_B=\frac{I_C}{hFE}$

Austin
#4
11-11-2009, 12:28 AM
 Ronscott1 Junior Member Join Date: Nov 2009 Posts: 15

Hobbyist-
Those are the guidelines I chose to follow. There is just about no information in my textbook about this type of biasing, let alone there is not much information available on the web. I am not really sure what guidelines to use. I just could not find any except for voltage divider bias in my text. Do you know of any?

Austin- Ideally what should I try to bias the base around?
#5
11-11-2009, 12:37 AM
 ELECTRONERD Senior Member Join Date: May 2009 Location: Outer Space Posts: 1,145

Quote:
 Originally Posted by Ronscott1 Hobbyist- Those are the guidelines I chose to follow. There is just about no information in my textbook about this type of biasing, let alone there is not much information available on the web. I am not really sure what guidelines to use. I just could not find any except for voltage divider bias in my text. Do you know of any? Austin- Ideally what should I try to bias the base around?
Ron, a good resource is our current AAC e-book. Just go under "Volume III Semiconductors" and scroll down to the BJT biasing calculations. When beginning to bias transistors, Ohm's Law is virtually the only technique necessary, besides understanding linear region, saturation, and gain. Using the formula I suggested, $I_B=\frac{1mA}{300}=3uA$. That's a very small Ib current. By seeing the formula alone, you can see that Ic, Ib, and β are all related.

Austin
#6
11-11-2009, 01:06 AM
 hobbyist Senior Member Join Date: Aug 2008 Posts: 716 Blog Entries: 21

It looked like in your equation, for RB value, you were using the full 9V. instead of the drop across RC.

I may be looking at your equation wrong. Though too.

Is imidpoint at collector with respect to emitter or at the collector with respect to ground? That's my main question.
#7
11-11-2009, 04:28 AM
 t_n_k Senior Member Join Date: Mar 2009 Posts: 4,956

Quote:
 Originally Posted by Ronscott1 But when I plugged these numbers back into the equation Ie=(Vcc-Vbe)/Rc+Re+Rb/βdc, I got Ie=1.8mA. Is there something that I am doing wrong? How should I go about calculating these resistor values? Also are there any specific guidelines that I should follow?
Hi Ron,

It's those wretched parentheses ....

$Ie=\frac{(Vcc-Vbe)}{(Rc+Re+\frac{Rb}{(1+\beta)})}$

$Ie=\frac{(9-0.7)}{(3600+900+\frac{1.14e6}{(301)})}$

$Ie=\frac{(8.3)}{(8287.4)}$

$Ie=1mA$
#8
11-11-2009, 03:57 PM
 Ronscott1 Junior Member Join Date: Nov 2009 Posts: 15

Thanks so much for your help guys. Hobbyist - I am pretty sure that midpoint is with respect to ground.
Best,
Ron
#9
11-11-2009, 05:12 PM
 Audioguru Banned Join Date: Dec 2007 Location: Ontario, Canada Posts: 9,411

Quote:
 Originally Posted by ELECTRONERD Your schematic diagram is rather strange, since Rb usually connects above Rc to the Vcc source.
No.
That is the worst way to bias a transistor since the wide range of beta, the different base-emitter voltage and different temperature all determine its amount of needed base current.

When a single base resistor connects to the collector then it provides AC and DC negative feedback.
If the beta or temperature is high then the transistor tries to conduct more which reduces its collector voltage that reduces the base current. Then the transistor does not turn on too much. If the beta or temperature is low then the opposite happens and the transistor does not turn off too much.
#10
11-11-2009, 08:37 PM
 hobbyist Senior Member Join Date: Aug 2008 Posts: 716 Blog Entries: 21

Quote:
 Originally Posted by Ronscott1 Thanks so much for your help guys. Hobbyist - I am pretty sure that midpoint is with respect to ground. Best, Ron
See post #7.

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