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  #1  
Old 11-10-2009, 11:45 PM
Ronscott1 Ronscott1 is offline
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Default 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
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Old 11-11-2009, 12:07 AM
hobbyist hobbyist is offline
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Are they the guidelines your suppose to use, or are they your choice to use?
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Old 11-11-2009, 12:18 AM
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ELECTRONERD ELECTRONERD is offline
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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

Austin
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Old 11-11-2009, 12:28 AM
Ronscott1 Ronscott1 is offline
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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?
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Old 11-11-2009, 12:37 AM
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Quote:
Originally Posted by Ronscott1 View Post
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, . That's a very small Ib current. By seeing the formula alone, you can see that Ic, Ib, and β are all related.

Austin
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Old 11-11-2009, 01:06 AM
hobbyist hobbyist is offline
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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.
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Old 11-11-2009, 04:28 AM
t_n_k t_n_k is offline
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Quote:
Originally Posted by Ronscott1 View Post
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 ....







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Old 11-11-2009, 03:57 PM
Ronscott1 Ronscott1 is offline
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Thanks so much for your help guys. Hobbyist - I am pretty sure that midpoint is with respect to ground.
Best,
Ron
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Old 11-11-2009, 05:12 PM
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Quote:
Originally Posted by ELECTRONERD View Post
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.
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  #10  
Old 11-11-2009, 08:37 PM
hobbyist hobbyist is offline
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Quote:
Originally Posted by Ronscott1 View Post
Thanks so much for your help guys. Hobbyist - I am pretty sure that midpoint is with respect to ground.
Best,
Ron
See post #7.

he confirms your answer of 1mA.
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