correction to Biasing calculations

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mnada

Joined Apr 5, 2012
26
Dears, I found the following in this page In the attached file, which I need your confirmation if what I think is true or not. I believe the marked should be RE
 

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adam555

Joined Aug 17, 2013
858
Not sure you might also want to correct a small typo I found in this chapter; it's nothing important. In the second line of the Emitter-bias formulas the first "β" is written with a lowercase "b":

 

adam555

Joined Aug 17, 2013
858
I think I found 2 more in that same chapter....

The first is in this paragraph:
"How can we improve the performance of emitter-bias? Either increase the emitter resistor RB or decrease the base-bias supply VBB or both. As an example, we double the emitter resistor to the nearest standard value of 910Ω."
In which I think "RB" should be "RE"

And in this paragraph:
"As an exercise, rework the emitter-bias example with the base resistor reverted back to 470Ω, and the base-bias supply reduced to 1.5V."
I think it should read "emitter resistor" instead of "base resistor".
 
Last edited:

adam555

Joined Aug 17, 2013
858
Correct, and the "Table below", isn't even a table.
I'm not trying to be picky -just helping-, but now that you mention the tables, when I reached the next one at a first glance I got the impression that "Bypass Capacitor for RE" was the table caption instead of the title for the next section; as it's right under it without any separation line.
 

adam555

Joined Aug 17, 2013
858
I think I found a minor mistake in the "Amplifier impedance" chapter; though maybe I'm wrong.

I'm not sure if this formula is right. You wrote it first with the "/IE"...
Thus, for the common-emitter circuit Rin is
Rin = βREE/IE
... and then further down without it...
Rin = βREE = 100(26) = 2600Ω
Shouldn't it be always "Rin = βREE", without the "/IE"?
 

LvW

Joined Jun 13, 2013
1,300
Also here:
"The value of internal resistance RE is a function of emitter current IE, Table below. "
"RE" should be "REE".
I don`t like at all the usage of REE.
I know what it means (it is 1/gm=Vt/Ic), however there is one general and important rule:
Don`t mix ohmic (static) resistances with dynamic (differential) resistances.
This of extreme importance for understanding all circuits involving non-linear parts (diodes, transistors).
Thus, all differential parameters should carry SMALL letters and we always should write rEE.

EDIT: Using this unfortunate symbol REE the gain of an emitter stage is
G=-Rc/(REE+Re)
Thus, it looks as the gain is simply determined by ohmc resistors following Ohm`s law.
According to my experience, some beginners indeed were following this false approach.



 
Last edited:

adam555

Joined Aug 17, 2013
858
Yes, that happened to me: the first time I came across "REE" I had no idea it was a dynamic resistance. But I already knew that when I read the "Biasing Calculations" chapter.
 

LvW

Joined Jun 13, 2013
1,300
Yes, that happened to me: the first time I came across "REE" I had no idea it was a dynamic resistance. But I already knew that when I read the "Biasing Calculations" chapter.
May I add another point?

Without emitter resistor Re the gain of the circuit (which is a voltage-controlled current source with a load Rc) is

G=-gm*Rc.

To me, usage of a transconductance gm is straight-forward and logical because this parameter describes the input-output transfer charcteristic of the BJT (slope of the Ic=f(Vbe) curve).
And therefore my question: Is there any good reason NOT to use gm but, instead, a symbol r,ee (or even R,ee) ? Does this make any sense or does it help to understand what`s going on within the BJT?
 

Dcrunkilton

Joined Jul 31, 2004
422
Not sure you might also want to correct a small typo I found in this chapter; it's nothing important. In the second line of the Emitter-bias formulas the first "β" is written with a lowercase "b":


Letter b changed to symbol Beta in image 03488.png at ibiblio.org

Credited to adam555 in the Contributors List.

Thanks adam555,

Dennis
 

Dcrunkilton

Joined Jul 31, 2004
422
I think I found 2 more in that same chapter....

The first is in this paragraph:
"How can we improve the performance of emitter-bias? Either increase the emitter resistor RB or decrease the base-bias supply VBB or both. As an example, we double the emitter resistor to the nearest standard value of 910Ω."
In which I think "RB" should be "RE"

And in this paragraph:
"As an exercise, rework the emitter-bias example with the base resistor reverted back to 470Ω, and the base-bias supply reduced to 1.5V."
I think it should read "emitter resistor" instead of "base resistor".
emitter resistor replaced by base resistor as proposed at ibiblio

Credited to adam555

Dennis
 

Dcrunkilton

Joined Jul 31, 2004
422
I'm not trying to be picky -just helping-, but now that you mention the tables, when I reached the next one at a first glance I got the impression that "Bypass Capacitor for RE" was the table caption instead of the title for the next section; as it's right under it without any separation line.
Added a bigspace tag to the source at ibiblio.org to correct this.

Credited to adam555

Dennis
 

Dcrunkilton

Joined Jul 31, 2004
422
I think I found a minor mistake in the "Amplifier impedance" chapter; though maybe I'm wrong.

I'm not sure if this formula is right. You wrote it first with the "/IE"...
... and then further down without it...
Shouldn't it be always "Rin = βREE", without the "/IE"?
I deleted the "/IE" from the line in the copy at ibiblio.org .

Credited to adam555 in the Contributors list

Thanks,

Dennis
 

Dcrunkilton

Joined Jul 31, 2004
422
I don`t like at all the usage of REE.
I know what it means (it is 1/gm=Vt/Ic), however there is one general and important rule:
Don`t mix ohmic (static) resistances with dynamic (differential) resistances.
This of extreme importance for understanding all circuits involving non-linear parts (diodes, transistors).
Thus, all differential parameters should carry SMALL letters and we always should write rEE.

EDIT: Using this unfortunate symbol REE the gain of an emitter stage is
G=-Rc/(REE+Re)
Thus, it looks as the gain is simply determined by ohmc resistors following Ohm`s law.
According to my experience, some beginners indeed were following this false approach.


I have changed 22 instances of REE to rEE in the text and more in the images : 03489.eps 03494.eps 03495.eps 03497.eps 13062.eps , read .png for .eps on AAC.

I won't touch gain G for now.

Credited to LvW in the Contributors List.

Thanks for your input,

Dennis
 
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