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[SOLVED] please help me transistor amplifier

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michael 1978

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CAN SOMEBODY TEACH ME,
HOW TO FIND DESIRED VOLTAGE GAIN OF CE A TRANSISTOR AMPLIFIER
WICH IS FORMULA, I KNOW THE FORMULA IS rc:rl/re
BUT I WANT FOR EXAMPLE A VOLTAGE GAIN OF 50

FOR EXAMPLE

INPUT DESIRED DATA:
DESIRED VOLTAGE GAIN 2.0 WICH I WANT TO KNOW
F_MIN 10 HZ
Z_IN = 50 K
Z_OUT = 2k
POWER SUPPLY 12V

NOW HOW IS THE FORMULA TO FIND
R1 R2 I KNOW TO FIND
BUT RC AND RE I DONT KNOW TO FIND, THAT IS MY PROBLEM
 

goldsmith

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WICH IS FORMULA, I KNOW THE FORMULA IS rc:rl/re
Hi Michael
I hope you know when RE has been bypassed ( completely ) gain is RCllRL/re but when you've two sectional RE ( first one bypassed and the other one without bypass capacitor ) the gain will be RCllRL/RE2+re

INPUT DESIRED DATA:
DESIRED VOLTAGE GAIN 2.0 WICH I WANT TO KNOW
F_MIN 10 HZ
Z_IN = 50 K
Z_OUT = 2k
POWER SUPPLY 12V

NOW HOW IS THE FORMULA TO FIND
R1 R2 I KNOW TO FIND
BUT RC AND RE I DONT KNOW TO FIND, THAT IS MY PROBLEM
Well , as i can remember i've suggested you , to referring some textbooks or some articles .
Anywhere , did you select any transistor for your aim ? for first step you've to select a transistor and then going through the other design steps .
Best Wishes
Goldsmith
 

michael 1978

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Hi Michael
I hope you know when RE has been bypassed ( completely ) gain is RCllRL/re but when you've two sectional RE ( first one bypassed and the other one without bypass capacitor ) the gain will be RCllRL/RE2+re



Well , as i can remember i've suggested you , to referring some textbooks or some articles .
Anywhere , did you select any transistor for your aim ? for first step you've to select a transistor and then going through the other design steps .
Best Wishes
Goldsmith

goldsmith i know the gain is rc:rl/re , but when you design a gain first, after rc rl re are unknow value
ok is possible for you one more time to design one amplifier, please you show me first in similary way
 

LvW

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goldsmith i know the gain is rc:rl/re ,
Michael, it would be a good idea to draw a circuit and to show us the meaning and location of the parts you call rc, rl and re.
(Of course, I have a guess - however, I can`t be sure of it).
Otherwise it makes no sense to discuss with you.
For example, my formula is

A=-g*Reff/(1+g*Re)

If you are not familiar with my symbols you perhaps understand what I mean.
 

michael 1978

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Michael, it would be a good idea to draw a circuit and to show us the meaning and location of the parts you call rc, rl and re.
(Of course, I have a guess - however, I can`t be sure of it).
Otherwise it makes no sense to discuss with you.
For example, my formula is

A=-g*Reff/(1+g*Re)

If you are not familiar with my symbols you perhaps understand what I mean.


do you understand wat i want, like you give me example to desigin amplifier with gain of 60, wich formula you use
http://obrazki.elektroda.pl/3235465600_1352929030.jpg
 

LvW

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Michael,
if you really want to help you, you should spent some time to define your amplifier - based on a clear description and unequivocal symbols.

I don`t know the meaning of the following line (see your attachment):

rc,rl=r2:r5,25m:r1.

perhaps you can explain it to us.
Besides the fact that the attachment contains only symbols with capital letters I cannot find a part called rc or Rc at all.
 

michael 1978

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Michael,
if you really want to help you, you should spent some time to define your amplifier - based on a clear description and unequivocal symbols.

I don`t know the meaning of the following line (see your attachment):

rc,rl=r2:r5,25m:r1.

perhaps you can explain it to us.
Besides the fact that the attachment contains only symbols with capital letters I cannot find a part called rc or Rc at all.
ah yes easy do you see the image wich i put it, , rc is called of is r2, rl load i called of is r5, and re is called of is r1, easy rc is connectet to collector, re to emitter, and rl to output, after come the formula rcllrl/re , do you understand now, look once if you want image
http://obrazki.elektroda.pl/3235465600_1352929030.jpg
 

goldsmith

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Hi Michael
Well , let me explain what you can do ! you can earn your required gain , easily ! with a simple calculation . consider please that you've designed a CE amplifier . for your required Ro and Rin . ( do you know how to set it for your required zo and zin ?if no i can explain it step by step . )
then you can set your gain !
For example you need gain of 20 . and gain of your amplifier is around 75 then you can go through this formula !
20= RCllRL/RE2+re which RE2 is the resistor that shouldn't be bypassed to reduce the gain ( with creating a series current , negative feedback )
I hope you got the idea !
Best Wishes and Good luck
Goldsmith
 

LvW

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ah yes easy do you see the image wich i put it, , rc is called of is r2, rl load i called of is r5, and re is called of is r1, easy rc is connectet to collector, re to emitter, and rl to output, after come the formula rcllrl/re , do you understand now, look once if you want image
http://obrazki.elektroda.pl/3235465600_1352929030.jpg
I hope you now can see that is was not so easy to translate your symbols.

Art first, you should learn to distinguish between static (ohmic) resistors which are characterized by capital letters (R) and differential resistances (which are also very important), which are written as small letters (r).

Thus, your gain formula as shown above is

gain:G=(Rc||Rl)/Re

However, please note that this formula is not correct because
*the minus sign is missing (inverting operation)
*The transconductance g of the BJT is neglected.

The correct gain expression is

G=-gReff/(1+gRe)=-Reff/(1/g + Re) with Reff=Rc||RL

As you can see, the transconductance g may be neglected only for Re>>1/g . The validity of such a simplification must be checked from case to case.
As you probably knows, the value of g must be chosen and depends on Ic (g=Ic/Vt).
Thus, your design starts with selection of Rc, Re and Ic (assumption: supply voltage fixed) .
 

michael 1978

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Hi Michael
Well , let me explain what you can do ! you can earn your required gain , easily ! with a simple calculation . consider please that you've designed a CE amplifier . for your required Ro and Rin . ( do you know how to set it for your required zo and zin ?if no i can explain it step by step . )
then you can set your gain !
For example you need gain of 20 . and gain of your amplifier is around 75 then you can go through this formula !
20= RCllRL/RE2+re which RE2 is the resistor that shouldn't be bypassed to reduce the gain ( with creating a series current , negative feedback )
I hope you got the idea !
Best Wishes and Good luck
Goldsmith
thnx for reply
so goldsmith only in this way you can change a gain?
 

goldsmith

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thnx for reply
so goldsmith only in this way you can change a gain?
No ! there are gang of ways that you can go through them ! one of the best methods , is the way that " LvW " is explaining .
I've a sense ! i sense that you want learn really . but you don't know where to start and you are a bit confused . am i right ?
 

michael 1978

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No ! there are gang of ways that you can go through them ! one of the best methods , is the way that " LvW " is explaining .
I've a sense ! i sense that you want learn really . but you don't know where to start and you are a bit confused . am i right ?
yes you right, but you know i learn a lot of transistor, and i stop to voltage gain, i cant go next to lesson, becaue next lesson in the book is about jfet transistor and i like it first to learn good transistor, but anyway i learn enough so i have to go now to jfet to learn

thnx for help

- - - Updated - - -

I hope you now can see that is was not so easy to translate your symbols.

Art first, you should learn to distinguish between static (ohmic) resistors which are characterized by capital letters (R) and differential resistances (which are also very important), which are written as small letters (r).

Thus, your gain formula as shown above is

gain:G=(Rc||Rl)/Re

However, please note that this formula is not correct because
*the minus sign is missing (inverting operation)
*The transconductance g of the BJT is neglected.

The correct gain expression is

G=-gReff/(1+gRe)=-Reff/(1/g + Re) with Reff=Rc||RL

As you can see, the transconductance g may be neglected only for Re>>1/g . The validity of such a simplification must be checked from case to case.
As you probably knows, the value of g must be chosen and depends on Ic (g=Ic/Vt).
Thus, your design starts with selection of Rc, Re and Ic (assumption: supply voltage fixed) .


ah so to design first i need to start with selection of rc, re and ic thnx for reply
 

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