# Multistage using BJT

#### Communications Student

Joined May 3, 2007
10
Plz guys I need ur help, I have a project,I need to design a multistage amplifier using BJT whith a voltage gain 100, and Rload=1Kohm, and it will operates in the frequency range from o to 5Mhz

Any ideas????

#### 03GTS

Joined Jan 5, 2007
12
i might have something that will help you but i will have to look for an old handout i have. i'll get back to you

#### mozikluv

Joined Jan 22, 2004
1,435
here's a simple schem of multi stge bjt amps. i have deliberately left out the values for you to find out. just use the appropiate formulas to arrive at the desired function.

moz

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#### Communications Student

Joined May 3, 2007
10
Thankss soo much 03GTS and Mozikluv, and I'll begin to work it out wiz the configuration tht u attached Moz, and I appreciate ur efforts o3GTS hoppin u find ur handout

#### Communications Student

Joined May 3, 2007
10
Hey guys nothing new?? the dead line for the project is 19/5 ((((

#### thingmaker3

Joined May 16, 2005
5,083

This forum is not here to do your schoolwork for you.

If you have specific questions, we will be happy to help you work through problems. You must do the work, though.

Do you have a specific question about how to compute the component values?

#### Communications Student

Joined May 3, 2007
10
Ok moderator I'm sorry but really I didn't mean to be tht lazy and tht dependent on others, so as it's my very first time to post in here, don't be too harsh on me

So guys plz I need ur help, those are the values I computed based upon the pdf file above attached by Mozikluv:

R1=6oo ohm, R2=R3=50 kohm, R4=95kohm,R5=483.33 ohm,R7 =7.1 kohm
Problem is I can't compute R6,plus in the second BJT I have very small base current : I base=1.5 micro A, I collector= I emitter = 0,45mA (as I took hFE=300) , and also I don't know the values of the capacitors(((((((((((((((

#### mozikluv

Joined Jan 22, 2004
1,435

Review your emitter bypass cap. lesson

biasing your second Q (pnp) is practically the same as your first Q

moz

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#### Communications Student

Joined May 3, 2007
10
Thankss so much MOz for ur kindess and ur help, I'll work it out today with the help of ur formulas, plus I asked my teacher assistant today at college, he told me tht i have to buy the BJT first, adn measure its hFE using the Avo-meter and not just assume it any value in my design.

#### Communications Student

Joined May 3, 2007
10
in the formulas u sent me : Av = Vo/Vs = gm1 gm2(R4||rπ2 ) (R7||R8 ) ( R2/(R2 +R1))
Input resistance = R1||R2||r pi 1

how would i compute gm which is the differentiation of Id over Vgs ( JFET) while the required project and the configuration u attached is a BJT ??
and can I use anozer BJT other than 2N3904 and 2N3906 ??

#### Communications Student

Joined May 3, 2007
10
Thankss in advance really u help me lots guyzz

#### mozikluv

Joined Jan 22, 2004
1,435
1. gm = ICQ / VT

2. yes you can sub that with other transistors

moz

#### mannooha

Joined May 14, 2007
1
for communication student:
first: Dr\M.Mesel7y will be very sad
second: gm=1/re

#### Communications Student

Joined May 3, 2007
10
Thankss Moz

#### omar_megane

Joined May 16, 2007
1
The pdf attached contains a 2-stage CE BJTS with gain=10 and Rl=10k and a very good Q-point position.
I have a question, What is the advantage of putting the so called "swamping resistance" in the emitter side of the CE BJT amplifiers???

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