# Common Emitter Amplifier

Discussion in 'Homework Help' started by VVS, Dec 9, 2007.

1. ### VVS Thread Starter Active Member

Jul 22, 2007
66
0
Hi i have a feq questions regarding the Commen Emitter Amplifier.
Could some1 please explain to me why the first circuit is not suitable for biasing,
why the 2nd circuit is and what the function of the bypass capacitor is in the 3rd circuit?
I tried to find information about it in the internet and books, but I couldnt find anything useful.
thanx
VVS

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2. ### beenthere Retired Moderator

Apr 20, 2004
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1. The circuit is biased. What do you mean by "not suitable"?

2. The 2nd circuit is biased, but has no signal input.

3. In the tube days, the emitter bypass cap would have been called an AC neutralization capacitor. Given the effect on Xc with increasing frequency, what do you think it might be doing?

3. ### cljohnson72 New Member

Dec 11, 2007
3
0
I am studying this topic now.

My best guesses:

1. No voltage divider for VB

2. Voltage divider bias

3. The bypass capacitor shorts the signal around the emitter resistor, R1, in order to increase the voltage gain.

Without bypass capacitor : Av = R2/re+R1 where re = 25mv/IE
With bypass capacitor : Av = R2/re

4. ### sndpgr Member

Jun 22, 2006
23
0
1.the circuit is too much dependent on the value of 'beta' , calculate the emitter current and vary the value for 50<beta<150 you'll know

2. the 2nd circuit is called the voltage divider bias.if that's what you were asking.

3.The bypass capacitor helps in increasing the voltage gain at signal frequencies while allowing Re to take over at DC i.e provide stable biasing

5. ### VVS Thread Starter Active Member

Jul 22, 2007
66
0
i really have no idea what the capacitor might be doing. i understand that the AC component would go through it at low frequencies and neglect the Resistor. But apart from that i have no idea. How does shorting the emitter resistor increase the voltage gain??
and sndpgr. what should i vary from 50 to 150?? And how does the emitter resistor alone improve biasing?? thanx for your comments people!! =)

6. ### bbroach New Member

Dec 12, 2007
3
0
May I suggest a good circuit text for beginners? It's targeted at bench techs and not strictly rigorous, but it's fundamentally sound & will give you a good feel for BJTs and basic principles like feedback and frequency response without burying you in math & device physics.

Practical Circuit Analysis of Amplifiers, by Lorne MacDonald

regards, billb....

7. ### sndpgr Member

Jun 22, 2006
23
0
Al right, the emitter resistor is there to stabilize the circuits as beta variations,but R1(emitter amplifier) decreases the gain to R2/(R1+re),so we short circuit the resistor at signal frequencies to keep the gain R2/re.

Sorry you probably got me wrong vary the value of beta in 1st transistor in a range ,as an example 50-150 and see how it changes the value if Ie
How does emitter resistor alone bias the circuit?

well, if you calculate Ie in ckt '1' the value is (beta+1)(Vcc-Vbe)/Rb and if you add R1 it becomes (Vcc-Vbe)/(R1+Rb/(beta+1)). I hope this will clear every thing.

Or you can refer to Boylestead chapter 4 called DC biasing.

Don't worry I myself had trouble understanding these things.

8. ### VVS Thread Starter Active Member

Jul 22, 2007
66
0
cheers thanx a lot!!