# Help with a common Emitter Amplifier

Discussion in 'Homework Help' started by janner, Nov 16, 2009.

1. ### janner Thread Starter New Member

Nov 15, 2009
6
0
Hi Guys,

This is my first post on here so first of all "Hello!"
Right down to business. I have to design a two stage common emitter amplifier with a gain of 30, I have been given loads of notes but Im needing someone to point me in the right direction with it. Firstly im trying to just design a single stage amplifier get it working then basically just join two together. I know that then I can get the gain by just using 1/beta in the feedback.

I have been told that I can choose the supply and then choose the Ic. I used a supply of 15V and then an Ic of 4mA. The formulas I have been given to get the rest of the circuit are as follows
1, Choose Ic I used: 4mA
2, Make Ve 10% of Vcc I calculated 1.5V
3, Re = Ve/Ic I calculated 375 ohms
4, Rc = Vcc/2Ic I calculated 1.875K ohms
5, R2 = 10 x Re I calculated 3.75K ohms
6, R1 = (Vcc-(Ve+0.7))x R2 I calculated 21.8K ohms
Ve+0.7

Ok so I have done all my calculations and then built the circuit on multisim. The circuit inverts the output but does not ampilfy it? The output voltage is extately the same magnitude as the input I have attached a copy of my circuit with the wave forms on it but if any other info is needed please just leave a message.

This does seem a monster first post but any help anyone could give me would be much appreciated. Thanks 2. ### hobbyist AAC Fanatic!

Aug 10, 2008
885
87
I must say this calculation is a new one to me.

But if it works that's good. (the equation that is)

VB / R2 = ID

and

(VCC - VB) / ID = R1

So some how I'll need to rearange these equations to see if I can get that #6, equation.

Be an interesting algebra problem.

3. ### janner Thread Starter New Member

Nov 15, 2009
6
0
Aye the equation is divided by (Ve+0.7) it just came out abit wrong when typing sorry.

I got it to work I just removed the RL resistor I had in the circuit. so thanks anyways guys.

4. ### hobbyist AAC Fanatic!

Aug 10, 2008
885
87
(VE + 0.7v.) = VB

so (VB / R2) = divider Curent.

The R1 value is {(VCC - VB) / divider current.}

Are you sure you got your R1 value right?

Maybe I need to rearange and substitute the equations to get what they gave you for R1 equation.

5. ### janner Thread Starter New Member

Nov 15, 2009
6
0
no sorry the calculation for R1 is

R1=

(Vcc-(Ve+0.7)) xR2
Ve+0.7

so I got =

(15-(1.5+0.7)) x 3750 = 21.8KΩ
1.5+0.7

Like i said I have the circuit amplifiying with a gain of 5 but I want to make it much bigger so I need to add in a Capacitor (Ce) so Im off to do more reading on how to do this beacuse as soon as I put one in the circuit my output goes very wrong!

6. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,349
730
Do you need an AC gain of 30 or a DC gain of 30?

The Gain of 5 that you are getting currently, is that an AC measurement?

7. ### janner Thread Starter New Member

Nov 15, 2009
6
0
AC I think?? its a AC signal source. I was just told a gain of 30.

I have measured my gain of 5 as AC

8. ### thatoneguy AAC Fanatic!

Feb 19, 2009
6,349
730
Can you post your schematic as it currently is?

A .png screenshot works the best.

9. ### janner Thread Starter New Member

Nov 15, 2009
6
0
That's it i think? File size:
46.6 KB
Views:
30
10. ### Audioguru Expert

Dec 20, 2007
10,789
1,212
The gain of the transistor without a load is slightly less than Rc/Re.
Increase the gain by increasing Rc or reducing Re then re-bias it.

You can also increase the gain without re-biasing it by connecting a resistor in series with a capacitor from the emitter to ground. This reduces the effective value of Re at AC.