# Common Emitter Class A Amplifier

Discussion in 'Homework Help' started by pkrcat, Dec 11, 2009.

1. ### pkrcat Thread Starter New Member

Dec 7, 2009
4
0
Hi there everybody, I'm doing a course on electronics and I've been landed with an assignment I'm really stumped by. I've put quite a lot of time into trying to understand this but as I'm working off dribs and drabs from my college tutor along with some vague web tutorials I'm kind of at a loose end.

I'll post what I've got so far along with a schematic

The task is to design and built a common emitter class A amplifier using the
BJT Q2N2222 transistor and the following specifications:

Vcc = 18v
Lower cut-off frequency must be 210 Hz
Upper cut-off frequency must be 500 kHz
Gain β = 95
Rout = 17kΩ
Vin = 7mVrms
The collector current at operating point Icq is 10mA

I've tried the following calculations but the results indicate they seem to have gone quite badly wrong. .

Vce = 0.5 x Vcc = 9V

Ic = Vcc - Vce / Rout
= 18-9 / 17000
=5.29 e-4 A

I've been told to use Ve = 3V, I really have no idea why, I'd like to know where this comes from

Vout = Gv x Vin
= 95 x 7e-3
=0.665Vrms
=0.940 Vpeak

Total voltage swing = 0.94 x 2 = 1.88V peak to peak

Rc = Vcc - Ve - Vce / Ic
= (18 - 3 - 9) / 10e-3
=600Ω

Re = Vce / Ic
= 3 / (10e-3)
=300Ω

Next I need to calculate IBq, I've been told that you should use the maximum gain for the transistor being used, 180, giving the equation;

IBq = 10e-3/180 = 5.56e-5

Unfortunately this is where it all goes pear shaped and my R1 and R2 values are a bit crazy.

Below are screenshots of what I simulated, even if they are way, way off I'll just put them here anyway

http://yfrog.com/jascreenshotczj
http://yfrog.com/j0schematicj

Sorry about the long post, any help with this would be very greatly appreciated

Thanks guys

Last edited: Dec 12, 2009
2. ### hobbyist AAC Fanatic!

Aug 10, 2008
885
87
I'm not very much help compared to other more experianced posters on this board.

But I'll try to get you on the right track, since you said your way off base with this at the moment.

So here goes:

First your RL is 17K ohms, that would be the Load itself, not the output impedance.

Your VC should be 9v. (half the supply voltage) which is the voltage from ground to the collector terminal, when using a emitter resistor for stabilization, then the VC will be (VCE + VE), = 9v.

So your RC (collector resistor) shouild be around 10 x less so as to develope a substantial signal across the load. (less is better, if feasible with respect to the gain needed)

According to the parameters given the ICQ is given so RC will be a set value calculated by (Vc / ICQ)
where VC = (1/2 x VCC)

Once you get the RC established then using the VE of 3v. specified should calculate out for the emitter resistor (RE) needed.

You may need to juggle the RC value to get a substantial gain with respect to the emitter resistor calculated.
You may have to juggle both resistor values to achieve proper gain and voltages needed.

Only calculations and prototyping will tell.

If you use voltage divider biasing, then make the bottom ground resistor to base (RB1) around 10 x greater than RE.

Heres how I prototyped it:

VC = (VCC / 2)
ICQ = 10mA.
RC = (VC / ICQ)
VE = 3v.
RE = (VE / ICQ)
RB1 = (10 x RE)
VB = (VE + Vbe)
ID = (VB / RB1)
RB2 = {(VCC - VB) / ID}

This will give a very small DC GAIN (RC / RE)

Then calculate the proper value Capacitor needed to bypass the emitter resistor (RE), so as to aquire the proper Vout across the load. (within the bandwidth specified)

see if this helps.

Last edited: Dec 11, 2009
3. ### pkrcat Thread Starter New Member

Dec 7, 2009
4
0
Thanks for your reply, I should have mentioned that RL is the output impedence as it is being used to feed a second stage, it is at that that point the dc gain needs to be 95

4. ### Audioguru Expert

Dec 20, 2007
10,794
1,216
The beta of the transistor is said to be 95, not the DC gain of the entire circuit.
The circuit is an AC amplifier, not a DC amplifier.

RL is a load impedance, not an output impedance. RL is probably the input impedance of the circuit that is driven from this transistor.

5. ### pkrcat Thread Starter New Member

Dec 7, 2009
4
0
Thanks, β= 95 is what I was trying to say, confusing myself here.

The output impedence is definitely meant to be 17kΩ as seen in the screenshot. I shouldn't have called it RL, I'll refer to it as Rout from now on. This is what the end product should more or less look like

6. ### pkrcat Thread Starter New Member

Dec 7, 2009
4
0
Ok, I've tried using the calculations above but my trace isn't looking pretty at all, I have the values below

VAC = 0.7mV
Vcc = 18V
Vc = 9V
Ve = 3V

β = 95

Rout = 17k
Rc = 900Ω
Re = 300Ω

RB1 = 3kΩ
RB2 = 12kΩ

ICQ = 10mA
ID = 1.2mA
VBE = 0.6V
VB = 3.6V

C1, C2 = 7.58 e-5F
CE = 5.05e-5F

7. ### hobbyist AAC Fanatic!

Aug 10, 2008
885
87
Your problem is in the base resistors
there backwards 3K should be the bottom and 12K the top resistor.

When I said RB1 and RB2, that was the nomenclatures I gave them.

That's where you got messed up, your simulator put the R1 and R2 in different locations.
The base to ground resistor should always be smaller than the base to supply resistor, for a linear amp stage.

A good procedure is after a stage is built, check the voltage bias at both the collector terminal and the base terminal, to see if there close to calculated values.

Last edited: Dec 12, 2009