Basics of Common Emitter amplifier.

[Averagesupernova]

It's a lot more defined than beta which can typically run from 50 to 200 with small signal transistors. Gain figured using R'e is more predictable. R'e is the AC impedance of the base-emitter junction. Take a handful of transistors with a well designed voltage divider biased class A setup using an emitter bypass capacitor and the AC gain will be fairly consistent from one transistor to the next to the next, etc... The beta can vary quite a lot and the AC gain will be consistent. As far as building things to see what happens, this is exactly what I did in school a long long time ago, this is how I know.
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Varying beta from one transistor to the next will more likely affect the Zin of the amplifier than anything else. And if the voltage divider bias resistors are set low enough they will 'swamp' this out. If the previous stage driving it has a Zout that is a decade or so below the targeted Zin then it won't likely matter.

 

[technician]

I agree with Averagesupernova ideas and approach

 

[sophiecentaur]

I am a practical man!... when I connect an ohm-meter (+ to base, - to emitter) I get a 'low' reading... more than zero but less than about 100Ω. I think this is a sort of resistance inside the little metal can (OC71, BC107, 2N3055...). I will bet it is on a data sheet somewhere.
When I build one CE stage with Rc = 500Ω and Re = 100Ω I know the DC gain is x5 (Rc/Re)
With a bypass capacitor the AC gain is much higher... a few (undefined) hundred... consistent with Rc/(my ohm-meter reading)
Defined or not... I know what I use. It is easy to test these ideas...build them and find out what happens.

If you measure the gain then that will tell you the effective value of r' - for that particular transistor. Another transistor could behave differently. That's why we use feedback in most cases because the feedback defines the gain much better.

 

[Averagesupernova]

When a 'swamping resistor' is used, which often is a couple hundred ohms or less in series with the main emitter resistor, it is used to make the gain even more stable yet, although reduced. From what I recall though, R'e is pretty stable from one transistor to the next. Although it may vary some depending on quiescent emitter current. If the amp is set up right, the quiescent current is also very stable from one transistor to the next.

 

[sophiecentaur]

I thought that "quiescent current" was a term used in Class B operation.

 

[shayaan_musta]

That diagram makes things easier, thanks.
R_e provides DC feedback which sets the current through the transistor. 'Looking into the base', that 220Ω will appear as approximately 220βΩ (say 22k), which will appear in parallel with R₁. Almost negligible but it changes the effective value of R1 to about about 2.4k, giving a base voltage of about +1.45V (do you see the potential divider chain?). That produces an current through R_e of (1.45-0.7)220 or 3mA (the 0.7V is the diode drop across the be junction). This 3mA , going through R_c gives 6V across it (the DC value).. Leaving the collector at 12V above ground.

Because of the capacitor C_e there is no AC feedback so the AC gain is not defined but, if you assume again the β is 100, then this is the best you can do. A reasonable voltage swing about the collector standing volts would be +-5V as you say. The actual voltage gain will depend on the value of R_L but assume the Load is ten times the valus of R_c and you can 'ignore it'. So an input of just 100mV peak to peak will appear as a voltage swing of 10V peak to peak at the collector.

Hello sophiecentaur!

Thanks for this.

But my answer is still there where I was started it newly.

Kindly help me out with three basics questions i.e.

firstly, why coupling capacitors, bypass capacitor and emitter resistor are used?
secondly, what do coupling capacitors, bypass capacitor and emitter resistor do?
thirdly, what would happen with output response if they will not at there desired place?

Its a request that help me out with this. So that I could design a simplest CE amplifier.

Thanks a lot.

 

[Averagesupernova]

Shayaan, based on my experience I would say after you have read this thread you should know the answers to these things. Mostly, the resistors are there to set up the proper Q-point. A coupling capacitor is there for the obvious, to couple 2 stages together. The emitter bypass I am not going to tell you specifically since it has been covered enough times in this thread so it should be obvious. Your answers in your first post are mostly correct although they are pretty generic. If I were an instructor asking those three questions and those were the answers I got you would not fail, but you certainly would not get a top grade. Now forgive me if you are not in school and these are just questions. However, if you are in school, the way you have asked these questions tells me that you have not been paying attention to the transsitor theory taught up to this point or you are going to a school that is doing a poor job of teaching it. My 2 cents.

 

[technician]

I agree with averageSupernova. There is more than enough information here to be able to design and build a simple amplifier.
Have a go at making one and see what happens by experimenting. Things will go wrong but that is the best way to learn.

 

[sophiecentaur]

You have asked what is the purpose of various components. Draw the circuit leaving them out (short or open circuit as appropriate) then work out what will happen to the various voltages around the circuit.

 

[shayaan_musta]

Shayaan, based on my experience I would say after you have read this thread you should know the answers to these things. Mostly, the resistors are there to set up the proper Q-point. A coupling capacitor is there for the obvious, to couple 2 stages together. The emitter bypass I am not going to tell you specifically since it has been covered enough times in this thread so it should be obvious. Your answers in your first post are mostly correct although they are pretty generic. If I were an instructor asking those three questions and those were the answers I got you would not fail, but you certainly would not get a top grade. Now forgive me if you are not in school and these are just questions. However, if you are in school, the way you have asked these questions tells me that you have not been paying attention to the transsitor theory taught up to this point or you are going to a school that is doing a poor job of teaching it. My 2 cents.

As you said that my answers are right. I think so. But when I asked it to an experienced electronic engineer(33 years in electronic engineering) he said me nothing is right and ask me to go and refer to book. Then I got confused and move to physics forums.

Thanks.

 

[skeptic2]

It might be worth mentioning that this is not the only type of common emitter amplifier. In fact it could be argued that this isn't a true common emitter because the emitter is not really common to both the input and output sides.

Attached is an example of another common emitter arrangement which may have some advantages over the other configuration depending on intended application. They are:

1. It tends to have higher gain than the version with an emitter resistor
2. It is more immune from power supply noise than the other version.
3. At RF this is the preferred version as it minimizes the potential for oscillation.