How do I choose the collector resistor for a single stage CE amplifier?

[Danish_Khatri]

I have been given a basic electronics project for design of a single stage common emitter amplifier. The problem is that this the first time I am going to carry out such an assignment and have no idea how to start. I do not know how to make choice for the resistors and I need help in this regards.

Thanks.

 

[Studiot]

Why not start with the CE amp we discussed in an earlier thread?

https://www.physicsforums.com/showthread.php?t=401308&highlight=transistor

 

[uart]

State what type of signal you want to amplify. What source? What load? Then it is easy to help you.

If the above has not yet been specified and you get to choose, then a very good choice would be to make a simple line-out to head-phones amplifier. This is a good choice because :

1. It's fairly easy.
2. You can get a workable unit with just a single CE stage (per channel) and though it won't be up to audiophile standards you should be able to make something that works and dosen't sound too bad (if you design it right, and if you don't design it right it will sound terrible).

 

[Danish_Khatri]

dear I have posted this question on that thread. let's continue the dicussion there.

 

[Danish_Khatri]

Thanks for your help uart. This is a nice method. However, first step in the design of amplifier will be to choose the operating point for it. what should be the criterion for choice of operating point.
My idea is that I use characteristic curves for the transistor i m going to use (that is 2N3904), choose a particular Q point and design the circuit for corresponding values of collector current and base current.
Is there a better approach than this one?

 

[uart]

Thanks for your help uart. This is a nice method. However, first step in the design of amplifier will be to choose the operating point for it. what should be the criterion for choice of operating point.
My idea is that I use characteristic curves for the transistor i m going to use (that is 2N3904), choose a particular Q point and design the circuit for corresponding values of collector current and base current.
Is there a better approach than this one?

That's ok but you should choose your operating point with respect to the actual load impedance that you're driving. If your goal is just to hook up a signal generator and observe the result on a scope then you can choose just about any operating point you want. But if you want a real circuit driving a real load with given target gain, source and load resistance and input and output levels then you should consider those things when choosing an operating point.

I see you're using a 2N3904, one of the BJT's in the student PSpice evaluation library. Does this mean that you are only going to do it in spice or are you actually going to build the circuit?

 

[Danish_Khatri]

I am actually going to build the curcuit and my objective will be to observe the output on osscilloscope not driving a speaker or something else.

 

[uart]

It would still be a good idea to have target values for input resistance, load resistance, voltage levels (swings) and voltage gain in mind before starting.

 

[Studiot]

The other thread was about saturation. This time you will be operating the transistor in the active region (hopefully).

It is possible to start simple and build up to your complete amp, considering the reasons why each component is there and what its value should be. This will lead to a good understanding of how the circuit works, as well as or instead of a lot of maths.
A good way to design an amp is to separate the DC and the AC (signal) analyses and I propose that approach here.

Danish
In the other thread I put up a quick sketch for discussion. Are you in a position to add sketches? If not I will do some.

Uart
There is plenty of scope for cooperation in this enterprise.

 

[Danish_Khatri]

Studiot
I am now well aware of analyses of a transistor circuit. only thing I am facing problem with is that what should be the criterion for choice of the Q point. Once I am clear with this thing, I am sure I can design the complete circuit.

 

[Studiot]

As you wish.

The choice of collector resistor depends upon whether you want current output or voltage output, and further the size of the voltage swing required.

If a current output is desired then R_c must be large compared with the external load resistance.

If a voltage output is required then R_c must be small compared with the external load resistance.

The larger you can make R_c the larger will be the voltage gain.

R_c is selected to set the desired quiescent point and everything else follows. You can plot the R_c load line on the collector current / collector voltage characteritic curves. For example to maximise the voltage swing set the Q point midway between the rails.