Clearly, the guy who wrote the requirements for the project has a Master's Degree in vague writing:

In this project, your goal is to design and implement an audio amplifier by using basic components like
transistors and resistors.

I guess it would have taken a extra two minutes to actually write what the specs are, like perhaps saying only discrete components and no integrated devices.

Can't use op-amps? I don't know how much more basic you can get than an op-amp....

whatever. Interesting the students have to design the circuit using stone knives and flint rock tools but are expected to submit a Spice model of the circuit.....

 

[QUOTE=sbin19;549383]Maybe I did not express myself clearly.

1V AC input (music is given from mp3 player or computer ) .

We need to amplify this sound as we drive 8 ohm speaker ( at least 6W is wanted)

I am sure that at the output stage will be Class AB amplifier. Thats my choice as it has no distortion. However, at the input of Class AB ( or any Class x) I need 20Vpp AC signal so that at the load 6Watt condition is satisfied.

The problem is that I do not know how to amplifiy input signal ( from mp3) to 20Vpp AC by using standart CE or CS amplifiers. I do not think it is possible because according to my knowledge CS amplifer has a small signal condition. If input is larger than some value, say 200mV , then distortion occurs. Am I wrong?


I need another way maybe..

The project is here , 7th project, if you interested.

http://www.mems.eee.metu.edu.tr/courses/ee313/Project combined.pdf

Thank you all..[/QUOTE]

Your "knowledge" is wrong !! You have a fundamental misunderstanding of the operation of these devices.
Some intensive revision of Semiconductor operating conditions will set you on the right track.

CE & CS amplifier stages may be made to operate with very much greater input voltages than those you quote.

Complete commercial multi stage Audio Amplifiers may normally be designed to operate with quite low input voltages,but this is because many audio sources produce quite small voltages.
If you need an Amplifier which will be driven by a higher input voltage level,you would normally use less stages.

 

Clearly, the guy who wrote the requirements for the project has a Master's Degree in vague writing:

I guess it would have taken a extra two minutes to actually write what the specs are, like perhaps saying only discrete components and no integrated devices.

Can't use op-amps? I don't know how much more basic you can get than an op-amp....

whatever. Interesting the students have to design the circuit using stone knives and flint rock tools but are expected to submit a Spice model of the circuit.....

Project (1) is supposedly for a "Gilbert Cell",which they refer to as a "variable gain amplifier".

I suppose it is a fair enough description of a Gilbert Cell,but it completely misses the most important thing about such a circuit,which is its usual use as a mixer/frequency converter.

I guess this is left for the student to find out,but it seems a bit unfair.

 

First of all, I would like to thank you all for helping and your feedbacks. I am also sorry as I did not give the specifications clearly.

In the lab, there is a 12V DC supply available. In this project, opamp is available but it is not recommended. Everything must be as simple as possible. I should use basic components. Transistors, resistors and so on. I actually do not know how music stuff is working but I feel that I need to get low distorted signal at the end to hear qualified music.

Is this what you're thinking you both have to do and won't work because of distortion?

In the class, as I remember, we saw that some conditions so that we can use small signal models to BJT and MOS single stage amplifier circuits. Above this specific conditions, single stage was not working properly and gave distortion ( maybe I knew wrong , that is why I am asking)

In BJT; Vbe<5mV is the small signal (ss) condition.
In MOS; Vi < 0.2/(VGS-Vt) is the condition. Vi< 200mv

By the way, I have just learned what is feedback. ( I will study differential amplifier in two days)

All in all, If the my ss knowledge is wrong, then I should correct them. You claim that single stage amplifer (bjt or mos) can give 20Vpp sine signal at its output. Am I right?

Againt thank you all for helping..

 

Yes, a single stage bjt amplifier can give 20 volts p-p output, or 40, or 60. It just depends on which transistor and what you give it to work with.

It is still very unclear why you think a BJT transistor can not be allowed to have an input more than a few millivolts. The emitter of a bjt will always be a few tenths of a volt different from the base voltage. That is the voltage that matters. When you apply a signal to the base, the emitter voltage will follow the base voltage in either direction. Only when this doesn't happen do you get distortion, and that is a function of the circuit that you place around the bjt.

Now I am going to add a drawing of a low distortion transistor amplifier that works with 20 volts AC applied to its base.

 

The emitter resistor provides negative feedback so that the input can be a very high signal level. Then the gain is very low and the distortion is also very low.

The collector can have a resistor in series and connected to the positive supply and can provide voltage gain.

 

Yes, a single stage bjt amplifier can give 20 volts p-p output, or 40, or 60. It just depends on which transistor and what you give it to work with.

It is still very unclear why you think a BJT transistor can not be allowed to have an input more than a few millivolts. The emitter of a bjt will always be a few tenths of a volt different from the base voltage. That is the voltage that matters. When you apply a signal to the base, the emitter voltage will follow the base voltage in either direction. Only when this doesn't happen do you get distortion, and that is a function of the circuit that you place around the bjt.

Now I am going to add a drawing of a low distortion transistor amplifier that works with 20 volts AC applied to its base.

I was a bit bemused,too!

It is obvious to us that BJTs & FETS are used every day to amplify quite large voltages,so my first reaction was to think;"How could he have come by that impression"?

I believe the following is close to his thought process:-

"The transistor is biased "on" with 0.7V between base & emitter,so it seems intuitive to assume that a signal of more than a few millivolts will cause cutoff on the negative half cycle,thus,limiting the signal level the device can amplify"

The fallacy here is that the input signal in a real amplifier with an emitter resistor is impressed across the base-emitter junction & the emitter resistor in series,with the base to emitter voltage remaining at 0.7V approx.
( Of course this is just reiterating what you have already pointed out)

I am not going to attempt an in-depth analysis here,as it's too long since I did this stuff,but once the OP understands his basic misunderstanding,the sooner he can move on.

 

I think what OP means is that, the more feedback you introduce to the emitter of a common emitter amplifier by increasing the emitter resistance, and decreasing the vbe small signal voltage, smaller of a gain you would obtain out of the system, though the input signal range would be increased due to the smaller vbe. However, since he has to preserve both the gain an the input signal range at the same time, he cannot increase the emitter resistance too much to improve the input signal range, since he would be losing from the gain by the same amount (amount of feedback = 1+gmRc).

 

A transistor with high gain has severe distortion. Adding an emitter resistor reduces gain and reduces distortion.
Divide the total gain between two transistors and each transistor can have an emitter resistor for high total gain, good bias stability and low distortion.
Or use an opamp that uses many transistors for very high gain and very low distortion.