small signal Amplifier help.

[yasir_ali]

Hi every one.
i am working on small project. I've designed an oscillator using op-amp. the problem is the op-amp can supply 20mA of current but i have to take current to 4A. so i have to use a current Amplifier. i have decided to use a series of common collector amplifiers as the current Amplifier. but the Problem is that i am getting 8v RMS with 20mAout form my oscillator. 20mA is OK but i can't just apply 8v across base-emitter junction of a transistor. i was thinking to use 2N3904 transistor as the amplifier but look at the data sheet. The data sheet tells us that it can only handle 6v at the base-emitter junction. so tell me how to find a transistor according to my requirement?

i am attaching the data sheet (TO-92 case)

 

[MikeMl]

What voltage (Peak-to-Peak) do you want across your 4A load? What frequency?

I'm thinking you need an audio power amplifier capable of several watts output?

 

[audioguru]

The max allowed output current of a little 2N3904 transistor is only 200mA and it performs poorly above 100mA.
Like most silicon transistors its reverse-biased emitter-base breaks down at 6V but most circuits never reverse-bias the junction, it is usually forward-biased.

A common-collector transistor has its input signal at its base and at ground, not at its base-emitter junction. Its emitter voltage will always be about 0.7V less than its base voltage for an NPN transistor. The 0.7V is forward-bias, not reverse-bias.
The transistor will get extremely hot with a current of 4A. Use a push-pull circuit instead like an audio amplifier IC.

 

[yasir_ali]

@ MikeMl
i want the output voltage same as it is at the input of the current amplifier stage. the stage should only amplify the current to 4A and leave the voltage at almost on the same point. the frequency of the operation is 300Hz.
@audioguru
thanks audioguru. But i will not use a single stage common collector Amplifier, but i was thinking to use a series of number of stages to get the required 4A. is it possible or should i use this push pull Amp?

Actually look at the form link, what i am working for.

https://www.electro-tech-online.com/threads/current-amplifier.105997/

in this form, in post number:26 Mr.Eric and I finally reached to a block diagram and i am going to design that circuit. so after looking at that you'll have a complete idea that what i am asking or looking for.


Thanks

 

[MikeMl]

So you want 8Vrms @ 4A, which is E*I= 32W.The load resistance is E/I = 8/4 = 2Ω.

I still say you need an audio power amp like the car audio freaks use for driving speakers.
Or get a power **broken link removed**, available at DigiKey for ONLY $138.00


You didn't answer the question about what frequency you are operating at?

 

[audioguru]

The high power output is AC so of course you need a push-pull amplifier.

 

[yasir_ali]

@ MikeMl
i told that the frequency of operation is 300Hz. and Thanks for you help.

@audioguru
Thanks Audioguru. i'll also work on this push-pull Amp.