Dear Parmin
That was an informative post as usual; it briefs the important factors affecting the design of audio amplifier.
Dear kang2008
As small as you can observe, hearing aids are manufactured nowadays (as you can see in the illustration above) based on deferent techniques of ear insertion and all are small enough to fit in 5mm x 5mm.
Hearing aid electronics are custom manufactured and
they are not made out of on shelf component. If you are interested in how they work
(without thinking of making one yourself) , I can go through explaining more details about them.
For Example: The top of the line will contain a digital filter that you can be programed using a PC to get a 5 band equalizer. This is important because hearing problems usually come with irregular response that needs to be equalized to flat response over the audio frequency rang.
Thank you ...
Faros