I have to make 3 projects this sem-
1)to learn soldering techniques(I chose opamp as integrator)
2)analog 3)digital
In analog, I decided to make hearing aid. Ckt. diag. can be found at:"http://www.electronicsforu.com/electronicsforu/lab/ad.asp?url=/EFYLinux/circuit/aug2003/CI-2-hearing-aid.pdf&title=Low-Cost%20Hearing%20Aid".(Pls check the ckt. from there. I don't know to upload an image.) I am thinking to make a volume control as an addition in it. I checked out the gain practically. The 1st stage is the pre-amplifier(My sir told that the initial amplifier stage(s) having less gain are said to be preamplifier).So, I have thought to add a pot b/w C2 & R4(middle terminal to c2, left to gnd & right to r4).
Q1.Now, whether I sholud use a linear/log pot & of what value?(pls expln). 2.Also, in the ckt. explntn, it is given that c4-r6 & c5 are used to decouple the power. what does this decoupling means?
3.Also, in theory it is taught that pd biasing is used in amplifiers. But here it is nowhere seen. Instead fixed biased(least stable biasing technique) & c-b biasing are used.what nay be the reason?
4.although the o/p of 2nd stage is more than 3V for an i/p of (say) 5-10mv, why the next 2 stages are used?
5.Also the last stage is a pnp, why?
PLs explain these things. thanx in advance!!!

 

Don't make that project. It is extremely poorly designed.
Its second and third transistors are biased the way we were taught never to bias a transistor like that.

The transistors do not have emitter resistors so each transistor must be selected for beta for them to work and the temperature must not change.
The output has DC in the earphone that might damage it.
There will be horrible distortion.

Make one of these circuits instead since they are properly designed:
http://www.talkingelectronics.com/projects/HearingAid-2/HearingAid-2.html

 

The term "Decouple" in small signal amplifiers refers to removing any DC Bias linking between stages by feeding the signal from one stage to the next through a device that doesn't allow DC voltage to pass, such as a series capacitor.

I would also suggest the circuit posted by Audioguru instead of the one originally attached.