# hearing aid

Discussion in 'General Electronics Chat' started by theertham, May 1, 2012.

1. ### theertham Thread Starter New Member

Apr 28, 2012
26
0
hi
can you explain how to calculate voltage gain of each stage...... i have tried a lot to calculate but in many books they have given difficult formulas and many assumption so i couldnt do it....actually i have tried with hardware i am getting sine wave for very low input and also able to hear using mic and earphone....... but i dont know why there is a phase shift in the last output stage about 90° phase shift.........

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2. ### crutschow Expert

Mar 14, 2008
22,802
6,750
That is not a good design since there bias point of the transistors depends upon their gain. Normally you want a bias circuit with an emitter resistor that stabilizes the bias point.

The typical current gain of each stage is the typical beta or Hfe of the transistor. Since the common emitter amp has a very low AC input impedance, you can just multiply the current gains of all the stages together to get the approximate total current gain. You multiply that by the earphone resistance to get the input current to output voltage gain. The input current is approximately equal to the input voltage divided by the transistor input impedance Rb (Rb = 0.025/Ic).

What is the frequency at which you measure the phase shift? Such a shift indicates you are near either the upper or lower -3dB point of the amp frequency response.

3. ### Audioguru Expert

Dec 20, 2007
11,217
1,330
That is the worst "amplifier" that I have ever seen.
It even wastes battery power by offsetting and heating the earphone with DC.
It has no volume control so it will probably have severe clipping distortion all the time.