hi all,

i had already designed a microphone amplifier from two op amp stages with finally an output stage (common collector)

the problem is the impedance of the loudspeaker is 8 ohm,so when i placed it as R load of the common collector it works fine but with dc shift in the output so i put a capacitor in series with it to remove dc but when i put that capacitor the output gone and be unpredictable

this is the schematic of the circuit from multisim

 

You have the final transistor biased to only a couple of mA emitter of current. The level obtainable into an eight ohm load will only be a matter of millivolts before the onset of severe distortion. At this low current, the output impedance at this current will be more eight ohms, so the gain will also be reduced.

It would be better to bias the transistor for at least some tens of mA, but this would increase the power dissipated. A small transistor may only be rated for a few hundred mW. Preferably, use a complementary class AB output stage with decently rated transistors.

 

Thank you very much for Your replay...

i will try to increase Ie (the emitter current bias) and see the result,but if it won't work i will replace it with class AB output stage as you mentioned.

i will post here my results,
Thank you

 

The otehr problem is the 2k emitter resistor, it should be much less.
The transistor sources the current but teh 2k resistor gets rid of it and the difference in impedance is big.

 

A pair of complementary emitter-follower output power transistors have an output impedance of 1 ohm before negative feedback is added which reduces the amplifier's output impedance to 0.04 ohms or less. I attach the circuit.

Your first opamp is inverting with an input impedance of only 330 ohms which shorts the signal from the microphone. The opamp should be non-inverting with an input impedance of at least 10k ohms.
An electret mic is powered with a 10k resistor from a filtered positive 9V, not with only 1k and not with no filtering.

 

The otehr problem is the 2k emitter resistor, it should be much less.
The transistor sources the current but teh 2k resistor gets rid of it and the difference in impedance is big.

The resistor value is the problem*. The transistor is configured as an emitter follower, which gives a quite predictable voltage between the emitter and ground. The resistor connected between the emitter and ground defines the quiescent emitter current.

It is incorrect to say that "the transistor sources the current and the resistor gets rid of it", as the transistor is in series with the resistor. The same current must therefore flow through both devices (a capacitor blocks DC current from going to the loudspeaker). Your statement could only be right if the transistor was passing a large current, some of which was diverted away by a resistor in a parallel branching circuit.

Edit: *Or rather, it is the chief reason why the circuit works so very badly as it does. A simple emitter-follower with a resistor load is not good for this function anyway.

 

Thank you

I connect this output stage at the end , but i increase the output from the second opamp stage to be 2Volt in order to gain 1Volt on the 8ohm Resistor..

This is the output stage i used:

if there is any modification please reply me.

Thank you all specially Adjuster

 

1) You selected 100mA max transistors and 1A max rectifier diodes. They don't match so the output will have crossover distortion.
2) The output transistors also don't match since the "B" has much more current gain than the "A". So there will be even-harmonics distortion.
3) The little transistors have such a low max allowed current that the output to an 8 ohm speaker is 100mA peak which is only 0.04W RMS (which is almost nothing).
4) The small value 100uF output capacitor reduces bass frequencies below 200Hz which might be OK for speech.
5) You do not need the output capacitor since you have a dual polarity supply so there is no DC current in the speaker.
6) The very old opamp is spec'd with a minimum supply of plus and minus 5V but your supply is less. Why? If you use 4.5V batteries then they will drop to only 3V during their life which is much too low for that old opamp.

 

Thanks Audioguru

I tried to fix the problems that you mentioned and i found an IC which include that AB class on it and ready for use because i didn't find two transistors npn,pnp with the same Q-point.

this IC called L293D which include push pull amplifier.

 

An L293 is not an amplifier. It has 4 push-pull switches inside. A switch is not an amplifier.

Examples of class-AB amplifiers are the TDA2003 for cars and the LM386 low power amplifier powered by a 6V or 9V battery. There are hundreds of amplifier ICs available.

Amplifiers are usually DC-coupled (no internal coupling capacitors).

Here is a simple low power amplifier. With a 6V supply its max output is 0.32W into 8 ohms.