MOSFET AMPLIFIER

[shaiqbashir]

Hi Guys!

i have posted my circuit below which u can see. I have two questions here:

1) Which MOSFET will be the most suitable one for this amplifier?
2)What will be the magnitude of the input signal generated by the microphone?


Let me clearify my questions to u!

well! the input signal source given in my circuit is actually a microphone.  I want to know that what is the magnitude of the greatest signal that can be generated by the microphone? how can i find that? how much my circuit can bear? my circuit gives the best result when it is not more than 0.3V. so how can i limit the microphone signal not to exceed the limit of 0.3V?

Please help me as soon as possible. and tell me some MOSFET which can replace this IRFE130 because it is not available here. Secondly! if i want to connect IRF840 or IRF510 in place of it, what changes i have to make?

Thanks in advance!

View attachment 37280

 

[audioguru2]

Hi Shaiq,
Instead of having a microphone preamp with plenty of voltage gain that is matched to the microphone, driving an efficient power amplifier also with voltage gain to drive a speaker, you have sketched a circuit without much voltage gain, that will draw a massive current from the supply, and will short-circuit its input. Its output capacitor is much too small to feed audio to a 4 ohm speaker.

1) The output level from a microphone depends on loudness (inside a drum is really loud!) and distance of the sound to the microphone. Talking at a normal voice level about 10cm from it would produce about 10mV into a 10k ohm load.
2) Your circuit has a load on the microphone of only about 15 ohms, a dead short!.
3) The 100 ohm resistor shorts the Mosfet's gate to its drain, biasing it so its DC drain voltage will be from about 3V to about 5V. Therefore the 10 ohm resistor will have 10V to 12V across it and the supply current is about 1.1A all the time. Audio power amplifiers usually use a complimentary transistor circuit biased in class-AB for low supply current when the sound is not very loud.
4) The 20uF output capacitor will cutoff frequencies to the 4 ohm speaker below 1818Hz.