Hi guys, im having a problem with impedance matching of the mp3 player and the design of my amplifier. i know how to design an amplifier coz im quited good in the discussion but i cant start designing without matching the impedances.

anyway, heres my problem:

I was informed that the output impedance of an mp3 player ranges from 16-32 ohms. Therefore i need an amplifier that has an input close to this value. The problem is i must used a JFET as an amplifier because it is the requirement of my teacher. I cant lower down the input impedance of the JFET to match the impedance of the mp3 player.... ( This is my first amplifier project. If you have techniques on how to solve this problem, please inform me. THANKS!

PLEASE HELP!

 

You don't need to match impedance in this situation. The time to match impedance is when you are trying to transfer the maximum power as in RF or very low level transducers. Any Audio amplifier will have a high enough input impedance to work.

 

The MP3 player probably won't be much affected by a higher load impedance, but if you want to make sure, why not just connect a resistor across the amplifier input. Something like 27Ω 1/2 watt should cover it.

 

thanks jaclement.. but i had a follow up question so i can start now my design.....

my question is: what must be the range for the input impedance of my amplifier??? should i go with self bias configuration with an input impedance of 1MegaOhm??? i mean does the input impedance of my amplifier doesnt have an effect on the quality of the sound???...

anyway, thanks for the time.. )

 

@Adjuster: is it okay if i connect 27 ohms to the "gate resistor" in my self bias configuration???

 

It's probably not critical. If you bother with the resistor at all, you might be better to put the resistor right at the input of the amplifier. Probably you should then have a small blocking capacitor, just in case the player is one of the "el cheapo" efforts that puts a bit of DC on the output (unlikely, but let's cover it). Then you would have whatever gate bias resistor you were going to have anyway.

Providing a "correct" load is probably not so important here, but if you do leave off the 27Ω, that blocking capacitor becomes a bit more important. Commonly, the output from a small amplifier as in an MP3 player is a pretty big electrolytic capacitor. Your 1MΩ (or whatever) gate bias circuit might have trouble dealing with the charge and leakage from that.

So: 27Ω in parallel with the input (optional), then a DC blocking capacitor, then your bias resistor.

 

If you use a dc blocking capacitor, you should load the circuit after the cap so that you don't have a long time constant for whatever dc is beng blocked by that cap. Otherwise, you may find the input of the amp is blocked(biased to saturation by that dc offset) until the cap discharges. Try for a time constant compatible with the low frequency response of your loudspeakers.