Why it's so important to know the input and output impedance of a circuit (for max voltage transfer)

[@danielraducu9073]

i just love your channel, please lean towards rf electronics, my biggest dream is to be able to design radios and calculate values for components. A superheterodyne from top to bottom explained, from the input band, transistors impedance and impedance calculated for if filters would be a great series ... please try if you can...

[@electronicsainteasy5253]

Thanks for watching. I guess I could go into that. The good thing about superheterodyne radios is that any book on electronics will have a good description of what's involved. Of course, it won't have all the details on how to design the whole thing but all the steps will be explained. Then, it's a matter of studying each stage and designing each stage. I was gonna go into a different direction, like designing a synthesizer, but designing a radio is cool too. Food for thought, as they say.

[@bitlong4669]

So if my load is 4ohm speaker, then Rout needs to be less than 0.4ohm? Let’s say it’s a 10W speaker. So voltage to drive it would be v=sqrt(PR)=~6V. Therefore amplifying transistor needs to handle i=6/0.4=15Amps….. is my thought process correct?

[@electronicsainteasy5253]

For 10 watts you need about 5 amps to drive a 4 ohm speaker. So you would need a power transistor. Google 10 watt power amp transistor and you should find proper transistor to use then look at the datasheet for that Q and you might also get some sample schematics on the ds. Instead of Q, you can use ic like the LM 1875.

[@bitlong4669]

@@electronicsainteasy5253thanks. Just looked it up. Playing with bjts was more for education purposes also got quite a few on hands including them beefy ones. I like the 1875 too, will pick it up next time I’m ordering parts. What about using mosfets(or any fets for that matter)? They have high input impedance too right? Are they used for audio?

[@electronicsainteasy5253]

Note that what the video describes is for maximum voltage transfer, not necessarily maximum power transfer as the 2 are mutually exclusive. In the case of a speaker as load, you actually want max power transfer, not max voltage transfer. To accomplish max power transfer, you need a buffer amplifier (a voltage follower, basically) with high input impedance and low output impedance. That way, having a relatively high impedance at the output of your circuit (feeding into the speaker) is not an issue anymore. You could use a transformer too to act as a buffer. I talk about that in one of the videos about tube amps.