Impedance matching question. - Page 1
Forum section: electronics / projects

author: regalzack
author_rank: Contributor
timestamp: January 22, 2014, 04:00:58 am
content: Trying to learn as much as possible(beginning EE student) I appreciate the help!So I recently purchased a Gentek SFG-1013 <http://www.gwinstek.com/en/product/productdetail.aspx?pid=5&mid=72&id=94>The source impedance is 50ohms and I would like to be able to attach it to an 8ohms source(speaker)Would it be a good idea to use a transformer to match impedance.(rather than creating parallel resistance)Assuming that it would be worth while, would these calculations be correct?I found a formula online Rs=(N1/N2)^2 *RlWhereRs=Source resistanceRl= Load resistance(N1/N2)=prim/sec turns ratiotells me that to match 50ohm source to an 8ohm load (at optimum efficiency) would require a 2.5:1 transformer.Would there be any disadvantage of getting say a 4:1 transformer and undershooting the source impedance a little?Thanks you so much!Zachary Herberholz

author: Mechatrommer
author_rank: Super Contributor
timestamp: January 22, 2014, 11:54:41 am
content: why do you concern at power loss due to impedance mismatch in a speaker? i know nothing about it but wont be a simple sound or speaker amplifier do the job?

author: grumpydoc
author_rank: Super Contributor
timestamp: January 22, 2014, 01:45:19 pm
content: Connecting a function generator to a loudspeaker directly seems an odd thing to do somehow.As Mechatrommer says if you want to do this building a small audio amplifier would be best - say an LM386 based one. You won't need too much power - you won't want to listen to loud pure tones for long.If you just want some indication that your sig gen works then put a 39 or 43 ohm 0.5W resistor in series with the 'speaker (less for impedance matching and more to make sure you don't damage the output of the generator). You'll get just under 50mW at the 'speaker but that should be enough to make a small loudspeaker make a noise.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: January 22, 2014, 01:47:54 pm
content: Oh gee.If your primary goal is learning something, then you're already on the right track.If you want to listen to the sound of your function generator, via an 8 ohm speaker. (up to 20khz only),then I would get an audio amp and put that in between.But be careful as I am sure by twiddling the wrong knob you will blow something.

author: regalzack
author_rank: Contributor
timestamp: January 22, 2014, 02:52:53 pm
content: Great info, thanks everyone.Yeah, my primary goals is to learn, I need to dabble with things and familiarize myself with function generators.The last thing I want to do is cause any damage to equipment.How do I avoid this by the way?------As far as a lm386 amp. I have one that I am trying to troubleshoot, if anyone feels up to it here is a link to the posthttps://www.eevblog.com/forum/projects/lm386-audio-amp-troubleshoot/Thanks you all so much,Zachary Herberholz

author: Rufus
author_rank: Super Contributor
timestamp: January 22, 2014, 07:05:52 pm
content: Quote from: regalzack on January 22, 2014, 02:52:53 pmIt would have to be a pretty crappy signal generator to be damaged by connecting an 8 ohm speaker load.The voltage across the speaker will be reduced. 1v out of the generator will produce 1 * 8/(8 + 50) volts across the speaker. The generator may already assume a 50 ohm load so 1v set on the generator is actually 2 volts into an open circuit and would be 2 * 8/(8 + 50) volts into your speaker. The generator may have a setting for what load is assumed.The speaker won't be exactly 8 ohms and what it is will vary a lot with frequency, by measuring the actual voltage you can calculate what the speaker impedance is at particular frequencies.

author: G0HZU
author_rank: Super Contributor
timestamp: January 22, 2014, 08:39:38 pm
content: I'm not really into hifi etc but aren't you supposed to feed the speaker from a (near) voltage source rather than try to match it?So I would agree that you need to feed the speaker from an audio amp with feedback etc to give an extremely low output impedance. eg ballpark 0.1 ohm or less.A crude alternative would be to use a strongly biased emitter follower as a simple active buffer with lo Z output but this will give a fair bit of distortion.

author: dannyf
author_rank: Super Contributor
timestamp: January 22, 2014, 09:06:48 pm
content: QuoteGenerally yes, to maximize the energy transferred to the load.The output impedance is generally thought to be serial, btw.QuoteTrue. But do you need to match the impedance?Quotevs. a 2.5:1 transformer? less output power under the same drive.I think you may want to think what you are trying to do first. Most signal generators are not designed to drive a tough load like a speaker but they can drive them safely (not incredibly efficient, given their output impedance).