Measuring output impedance - Page 1
Forum section: electronics / beginners

author: w2aew
author_rank: Super Contributor
timestamp: April 06, 2014, 07:18:52 pm
content: My latest video shows a few quick methods for measuring the output impedance of a function generator, amplifier, or other circuits. These methods are useful for low frequency applications, where the output impedance is basically resistive.Enjoy!

author: SeanB
author_rank: Super Contributor
timestamp: April 06, 2014, 07:20:56 pm
content: Very nice video. Thank you.

author: David Aurora
author_rank: Frequent Contributor
timestamp: April 07, 2014, 04:07:13 am
content: I've only just gotten onto your videos recently but they're awesome, thanks! Between yours and Dave's videos I've learned so much

author: BravoV
author_rank: Super Contributor
timestamp: April 07, 2014, 06:15:19 am
content: Alan, thanks, great video !Question, is this method applicable to measure power supply's output impedance ?

author: The Electrician
author_rank: Frequent Contributor
timestamp: April 07, 2014, 10:10:49 am
content: At about 9:39, the very last mathematical operation you perform on the calculator is divide, even as you are saying "multiply".

author: HighVoltage
author_rank: Super Contributor
timestamp: April 07, 2014, 12:07:35 pm
content: I like your calculator.Good old school tool.

author: hgg
author_rank: Frequent Contributor
timestamp: April 07, 2014, 12:13:10 pm
content: Nice and helpful video as always!

author: w2aew
author_rank: Super Contributor
timestamp: April 07, 2014, 12:47:12 pm
content: Quote from: The Electrician on April 07, 2014, 10:10:49 amYes - I mis-spoke. But the equation shown, and the calculator operation as shown, are correct.

author: w2aew
author_rank: Super Contributor
timestamp: April 07, 2014, 12:48:26 pm
content: Quote from: BravoV on April 07, 2014, 06:15:19 amYes, of course the power supply's output impedance should be very low, and might be difficult to measure. Obviously replace the "scope" with a DMM...

author: electronics man
author_rank: Frequent Contributor
timestamp: April 07, 2014, 12:50:44 pm
content: Another nice video, very helpfull

author: KD0CAC John
author_rank: Frequent Contributor
timestamp: April 07, 2014, 01:36:20 pm
content: Thanks againBetween you , Dave & signal path blog [ there others but ] I am always eager to see if you guys have a new video .

author: w2aew
author_rank: Super Contributor
timestamp: April 08, 2014, 01:27:49 am
content: Quote from: KD0CAC John on April 07, 2014, 01:36:20 pmThanks John - I'm honored to be grouped in such company. ;-)

author: AlfBaz
author_rank: Super Contributor
timestamp: April 08, 2014, 04:32:37 am
content: "I see later"Cringe-worthy

author: AlfBaz
author_rank: Super Contributor
timestamp: April 08, 2014, 04:39:10 am
content: Awesome videos AlanI'm curious though, any examples of circuits or devices that exhibit changing output impedance with load?I'm thinking maybe diodes/zeners

author: The Electrician
author_rank: Frequent Contributor
timestamp: April 08, 2014, 05:09:20 am
content: Next you could show how to measure an AC output impedance.

author: BravoV
author_rank: Super Contributor
timestamp: April 08, 2014, 07:40:51 am
content: Quote from: The Electrician on April 08, 2014, 05:09:20 amYES !!! Thats the real meaty part that I've been waiting.

author: SeanB
author_rank: Super Contributor
timestamp: April 08, 2014, 06:29:52 pm
content: Quote from: AlfBaz on April 08, 2014, 04:39:10 amMany discrete amplifiers do that, and often many opamps and power amplifiers will behave badly as the load impedance decreases or the frequency rises, as either thermal limiting or current limiting interferes. A simple single transistor emitter follower has a varying impedance as the output rises and falls, low impedance on the rising voltage but goes high on the falling side as the load resistor is the only thing sinking current.

author: AlfBaz
author_rank: Super Contributor
timestamp: April 08, 2014, 10:07:58 pm
content: Quote from: SeanB on April 08, 2014, 06:29:52 pmMy brain is behaving badly, thanks

author: w2aew
author_rank: Super Contributor
timestamp: April 09, 2014, 12:05:30 am
content: Quote from: AlfBaz on April 08, 2014, 04:39:10 amThe simple common collector amplifier, also known as the emitter follower. The effective small-signal output resistance of the emitter is a linear function of collector current (vt/Ic)...

author: GK
author_rank: Super Contributor
timestamp: April 09, 2014, 12:29:57 am
content: That’s assuming that the base is driven by a source impedance of zero, which is seldom the case. The output impedance is then more accurately approximated by (Rsource/beta)+Re. Since beta/hfe is frequency dependant (f> hfe<) Zout can look inductive when the source resistance is significant.

author: w2aew
author_rank: Super Contributor
timestamp: April 09, 2014, 01:53:35 am
content: Quote from: GK on April 09, 2014, 12:29:57 amYes, true of course - I should've been more complete...

author: Silverio
author_rank: Newbie
timestamp: April 09, 2014, 04:50:54 am
content: Thanks! This is really useful

author: Dr. Frank
author_rank: Super Contributor
timestamp: April 09, 2014, 08:33:44 am
content: Dear Alan Wolke,once again an interesting and meaty video.An analogue video about How To determine the extremely high output impedance of (precision) current sources would also be very interesting, I think.Also, the extremely low output resistance of a PSU could be evaluated in a similar manner.Frank

author: krivx
author_rank: Frequent Contributor
timestamp: April 09, 2014, 09:59:14 am
content: Quote from: The Electrician on April 08, 2014, 05:09:20 amWasn't this shown? Just for a single frequency but you could repeat to cover the desired ranges.

author: The Electrician
author_rank: Frequent Contributor
timestamp: April 09, 2014, 01:32:21 pm
content: Quote from: krivx on April 09, 2014, 09:59:14 amI was thinking of cases where the device itself is not the provider of the AC signal, such as a power supply.