Measuring output impedance

[w2aew]

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!

[SeanB]

Very nice video. Thank you.

[David Aurora]

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

[BravoV]

Alan, thanks, great video ! 

Question, is this method applicable to measure power supply's output impedance ?

[The Electrician]

At about 9:39, the very last mathematical operation you perform on the calculator is divide, even as you are saying "multiply".

[HighVoltage]

I like your calculator.
Good old school tool.

[hgg]

Nice and helpful video as always!

[w2aew]

At about 9:39, the very last mathematical operation you perform on the calculator is divide, even as you are saying "multiply".

Yes - I mis-spoke.  But the equation shown, and the calculator operation as shown, are correct.

[w2aew]

Alan, thanks, great video ! 

Question, is this method applicable to measure power supply's output impedance ?

Yes, 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...

[electronics man]

Another nice video, very helpfull

[KD0CAC John]

Thanks again
Between you , Dave & signal path blog [ there others but ] I am always eager to see if you guys have a new video .

[w2aew]

Thanks again
Between you , Dave & signal path blog [ there others but ] I am always eager to see if you guys have a new video .

Thanks John - I'm honored to be grouped in such company.  ;-)

[AlfBaz]

"I see later"
Cringe-worthy 

[AlfBaz]

Awesome videos Alan
I'm curious though, any examples of circuits or devices that exhibit changing output impedance with load?
I'm thinking maybe diodes/zeners

[The Electrician]

Next you could show how to measure an AC output impedance.   

[BravoV]

Next you could show how to measure an AC output impedance.   

YES !!! Thats the real meaty part that I've been waiting. 

[SeanB]

Awesome videos Alan
I'm curious though, any examples of circuits or devices that exhibit changing output impedance with load?
I'm thinking maybe diodes/zeners

Many 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.

[AlfBaz]

...power amplifiers will behave badly...

My brain is behaving badly, thanks

[w2aew]

Awesome videos Alan
I'm curious though, any examples of circuits or devices that exhibit changing output impedance with load?
I'm thinking maybe diodes/zeners

The 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)...

[GK]

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.
 

[w2aew]

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.

Yes, true of course - I should've been more complete...

[Silverio]

Thanks! This is really useful 

[Dr. Frank]

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

[krivx]

Next you could show how to measure an AC output impedance.   

Wasn't this shown? Just for a single frequency but you could repeat to cover the desired ranges.

[The Electrician]

Next you could show how to measure an AC output impedance.   

Wasn't this shown? Just for a single frequency but you could repeat to cover the desired ranges.

I was thinking of cases where the device itself is not the provider of the AC signal, such as a power supply.