Mbira

Impedance Terminology and understanding?
« on: August 12, 2018, 03:49:47 PM »
Hi folks,
I am trying to better understand impedance.  Is it correct to say that impedance is a measurement at a specific point (as opposed to measuring impedance through a point?) 

Is impedance generally measured in relation to ground, or is it measured in relation to a previous point? 

Reading in wikipedia, they say "high impedance means that a point in a circuit (a node) allows a relatively small amount of current through, per unit of applied voltage at that point. High impedance circuits are low current and potentially high voltage, whereas low impedance circuits are the opposite (low voltage and potentially high current). "

So, in the diagram attached, am I correct in saying:

1) Point A is low impedance because the cathode has low voltage (tied to ground), but there is high current that is flowing to the plate. 

2) Ideally Point B should not be seeing hardly any current, and even though the voltage applied to the grid is relatively low, it is still much greater than the current, so it would be high impedance.

3) Point C confuses me because it has high DC and AC voltage and also high current....so what would this stage be considered? 

For some reason impedance confuses the heck out of me, so I apologize if these questions seem stupid, but I'm just trying to keep chipping away at this to get it. 
Joel Laviolette

Rattletree   |  https://www.rattletree.com
The Rattletree School of Marimba | https://www.learnmarimba.com


abbey road d enfer

Re: Impedance Terminology and understanding?
« Reply #1 on: August 12, 2018, 05:21:51 PM »
Hi folks,
I am trying to better understand impedance.  Is it correct to say that impedance is a measurement at a specific point (as opposed to measuring impedance through a point?) 

Is impedance generally measured in relation to ground, or is it measured in relation to a previous point? 
Impedance is measured between two points. It is the ratio of the voltage between these two points and the current circulating. One point can have a particular impedance referred to ground, and a totally different impedance referred to another point in the circuit.

Quote
Reading in wikipedia, they say "high impedance means that a point in a circuit (a node) allows a relatively small amount of current through, per unit of applied voltage at that point. High impedance circuits are low current and potentially high voltage, whereas low impedance circuits are the opposite (low voltage and potentially high current). "
Forget about it. this is more confusing than anything. It may be true in some specific circumstances, but cannot be used as a universal definition. It's like the size of a car; a small car is smaller than a big car, but how big is big car?

I can't see the picture.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
"The important thing is not to convince, but to give pause for thought." (B. Werber)
Star ground is for electricians.

Mbira

Re: Impedance Terminology and understanding?
« Reply #2 on: August 12, 2018, 05:42:13 PM »

I can't see the picture.

It's an attachment in the post.  Looks like there are 31 views so far, so it seems to be working.  Sorry-I don't see a way to upload the pic to be able to view it inline. 
Joel Laviolette

Rattletree   |  https://www.rattletree.com
The Rattletree School of Marimba | https://www.learnmarimba.com

scott2000

Re: Impedance Terminology and understanding?
« Reply #3 on: August 12, 2018, 06:15:06 PM »
I think just clicking on the pic counts it as viewed.... I get nothing but a 404 error......

I like this guy's videos....Haven't watched this one but maybe it'll have something helpful....

https://www.youtube.com/watch?v=L_9v1qOcato

Mbira

Re: Impedance Terminology and understanding?
« Reply #4 on: August 12, 2018, 06:55:58 PM »
I think just clicking on the pic counts it as viewed.... I get nothing but a 404 error......

Ah shoot....ok, let me try again:
Joel Laviolette

Rattletree   |  https://www.rattletree.com
The Rattletree School of Marimba | https://www.learnmarimba.com

squarewave

Re: Impedance Terminology and understanding?
« Reply #5 on: August 12, 2018, 10:49:29 PM »
Hi folks,
I am trying to better understand impedance.  Is it correct to say that impedance is a measurement at a specific point (as opposed to measuring impedance through a point?) 

Is impedance generally measured in relation to ground, or is it measured in relation to a previous point? 

Reading in wikipedia, they say "high impedance means that a point in a circuit (a node) allows a relatively small amount of current through, per unit of applied voltage at that point. High impedance circuits are low current and potentially high voltage, whereas low impedance circuits are the opposite (low voltage and potentially high current). "

So, in the diagram attached, am I correct in saying:
You're over thinking it a little. It's not that complicated (but it is one of the most important principles in not just electronics but physics and a lot of other things - even software).

Impedance is just a more abstract term for resistance. There are lots of analogies. Imagine a bicycle with gears where force on the pedals is like voltage and how fast the pedals are moving is like current. If you put it in a low gear, pedaling is very easy or you could say, it's impedance is low. In a high gear, you pedal slower (less current) but with more force (high voltage) so impedance is high.

Everything has impedance (rock is high impedance and air is low impedance). When measuring impedance in electronics, you simply look at the voltage vs current (aka "Ohms Law" R = V/I) between any two points. The impedance of a resistor is just it's resistance. Even a peice of wire has impedance. Not much but enough to matter with high frequency digital circuits. If you take a piece of magnet wire and wrap it tightly around a staw, you drastically change that wire's impedance. At high frequencies, it might have an impdedance of 100 ohms. But at DC it's impedance could be 0.1 ohm.

CJ

Re: Impedance Terminology and understanding?
« Reply #6 on: August 12, 2018, 11:34:54 PM »
usually when you say Z you are indicating that there might be something other than pure resistance impeding the flow of alternating electrons,  but there does not have to be L or C in the circuit to say Z.

if caps and coils did not have the property of having the current being 90 degrees out of phase with the voltage, then we might not need Z at all, simple resistance could be used to analyze circuits.

but since L and C do involve a phase relationship, we need to take that into consideration, and vectors are a very handy tool to do this.   

there are many complex formulas involving  Z, check out RDH4 if you need some reading to to catch some Z's.

luckily for audio, we only have a few Z's to worry about,  like 600 ohms and 600 ohms.  :D
well, there might be others, like tube plate Z and loudspeaker Z and microphone Z but compared to the EE's who used to design Marantz receivers, or Ham radio geeks, it is not that bad.

here is a simple vector diagram of resistance and L or C,  so 1 ohm R in series with 1 ohm capacitive reactance would give a Z of 1.414 ohms>
« Last Edit: August 12, 2018, 11:40:18 PM by CJ »
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

Mbira

Re: Impedance Terminology and understanding?
« Reply #7 on: August 12, 2018, 11:52:29 PM »
Would you guys be willing to look at the diagram I posted and tell me if my points are correct?  That will help me to understand if there are things that I'm not understanding. 

1) Point A is low impedance because the cathode has low voltage (tied to ground), but there is high current that is flowing to the plate.  Correct?

2) Ideally Point B should not be seeing hardly any current, and even though the voltage applied to the grid is relatively low, it is still much greater than the current, so it would be high impedance.  Correct?

3) Point C confuses me because it has high DC and AC voltage and also high current....so what would this stage be considered? 
Joel Laviolette

Rattletree   |  https://www.rattletree.com
The Rattletree School of Marimba | https://www.learnmarimba.com

scott2000

Re: Impedance Terminology and understanding?
« Reply #8 on: August 12, 2018, 11:59:09 PM »
This is an interesting thread with some good stuff.....

https://groupdiy.com/index.php?topic=30023.0


squarewave

Re: Impedance Terminology and understanding?
« Reply #9 on: August 13, 2018, 12:32:55 AM »
1) Point A is low impedance because the cathode has low voltage (tied to ground), but there is high current that is flowing to the plate.  Correct?
No. Impedance has nothing to do with DC voltages. The voltage could be high at still be high impedance. Impedance has to do more with how easy it is to push and pull current from a particular point in the circuit and that can occur at any voltage.

2) Ideally Point B should not be seeing hardly any current, and even though the voltage applied to the grid is relatively low, it is still much greater than the current, so it would be high impedance.  Correct?
No. Impedance has nothing to do with the relative currents between the grid and the plate / cathode. The device could be passing very low current and the grid would still be high impedance because the grid of a tube is actually an open connection. That's just how tubes work.

3) Point C confuses me because it has high DC and AC voltage and also high current....so what would this stage be considered?
Again, impedance is mostly not referring to DC (voltage or current). Resistance is to DC as Impedance is to AC. The impedance at the anode in that circuit is actually relatively high. The plate resistor in a typical gain stage is like 100K. If the circuit is biased properly that would mean that the tube would also have a DC resistance of around the same amount. But in practice the AC impedance is actually like 70k. Meaning if you tried to push / pull current from that point, it would behave like a 70k resistor to ground.
« Last Edit: August 13, 2018, 12:37:07 AM by squarewave »


scott2000

Re: Impedance Terminology and understanding?
« Reply #10 on: August 13, 2018, 12:48:40 AM »

here is a simple vector diagram of resistance and L or C,  so 1 ohm R in series with 1 ohm capacitive reactance would give a Z of 1.414 ohms>

... I knew doing tile for 20 years would come in handy after my knees gave up.......

abbey road d enfer

Re: Impedance Terminology and understanding?
« Reply #11 on: August 13, 2018, 02:03:33 AM »
1) Point A is low impedance because the cathode has low voltage (tied to ground), but there is high current that is flowing to the plate.  Correct?
Point A is tied to ground, so it has null impedance referred to ground. But it has high impedance referred to many other points in the universe.

Quote
2) Ideally Point B should not be seeing hardly any current, and even though the voltage applied to the grid is relatively low, it is still much greater than the current, so it would be high impedance.  Correct?
Point B has null impedance referred to ground because a grounded voltage source is connected to it.
Now if you disconnect this voltage source, point B has a very large impedance, because the grid impedance is the result of parasitics only.
However, this impedance is large compared to what? It is large in audio parlance because  anything below 600 ohms is considered low impedance, anything above 100k is high impedance. But is 601 ohms not low-impedance? The notion of high/low impedance must be considered in respect to the source impedance (for an input) or to the load impedance (for an output), and has no other significant meaning.
A 600cc 4-stroke engine is a small engine for a muscle car, but a big engine for a lawn-mower.

You should concentrate on knowing how to calculate a node's impedance because that's what's important.

BTW, you cannot say the voltage is higher than the current; that would be lke saying the car's speed is higher than it's fuel consumption.


Quote
3) Point C confuses me because it has high DC and AC voltage and also high current....so what would this stage be considered?
We don't know; it depends on the tube's parameters, the plate resistor, and the context. If you want to drive a loudspeaker directly, the impedance is too high, but if you want to drive a power amp, it's probably OK.
Who's right or wrong is irrelevant. What matters is what's right or wrong.
"The important thing is not to convince, but to give pause for thought." (B. Werber)
Star ground is for electricians.

Mbira

Re: Impedance Terminology and understanding?
« Reply #12 on: August 13, 2018, 02:23:34 AM »
Thanks for all your help guys.  I will try and parse through all the bits you have said that I don't understand and find the pieces that I slightly understand so I can grow from there.   :-)
Joel Laviolette

Rattletree   |  https://www.rattletree.com
The Rattletree School of Marimba | https://www.learnmarimba.com

ruffrecords

Re: Impedance Terminology and understanding?
« Reply #13 on: August 13, 2018, 03:46:40 AM »
Thanks for all your help guys.  I will try and parse through all the bits you have said that I don't understand and find the pieces that I slightly understand so I can grow from there.   :-)

I think the most important concept to take away is that impedance exists between two points so it does not make sense to say the impedance at point A is high or low because that  is only one point. You always need to refer the impedance between point A and some other point.

Cheers

Ian
www.customtubeconsoles.com
https://mark3vtm.blogspot.co.uk/
www.eztubemixer.blogspot.co.uk


'The only people not making mistakes are the people doing nothing'

Mbira

Re: Impedance Terminology and understanding?
« Reply #14 on: August 13, 2018, 03:55:52 AM »
I think the most important concept to take away is that impedance exists between two points so it does not make sense to say the impedance at point A is high or low because that  is only one point. You always need to refer the impedance between point A and some other point.

Cheers

Ian

Got that.  It makes sense since it is a type of resistance. 

A big thing for me is to try and understand things on a physical level....I think of current in terms of all those electrons moving, etc.  I can plug numbers in to solve equations, but that doesn't help me to understand the concept. 

It's like when a kid learns 2+2.  They need to look at two apples and then two more apples and then put the apples together.  Too often, people just want to explain things with abstract formulas first and so I get lost because I need to relate it to the physical world first. 

One of the physical world things that I think I need to learn more about is how in the video that scott2000 posted at 28:00 the guy starts talking about the inverse phase relationship between inductors and capacitors.  This is something I want to look in to the physics of it so I can better understand. 
« Last Edit: August 13, 2018, 03:59:08 AM by Mbira »
Joel Laviolette

Rattletree   |  https://www.rattletree.com
The Rattletree School of Marimba | https://www.learnmarimba.com

sahib

Re: Impedance Terminology and understanding?
« Reply #15 on: August 13, 2018, 06:00:15 AM »

…….. relationship between inductors and capacitors.  This is something I want to look in to the physics of it so I can better understand.

You are on the right track.

Resistance describes the relationship between the current and voltage in a circuit that is purely resistive. 

Impedance describes the relationship between the current and voltage in a circuit that has reactive components.

Best way to learn is to download a free book on electrical principals and start from scratch. Floyd's Electrical Principals is a very easy book.


JMFahey

Re: Impedance Terminology and understanding?
« Reply #16 on: August 16, 2018, 11:28:51 AM »
Hi folks,
I am trying to better understand impedance.  Is it correct to say that impedance is a measurement at a specific point (as opposed to measuring impedance through a point?) 

Is impedance generally measured in relation to ground, or is it measured in relation to a previous point? 

Reading in wikipedia, they say "high impedance means that a point in a circuit (a node) allows a relatively small amount of current through, per unit of applied voltage at that point. High impedance circuits are low current and potentially high voltage, whereas low impedance circuits are the opposite (low voltage and potentially high current). "

So, in the diagram attached, am I correct in saying:

1) Point A is low impedance because the cathode has low voltage (tied to ground), but there is high current that is flowing to the plate. 
No.
Point A is grounded, *shorted*  to Ground, so relative to Ground it has ZERO impedance.
Voltage is not "low", it´s ZERO, so you can´t calculate or measure Impedance using ZERO Volts.
Current through it is irrelevant to define any impedance because resistance is ZERO, voltage drop is ZERO.

Quote
2) Ideally Point B should not be seeing hardly any current, and even though the voltage applied to the grid is relatively low, it is still much greater than the current, so it would be high impedance.
Yes.
Quote
3) Point C confuses me because it has high DC and AC voltage and also high current....so what would this stage be considered? 
Impedance will basically be V/I .

Ok, ok, but is that *high*  or *low* Impedance?
High or Low is just a convenient label, the actual *number*  is what I calculated above.
Labels by themselves are fuzzy and don´t mean much .... remember the example you were given before: "what is a Large car? ... what is a small car? ... " .... same thing.

Quote
For some reason impedance confuses the heck out of me, so I apologize if these questions seem stupid, but I'm just trying to keep chipping away at this to get it.

Nothing studying and patience won´t cure .  ;)
Design - Make - Service Audio Equipment since 1969.


 

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