# Two Stage RC coupled amplifer

Discussion in 'Homework Help' started by howartthou, Jul 11, 2009.

1. ### howartthou Thread Starter Active Member

Apr 18, 2009
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Hi All

If any of you teach, and if you ever write tutorials or books, please, please don't assume we understand - we don't!

Now that I have vented please let me explain. I am reading about the attached amp circuit and one part poses a question:

Q. Trace the path for the d-c component of plate current for tube V2

A. The d-c path is from ground (the negative side of the power supply) through R5, through the tube, through R6 to the positive side of the power supply, and through the supply back to ground.

Hmmmm. "...and through the supply back to ground". Hmmmm. If you look a Ebb (which I assume us the supply) the positive and negative path are open! Only a-c can pass to the negative side and not d-c!

So, can someone please explain how dc can go "through the supply back to ground" when the + and - are open

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2. ### t_n_k AAC Fanatic!

Mar 6, 2009
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The schematic assumes a DC power supply is connected between the + and - terminals at Ebb. It's just not shown - probably for reasons of "clarity". Imagine a battery was connected there - then there is a complete path.

3. ### howartthou Thread Starter Active Member

Apr 18, 2009
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Oh of course, how silly of me not to see or imagine something thats not actually shown!

These omissions for the "sake of clarity" only help those that don't actually need it, and confuse those that are still learning the basics!

Sorry tnk, I am thankful for your answer, you are of course correct.

Do you think we can get authors to stop this "clarity" thing in diagrams. I hate it, it just confuses the hell out of me...

How can you teach that an "open" is shown that way and then expect readers to imagine invisible components so an open may or may not be an open depending on whether the author feels like "clarity". I cannot believe it!

4. ### Wendy Moderator

Mar 24, 2008
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Thing is, once you have the concepts down, it really is simplier. Programming books have the same problem, the basic concept was learned so long ago the writer doesn't even remember learning it. This is what makes writers for beginners such a needed commodity. Have you read the AAC book?

5. ### The Electrician AAC Fanatic!

Oct 9, 2007
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Go back and have a look at the circuit you posted with your thread titled "Amplifier General Questions".

You'll see a node labeled "+Vcc". It is assumed that the power supply is connectected there, and it is further assumed that the other terminal of the power supply is connected to ground, even though that connection is not shown.

It is assumed that the impedance of the power supply is very low, so that the impedance of the Vcc node to ground is therefore very low; it's essentially a short to ground.

It's the same potential as ground for AC currents.

It is intended that no AC voltage will appear at Vcc.

Now, having said all that, understand that it's only almost true.

In the real world, the impedance of a power supply isn't zero.

It is intended that it is close enough to zero that for engineering purposes, it may be considered zero.

It may be quite near zero, but it isn't exactly zero.

You can see that the designer of the circuit shown knew that, because he added R5 and C6. Those two components provide some extra filtering to further remove any AC voltages that may appear on the Vcc node before the Vcc DC voltage is used to power Q1, the first stage of the amplifier.

6. ### hobbyist Distinguished Member

Aug 10, 2008
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Any time you have current flowing, there has to be a COMPLETE path, so wether it is AC or DC, it still needs a complete path to flow, so wether it is a Transformer supply, or a battery there will always be a complete circuit path through the supply structure, so as for current to flow.

A voltage potential is just THAT, a POTENTIAL when there is NO complete circuit path.
But once that path is completed then the POTENTIAL device causes current to flow THROUGH it, wether it is a battery or a transformer or even static electricity.

ANYTHING with a electrical potential will have current flowing THROUGH IT once there is a COMPLETE path for current to flow.

Even when you rub your feet on a carpet and touch the metal fireplace screen, a electric potential built up between you and the metal screen , and the moment you touched it a complete path existed and a current flowed through you through the ground through the screen and back through you again, a complete circuit, you can never have current flowing without, a COMPLETE circuit path.

Any voltage potential becomes a patrh for current to flow THROUHJ, a supply voltage is NOT a open circuit, it is a inherent resistance with voltage potential at each terminal just waiting for a complete path to be made for it to release it's potential energy, in the form of electric current..

It is impossible for current to flow without a complete path for it to flow through, until the path is broken, so that's why yopu NEVER refer to any voltage source as an open circuit when current is FLOWING, AC or DC.

It is ONLY open when there is NO complete path for current to flow.

So if current is flowing then the supply has current flowing through it just as every component that makes up that COMPLETE path, that's why it is called a circuit, circuit essentially means a complete circle.

Last edited: Jul 11, 2009
7. ### howartthou Thread Starter Active Member

Apr 18, 2009
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Bill
If the ACC book is about abbreviations, ommissions, short-hand etc., then no I havent read it. To be honest I would rather read a book on electronics, especially one that has a chapter or so on "short-hand" for circuits, but also just a book that isn't so assuming. Fully agree writers for beginners, intermediates and advanced thing.

Electrician
If the other terminal of the power supply, which would be negative, is connected to ground, then is ground an earth wire or a chassis that would be connected to ground as in earth ground? I thought negative was ground because its at zero voltage. As you can see this ground thing, which I know is a reference point, still confuses me too...

Hobbyist
Funny you should go into a detailed explaination of potential. Something I read the other day made the point that there has to be a potential voltage difference for current to flow.

Also I cannot understand how a current flows through me to the metal fire guard if I am on a rug that is on a wooden floor. So how does the current flow through "the ground" if the ground is not a conducter

I get you point though about potential difference for current to flow...

8. ### t_n_k AAC Fanatic!

Mar 6, 2009
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I'd be interested in the response to this very good question as well .... I suspect it's a little more complex than one might think.

9. ### The Electrician AAC Fanatic!

Oct 9, 2007
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You can see a ground symbol under the bottom of R7. That node of the circuit is taken to be the reference node. It doesn't have to be connected to the earth but it could be. If the Vcc supply is a battery, the entire circuit could be in a plastic box with no connection to the earth, and it would still work.

Very often the reference node of a circuit is called "ground" instead of "reference node". It is often connected to the chassis, which may or may not be connected to the earth.

The negative end of the supply is connected to the reference node (ground) because the transistors are NPN, and the emitters are connected to the reference node. If the transistors had been PNP, then the positive end of the supply would have been connected to the reference node.

Ground does not have to be the most negative potential in a circuit; it's just usually the reference node.

10. ### hobbyist Distinguished Member

Aug 10, 2008
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It was a very POOR example.
It is probably just a static discharge, NO current flowing, I just threw that out there, to use as an example, that if current flows no matter what the potential source is there has to be a complete path or else it wouldn't be able to flow.

Static discharge and all that stuff I leave up to the experts to explain if there is a current flow or not.

But I see I made my point clear about any voltage source has to have current flow through it in order for there to be a circuit of current flowing.

I'll be more specific on this subject, when I say a current flowing I'm talking about a CONTINUOS current until the path is broken, a static discharge may have electrons flowing to equal out the imbalance but it is NOT a contunuous current flowing, a capacitor will supply a flow of electrons into a circuit it is connected to but because it is a passive element and can only absorb and discharge energy, but not deliver it continuously then current does not flow through it like an active source does.

Only active power sources have contunuous current flow THROUGH it, in order for a circuit to be a circuit.

Your original question was about the battery being open, and I think you were looking at the supply voltage as being a open in a circuit when current is flowing, but that's why I streessed in these posts that the power source is connected in series with every other component in that path and therefor the power source itself becomes a path for current to flow, THROUGH, when it is CONTINUOUS current.

Last edited: Jul 12, 2009

Feb 27, 2009
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