Hi all. New to this forum!! I have a reasonable understanding of electronics and so will be able to separate the wheat from the chaff on any replies to this post. If anyone has any ideas please feel free to reply. I can assure you if your advice is good then I will no doubt be able to repay the favour with regards to multiple other areas of electronics!


why is there never a mention with regards to electromagnetism when designing transistor amplifiers? The methodology is to bias the transistor using dc analysis then superimpose the ac signal! Now a time changing ac signal will cause a magnetic field, this field could cause the direction of current flow to change. Now even if the 'ac' signal does not cross into the negative half of the cycle it will still produce a magnetic field and could cause a change in the amount of current flowing. My first thought was that the magnetic field aspects are ignored as the input to the amplifier is extremely small, but the gain can be extremely large if one has a multi stage amplifier so the magnetic field could change the current flow in the circuit. Also, when using operational amplifiers there is never any mention of electromagnetism either. Am I missing something here?

 

The magnetic field is intimately tied up with the current flow.
You can't have one with out the other,so the presence of the magnetic field does not change anything.
Even external magnetic fields have no discernible effect on solid state equipment,except in the case of "Hall effect" devices.
They can effect vacuum tubes to some extent.

 

Hey thanks for your reply. Yeah i know that both magnetic fields and current are responsible for each other. DC current does not produce a time changing magnetic field but Ac current does. So when superimposing an AC signal onto a DC bias there should be an changing magnetic field introduced in the vacinity of the bias. My question was why does this not interfere with the DC current and cause this to change?

Also solid state equipment?

Not being a specialist in emag and equally not a physicist i'am generally not entirely comfortable in this realm.

thanks again!!

 

The magnetic field is too small to affect the circuit.

 

So by traditional operational requirements of transistor amplifiers the magnetic filed would affect the DC current. The key point being but it is too small to be able to notice any changes in DC current.

How would that work for multistage amplifiers - where the AC signal gets progressively bigger as more than one transistor is used - or even power amplifiers which have transformers connected into them for means of impedance matching with the load?

I know that these questions are a bit off field and there is not mention of electromagentic effects in the books i have read but I like to fundamentally understand if there is magnetic phenomenon at play, even if it is to small to be noticed.

Thanks very much!!

 

Still too small.
The closest I have come to seeing magnetic interference is when you locate the preamp circuit too close to the power transformer. In that case you will pick up AC line hum.
A sensible designer would always position the preamp and amplifier sections as far away from the transformer as possible.

 

Thanks you both for your replies!!

I kind of knew there must be some interaction between the DC current and the AC generated magnetic field. I put two and two together and assumed that it must be too small to have a noticeable affect but not being a physicist i was just really curious as to what effects the field could have. I spent six years studying electronics and emag is never mentioned and i honestly think some people probably dont even know about it. The only times i have came across emag is electricity distribution and shielding low noise electronics. If either of you have any electronics related questions then feel free to hit me up!! Or if you know more than i do, probably the case, then feel free to answer anymore of my crazy questions

Thanks again!!

 

I spent six years studying electronics*

*Sorry if this sounds nasty,but I would ask for my money back!

 

The EEE department in ones country with a 90% employment rate does not give one their money back.

While i claim i have not done extensive electromagnetism extensively I am still aware of it. If you read the post correctly then you would not have stupidly started to write about the connection between ac and magnetic fields. Do not believe the name, i actually do have a reasonable education, while not in the field of physics I do know electronics. If you are going to reply to my threads I would appreciate it if you did so with relevant information.

Only a physics buff would hit out with solid state equipment. Since posting this i have since received a more than acceptable answer which details the permeability of air being low and so magnetic field is not strong, although it does in theory have the ability to interfere with the DC bias, which was overwhelmingly obvious

 

The EEE department in ones country with a 90% employment rate does not give one their money back.

While i claim i have not done extensive electromagnetism extensively I am still aware of it. If you read the post correctly then you would not have stupidly started to write about the connection between ac and magnetic fields. Do not believe the name, i actually do have a reasonable education, while not in the field of physics I do know electronics. If you are going to reply to my threads I would appreciate it if you did so with relevant information.

Only a physics buff would hit out with solid state equipment. Since posting this i have since received a more than acceptable answer which details the permeability of air being low and so magnetic field is not strong, although it does in theory have the ability to interfere with the DC bias, which was overwhelmingly obvious

I am not a Physics buff,& I have made my living in Electronics for over 40 years.
Your postings all sound like someone who has little idea of Electronics,but knows a lot of buzzwords.

As you are knowledgeable in this field,why do you write questions which sound like they were penned by a bright 12 year old?
Is the idea to fool people into thinking you are dumb,so you can then get on your high horse about your high level qualifications?

 

Well first of all you are reading the questions so they do not sound like anything at all.

If you had read my question correctly then I'm sure someone with 40 years experience would have been able to realize that yes there is a magnetic field at play. Now what effect do the magnetic field have on the DC current and why.

Perfectly reasonable question to ask in this forum. I also do not think a 12 year old child would deduce such a question.

Also if you have 40 years in the business then my qualifications mean nothing as you have more knowledge and ability then i do.

 

I am not a Physics buff,& I have made my living in Electronics for over 40 years.
Your postings all sound like someone who has little idea of Electronics,but knows a lot of buzzwords.

As you are knowledgeable in this field,why do you write questions which sound like they were penned by a bright 12 year old?
Is the idea to fool people into thinking you are dumb,so you can then get on your high horse about your high level qualifications?

There is no need to insult people on this forum.

 

Well first of all you are reading the questions so they do not sound like anything at all.

If you had read my question correctly then I'm sure someone with 40 years experience would have been able to realize that yes there is a magnetic field at play. Now what effect do the magnetic field have on the DC current and why.

Perfectly reasonable question to ask in this forum. I also do not think a 12 year old child would deduce such a question.

Also if you have 40 years in the business then my qualifications mean nothing as you have more knowledge and ability then i do.

There are some very clever 12 year olds around!

I re-read your original question,& I still have trouble reading it.

OK,your question does make sense,but the phrasing is very hard to comprehend.
That said,sorry about the comments.

You received several answers which from an Electronics point of view are adequate,as any effect is so small as to be undetectable.

Perhaps it would have been more appropriate in the Physics section,where such effects may have been commented upon before.

 

Yeah, it was probably more of a physics based question!!

That being said however, the key to understanding something is the question "Why". Obviously there needs to be a reference point as one could keep asking why all the way back to the big bang!! Why does the field not substantially interfere with the DC bias.

I know now why it does not.

Remember a statement of fact is almost meaningless unless one knows "why" this particular fact is so

Thanks again for all the comments regarding this question!!

 

In the attached schematic, are you suggesting that the AC current will modify the DC current?

 

Magnetic fields are often associated with inductance. When current flows through an inductor, the energy is stored in a magnetic field.

 

Why have you got an AC source in series with a DC supply?

Also depending on the frequency of the AC then the impedance of the inductor could tend towards infinity. That circuit in its present form does not make a whole lot of sense to me.

The point i was making was this,

I have designed a transistor amplifier lets just say i common emitter. Now there are coupling and de-couping capacitors at play. As such i analyse the circuit and have so have DC values of current going through my circuit. Now when i have my signal that i want to amplify pass through the circuit then I have an ac current moving through my circuit. I was suggesting that this would create an magnetic field which if strong enough could interact with the DC bias and change the DC conditions of the circuit.

It was more of a theoretical concept than a practical one as in practice the AC generated magnetic field is just simply not strong enough to cause damage to the DC bias of my amplifier.

thanks

 

Why have you got an AC source in series with a DC supply?

Because your amplifier, with its DC bias and AC current, can be simplified down to several AC + DC + R circuits. Leave out the inductor if you want. I just put it there because any piece of wire has inductance.

Also depending on the frequency of the AC then the impedance of the inductor could tend towards infinity. That circuit in its present form does not make a whole lot of sense to me.

Sure. So what? Infinite AC impedance does not affect DC.

The point i was making was this,

I have designed a transistor amplifier lets just say i common emitter. Now there are coupling and de-couping capacitors at play. As such i analyse the circuit and have so have DC values of current going through my circuit. Now when i have my signal that i want to amplify pass through the circuit then I have an ac current moving through my circuit. I was suggesting that this would create an magnetic field which if strong enough could interact with the DC bias and change the DC conditions of the circuit.

It was more of a theoretical concept than a practical one as in practice the AC generated magnetic field is just simply not strong enough to cause damage to the DC bias of my amplifier.

thanks

I understand what you are saying (I think). I just don't understand why you think AC fields in your circuit could affect DC bias, regardless of the strength. AC fields do not induce DC currents.
AC current through (or voltage across) nonlinear devices (transistors, diodes, etc.) can upset bias, but this is accounted for in any thorough circuit analysis, and is not a result of EMFs. Perhaps even strong fields around them could cause distortion, but so far you haven't mentioned nonlinearities. Maybe that was your intention?

 

So you are saying that magnetic fields only affect AC current.

hypothetically If I have a AC generated magnetic field in my hand and bring it close to another circuit which has AC current superimposed on DC bias then the AC current superimposed could change but never the DC bias current?

Also I'm aware of how to form models of transistor circuits, but one would require DC blocking capacitors in the circuit to isolate the DC current from the AC source?

thanks

 

So you are saying that magnetic fields only affect AC current.

hypothetically If I have a AC generated magnetic field in my hand and bring it close to another circuit which has AC current superimposed on DC bias then the AC current superimposed could change but never the DC bias current?

That's correct.

Also I'm aware of how to form models of transistor circuits, but one would require DC blocking capacitors in the circuit to isolate the DC current from the AC source?

thanks

A pure AC voltage source has zero DC component. In general, the AC source may have a DC component. A series capacitor can be used to remove the DC component if in fact the DC component is not part of the signal.