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Amplifiers with AC Biasing Voltage

03/23/2015 8:27 AM

Generally in amplifiers dc biasing is used. What happens if ac biasing is used? I mean ac voltage of 9 to 12v from a stepdown transformer.

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#1

Re: Amplifiers with ac biasing voltage.

03/23/2015 8:32 AM

Generally, the fundamental frequency of the ac and a load of harmonics appears at your speakers. Either that, or it ceases to be an amplifier and becomes a smoke generator instead.

Did you or the other survivors put your earlier experiments on YouTube? How about this one as well?

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#2
In reply to #1

Re: Amplifiers with ac biasing voltage.

03/23/2015 8:59 AM

Song goes like " why are you gonna be so rude... Dont you know i am human too..." Wait a minute is OP human? ;)

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#3

Re: Amplifiers with ac biasing voltage.

03/23/2015 9:36 AM

DC is a myth if you really think about it. It is certainly an important concept to use and understand in engineering design. The classic example of DC is a battery. However, any battery ever made was at one time in the past not made. That voltage source did not exist all the way into the distant path (t=-∞). Similarly that battery has a finite amount of chemicals and a built in leakage path. That battery will not exist forever. That battery will not be a constant voltage source until +∞. A true DC voltage source exists from -∞ ≤ t ≤ +∞. A true DC voltage source is time invariant.

Now if you are going to continue to ask theoretical questions about electronics and you want me to continue to try and explain to you what you are asking, you have to demonstrate to me that you understand what I say. I say this because it is clear to me that you have no understanding at all why DC is an important concept to know and you know nothing about the meaning of biasing an amplifier, why it is done and the terms one needs to understand to even discuss this topic.

You have to crawl before you can run. You're trying to spear fish for sharks in the Ganges before you've had your first bath.

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#7
In reply to #3

Re: Amplifiers with ac biasing voltage.

03/23/2015 11:59 AM

I know the concept of dc biasing,about active region and dc load line all that thing. Here my questions is "ac biased amplifier results in amplification of signal when transistor is on in that ac" . That means the switching of transistor occurs. Am i correct???

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#8
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Re: Amplifiers with ac biasing voltage.

03/23/2015 12:32 PM

Try it and see for yourself.

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#10
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Re: Amplifiers with ac biasing voltage.

03/23/2015 1:11 PM

You know which words to repeat. You do not understand the meaning of these words at all.

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#29
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Re: Amplifiers with ac biasing voltage.

03/24/2015 9:42 AM

hey i know their concepts also. i am asking that this ac biasing will be a new one eventhough ithas many complications?

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#34
In reply to #29

Re: Amplifiers with ac biasing voltage.

03/24/2015 10:13 AM

Let me spell it out for you:

Amplifiers have a 'working range' that is less than the full voltage range they can handle. This 'working range' is also known as the 'linear range,' since this is where the Vout is an accurate representation of the Vin. Outside this range, the response is non-linear and the resulting signal is either distorted or outright 'clipped' (if you look at a 'clipped sine wave, you will see the rounded peaks are 'cut off' and the signal begins to look like a square wave).

You set the bias of an amplifier so that the signal you want to amplify is in the 'linear range.' We use DC for this because we need to 'offset' the AC signal so it is all within the linear range.'

If you use an AC bias, you are, at best, distorting the signal beyond recognition, and at worst, destroying the sensitive components within the amplifier.

I recommend a little reading on the subject before continuing:

en.wikipedia.org/wiki/Amplifier

Pay attention to the different 'Classes' of Amplifiers, mainly A, B, AB, and C. Those four are the most common ones used with analog signals, notice how they all use a DC bias, but bias at different points? Class A biases right in the middle, Class B biased 'low,' AB is two Class B amps working together to 'mimic' a Class A, and Class C is biased 'very low' to turn the input signal into a string of short, narrow 'pulses.'

Look at your starting signal, then look at what you want to turn the signal into, then choose the best class of amplifier for the job.

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#31
In reply to #10

Re: Amplifiers with ac biasing voltage.

03/24/2015 9:46 AM

I'm sorry, I just have to comment here.

Your statement reminded me of that line from The Princess Bride:

"You keep using that word, I don not think it means what you think it means."

(Said by the Spaniard, Iningo Montoya, to the Sicilian (whose name I do not remember) due to the latter's habit of shouting out 'inconceivable' when in denial of what is right before his eyes. (For those who are non-native in English, the word he should have used was 'impossible,' not 'inconceivable.' Impossible means 'This cannot happen/I do not believe this," while inconceivable means 'I cannot think of this,' usually in the form of moral or ethical realms, as in 'You want me to kill and eat a puppy in front of said puppy's boy? That's monstrous! It's inconceivable!")

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#18
In reply to #7

Re: Amplifiers with ac biasing voltage.

03/24/2015 1:27 AM

Oh snap, I thought you were talking about a vacuum tube amplifier..... In this case you won't need the type x filter capacitor.

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#4

Re: Amplifiers with ac biasing voltage.

03/23/2015 9:49 AM
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#5

Re: Amplifiers with ac biasing voltage.

03/23/2015 9:53 AM

Do a search and find the information yourself.

Is this just more homework?

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#6
In reply to #5

Re: Amplifiers with ac biasing voltage.

03/23/2015 10:04 AM

I seriously doubt that this is homework. Any teacher of electronics worth their salt would not put the terms AC and bias in this way. This question seems more like an AI trying to string words together to simulate a conversation on an engineering forum.

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#9
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Re: Amplifiers with ac biasing voltage.

03/23/2015 12:47 PM

Admin tends to delete all those.

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#12
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Re: Amplifiers with ac biasing voltage.

03/23/2015 2:55 PM

Lokeshloki isn't a bot based on my past discussions with him.

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#13
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Re: Amplifiers with ac biasing voltage.

03/23/2015 3:19 PM

I didn't say he was an AI bot. I was saying that here (s)he sounds like like a bot.

Must be the marmalade in the crankcase.

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#14
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Re: Amplifiers with ac biasing voltage.

03/23/2015 4:57 PM

Bots currently are not sophisticated enough to interact and respond in this particular manner. They are getting more sophisticated however, but that is a side issue for the future.

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#24
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Re: Amplifiers with ac biasing voltage.

03/24/2015 8:53 AM

Without bias, bots can be very spastic.

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#25
In reply to #12

Re: Amplifiers with ac biasing voltage.

03/24/2015 9:09 AM

Thank goodness for that.

If he were an AI, I'd have become more concerned for the industry. The 'baseline' AIs used today have better 'questioning' algorhythms than I've seen from him.

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#37
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Re: Amplifiers with ac biasing voltage.

03/24/2015 2:47 PM

It's not about the original question its about how a bot reacts on its own to the different answers posted forming both positive and negative responses and even showing human like initiative like arguing, posting anonymously to aid their argument or post negative responses or using private messaging for personal attacks.

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#38
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Re: Amplifiers with ac biasing voltage.

03/24/2015 3:42 PM

You're right, and I see the subtext of your post.

I'm sorry for my rude remarks.

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#40
In reply to #37

Re: Amplifiers with ac biasing voltage.

03/25/2015 4:05 AM

SURELY NOT!

ON CR4?

Unbelievable.....!!!

Were the PMs Anonymous or not? (Can you send and Anonymous PM on CR4? No idea!!)

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#41
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Re: Amplifiers with ac biasing voltage.

03/25/2015 2:39 PM

No the PM wasn't anonymous, it also wasn't directed at me I was merely notified after the fact as a general 'heads up'.

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#42
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Re: Amplifiers with ac biasing voltage.

03/25/2015 4:17 PM

Ah!!

I was only funning with you!!

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#11

Re: Amplifiers with AC Biasing Voltage

03/23/2015 1:14 PM

since you didn't determine what amplifier you mean O.P amp.or single transistor i assume

an amplifier of one transistor .the D.C biasing is used to power output circuit for example.

C-E(collector-emitter) in common emitter circuit and for input circuit here (base-emitter) to determine what class of amplifier do you want the transistor to work actually there are three classes of amplifiers "class A " is used to amplify the audio

"class B" to determine the amplitude of the signal. and "class C" is used to amplify the R.F signal and to create a single side band(ssb) in A.M radio transmissions.if you bias

the transistor with A.C volt you will not get operation of those classes that i explained it

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#15

Re: Amplifiers with AC Biasing Voltage

03/23/2015 10:34 PM

For some much needed enlightenment on this subject,read,digest and understand the following links,and all links associated with it on the referenced pages.

Then return and post intelligent questions for which the answers are not general,common knowledge of those well versed in the art.

https://en.wikipedia.org/wiki/Biasing

Good luck on your learning adventure.

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#16

Re: Amplifiers with AC Biasing Voltage

03/24/2015 12:10 AM

Is this like powering an oscillator with AC power? I'm sure this will work just as well.

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#30
In reply to #16

Re: Amplifiers with AC Biasing Voltage

03/24/2015 9:44 AM

your are really brave

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#35
In reply to #30

Re: Amplifiers with AC Biasing Voltage

03/24/2015 12:45 PM

Thank you for the compliment.

Cheers !

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#17

Re: Amplifiers with AC Biasing Voltage

03/24/2015 12:20 AM

If the question is to be taken seriously, the OP must define the use and conditions of operation of the amplifier.

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#19

Re: Amplifiers with AC Biasing Voltage

03/24/2015 3:37 AM

Biasing with AC? Errr, sure, why not? The biasing will follow the AC.

Can't see why you'd want to do that though.

What are you trying to do or understand?

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#20

Re: Amplifiers with AC Biasing Voltage

03/24/2015 5:33 AM

My thoughts are that the OP should learn all about DC biasing first, I think that will possibly make him understand why his question is probably not going to be answered in the way he imagines.

I don't know, but I would guess that a varying DC bias is used in some specific applications, but I personally have never heard of AC bias myself (somewhere maybe!!) as AC varies around a 0 voltage, which would imply both positive and negative biasing at the frequency of the AC!

But always prepared to learn something new!!

Best of luck!!

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#21
In reply to #20

Re: Amplifiers with AC Biasing Voltage

03/24/2015 6:42 AM

AC can have a DC offset.

Commonly found varying bias amplifier could be part of a feed back loop in an AGC circuit but it's not normally varying at AC mains frequency.

Then not all bias voltages set the gain, they can change DC offset too....

I am delving way back into the depths of my memory here so OP should check this.

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#22
In reply to #21

Re: Amplifiers with AC Biasing Voltage

03/24/2015 7:24 AM

I agree, but if the AC never crosses the 0 point, its a varying DC to my mind, even if at a specific frequency.....all the time!!

I like this little graph from Wiki:-

I hope this illustrates my "take" better....

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#28
In reply to #22

Re: Amplifiers with AC Biasing Voltage

03/24/2015 9:24 AM

You bring up some good points, Andy. However, when dealing with the math, it might be simpler to look at variable DC that never crosses zero as an AC signal 'riding' on a DC offset. You then 'separate' the two, run each one through the functions on their own (where the math will be simpler) then 'recombine' them afterwards to get the actual result.

It seems like more complication and more work at first, but in the end it's faster and simpler. Like learning that adding the taking the log values of two numbers, adding them together, and then getting the antilog of the result is the same as multiplying the two numbers. It seems long and complicated, until your electronic calculator dies and you cautiously pick up that old slide rule, which has two identical log scales you can slide against each other. You carefully glide the polished wood of the relic until the zero on one scale lines up with the two on the other, then read across from the three on the first scale to get the answer for two times three. With trembling hand you transcribe the result: five point nine nine nine. Then you decide to round it off to 6 since you don't need three digits of precision for this problem.

(Yes, that's an old (very old) college joke at the end. You want A-list humor from me this early in the morning? I'm not done with my coffee yet, gimme a break.)

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#32
In reply to #28

Re: Amplifiers with AC Biasing Voltage

03/24/2015 9:55 AM

Your technique is called superposition. Where one, in theoretical analysis, transfers one waveform source at a time through a transfer function of a circuit. This method works iff the transfer function is a linear, time invariant transfer function. This is a very useful and powerful analysis technique. It does have a few additional critical conditions and methodologies than what we've discussed here.

The problem posed in this thread is about changing the bias of an unknown amplifier with an unknown AC signal. It is quite likely that this bias change will produce extremely non-linear changes to the transfer function that will vary in time. However, since we know nothing about this amplifier or how the bias variation change gets applied to this amplifier, we know nothing about what will happen.

P.S. I did not misspell "iff". Another symbol I could have used is ↔

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#36
In reply to #28

Re: Amplifiers with AC Biasing Voltage

03/24/2015 2:14 PM

Your post is fine with me. Coffee ok?

I am able to understand both methods, but yours might be simpler for the OP....I had simply forgotten about it, thanks...

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#23

Re: Amplifiers with AC Biasing Voltage

03/24/2015 7:53 AM

The whole concept of biasing is to keep the amplifier in the active range of its ability to operate. That being said, superimposing an AC waveform is most likely to drive the amplifier to an OFF or Saturated condition thereby introducing noise into your signal that will be very difficult to separate.

Amplifiers are intended to increase the signal strength of a given signal. Everything else is considered to be unwanted noise. If you've ever listened to a public address or even a band with big amplifiers, that annoying hum of 60 or 50 Hz is superimposed AC biasing. I just can't imagine a use for that. What did you have in mind?

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#26

Re: Amplifiers with AC Biasing Voltage

03/24/2015 9:16 AM

The question is ridiculous.

Biasing is there to provide fixed stable conditions for the active components to operate in their correct region.

AC will not accomplish this... as it's not fixed and stable!

Del

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#27

Re: Amplifiers with AC Biasing Voltage

03/24/2015 9:22 AM

The speakers will hum. They will hum the frequency of your ac source or double that if you rectify to DC.

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#33

Re: Amplifiers with AC Biasing Voltage

03/24/2015 10:07 AM

It depends on the amplifier design and circuitry components.

You will most likely get an AC signal stacked on a DC signal (carrier wave) which will yield a very high value of intermittent noise feedback. (signal distortion)

If you are working with a guitar amplifier signal, the distortion result may well be what you want.

However, in circuits where clarity and finite control are mandatory, the noise will drive the signal into an unusable state.

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#39

Re: Amplifiers with AC Biasing Voltage

03/24/2015 5:52 PM

AC voltage is a waveform that swing between a maximum and minimum voltage value. If you want to drive an AC device like a speaker, you need to send an AC voltage to it. if your supply is 60 V DC you will bias the system at 35 V DC. now you can drive your speaker through an DC isolation device like a Capacitor or Transformer with a +25 V pulse to move out and make sound or with a -25 V pulse to move in and make sound. The one pulse will cause pressure on your ear drum and the other will cause depressurization on your ear drum and if you repeat this at different frequencies it is called sound. It mimic the vibration your voice cause on the transducer inside your microphone or the movement of the needle in the indents on a record on a gramophone, just the milli V or micro Volts you generate at the source will go to your speaker as sine waves of AC V. In our example depending on the volume control between 0 and 25 VAC that is 25 V positive and 25 V negative. If you increase the voltage the waveform will change to square waves and you call that distortion.

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