Amplifier concepts

Discussion in 'General Electronics Chat' started by sharanbr, May 25, 2015.

  1. sharanbr

    Thread Starter Active Member

    Apr 13, 2009
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    Hello All,

    I have some questions related to amplifier, which I have seen as being central to many important blocks.

    1) Is the amplifier's basic function to amplify the input?
    2) Does the input have to be voltage or can be current also?
    3) Does the input have to be DC or can be AC too?
    4) Is the amplifiers performance (or the poor performance) measured by how much the output is off with respect to input in terms shape of the output (not amplitude). In other words, is it undesirable for shape of the output change wrt to input?
    5) Probably a silly question - if amplifying the input is the main requirement, is it not possible to do this in a digital domain and then bring the output back to analog domain. Is it due to latency of processing associated with it?
     
  2. shteii01

    AAC Fanatic!

    Feb 19, 2010
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    1 yes
    2 yes
    3 yes
    4?
    5 digital is relatively recent development, analog is much older, therefore we have much more experience and investment in analog.
     
  3. tindel

    Active Member

    Sep 16, 2012
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    1) Yes.
    2) There are current-to-voltage amplifiers... also known as transimpedance amplifiers
    3) amplifiers are linear - meaning that they will respond perfectly to changes in input and provide an amplified version of the input on the output. At least theoretically.
    4) There are many things that determine performance of an amplifier - noise, quiesent power, absolute error (from input voltage and current offsets), non-linearity (distortion), slew-rate, stability, phase-shift, gain, bandwidth, etc.
    5) Absolutely you can do these things in the digital and analog domains.
     
  4. crutschow

    Expert

    Mar 14, 2008
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    In answer to 5):
    Yes, the trend is to do more and more analog functions in digital, witness modern TV and audio.
    HD TV does all the transmission and processing in the digital domain which is converted to analog at the screen and speaker outputs.
    Many new audio power amps are now digital (Class D switching), mainly due to their very high power efficiency which generally eliminates the requirement for any significant heat sinking on the power output stages.
     
  5. alfacliff

    Well-Known Member

    Dec 13, 2013
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    class c amplifiers are not linear. they are used to amlify cw or fm where the rf power is amplified, with no regard to amplitude. they are more efficient than class a, a1, or b amplifiers.
    most positioning drives for nc machines is now digital, the position info is digital froma scale or even laser, and after massaging the numbers, drive the motors directly as digital pulses.
     
  6. studiot

    AAC Fanatic!

    Nov 9, 2007
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    I am going to concentrate on (1), although the answer affects all the others.

    What do you mean by amplify?

    If you mean make it bigger, make what bigger, current?, voltage?, power?, something else?

    A transformer can make an ac voltage signal much bigger.
    Do you consider this as an amplifier?

    If not what is the difference between an amplifier and a transformer?


    The main thing that no one has mentioned is that amplifiers generally have their own source of electric power.
    Not only do they need an input signal to work they need a power supply and they draw from this supply and add something to the signal in making it somehow bigger.

    Are there any other things amplifiers do, ie principal uses?

    A piezo transducer pickup can produce a signal of a volt or two, but cannot drive a loudspeaker.
    Input the pickup signal into a suitable amplifier and you can drive a loudspeaker, at the same voltage.
    What is the difference here?
    Well the amplifier acts as an impedance transducer between the pickup and the loudspeaker.
     
    absf likes this.
  7. tindel

    Active Member

    Sep 16, 2012
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    Meh - considering the questioning of the OP I think my response was adequate... there's always exceptions to the rules...
     
  8. Himanshoo

    Member

    Apr 3, 2015
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    6
    you forgot about stability ....
    stability is a serious issue that employs concept of feedback..then often is a figure of merit for an amplifier..
    second selection of an amplifier also depends on the type of load its driving...hence loading effect should be considered....
    If loading is considered then amplifier coupling should also be considered...
    Every signal is dc or an ac...if specilization is concerned that bandwidth should also be concerned...since bandwidth and gain are inverse to each other so it becomes very crucial that how good an amplifier is in maintaining a fine balance between those two...

    hence the amplifier performance not only depend upon the shape of the signal but it also depends on the above mentioned parameters...
     
  9. sharanbr

    Thread Starter Active Member

    Apr 13, 2009
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    Dear crutchow,
    When I mentioned digital above, I meant specifically amplifier in digital domain.
    Take analog signal. Convert into digital. Scale up appropriately. Convert to analog.
    Is this as simple as above?
     
  10. sharanbr

    Thread Starter Active Member

    Apr 13, 2009
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    To put in simple terms, is amplification in digital domain is to multiply a given signal with a gain and then ADC conversion of digital signal ...
     
  11. WBahn

    Moderator

    Mar 31, 2012
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    1) Yes, hence the name. But the devil is in the details and it depends on what you mean "amplify". Probably the most universal meaning would be that it adds power/energy to a signal. This might be through increasing the voltage or by increasing the current or both. It's possible that one might increase while the other decreases but the power in the output signal could (should) still be higher than the power in the input signal.

    2) You can have any combination. Voltage amplifier, current amplifier, transimpedance amplifier (current in and voltage out) and transconductance amplifier (voltage in and current out). It's also possible to have amplifiers that involve things other than voltage and current, such as light or mechanical aspects, by combining transduction and amplification in the same device/circuit.

    3) The input is almost always AC. There is very little purpose to amplifying a signal that never changes. Be sure to distinguish this from the concept of AC-coupled versus DC-coupled.

    4) You are talking about "distortion" and that is one measure of an amplifier's performance. However, not all amplifiers are intended to be linear. Some are specifically designed to compress some portion of the input range and expand other portions (a process known as "companding").

    5) Yes. Possible and frequently done. Although in in the digital domain it is generally called "scaling" precisely because you are not adding power to the signal. That would happen, if it happens, when you convert it back to analog and that is where the "amplifier" would be located.

    Keep in mind that none of these are absolutes and, as with most things, the lines between "amplifiers" and other things have been blurred.
     
  12. #12

    Expert

    Nov 30, 2010
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    If digital processing chips only have a supply voltage of 3.3 volts or 5 volts, how can they amplify the amplitude of a signal? Everything they do is within the range of their tiny power supply voltage. The concept of adding some, "ones" to a digital signal and calling that, "amplified" escapes me.

    Another point to make is that a class D power amplifier does not really, "convert back to analog". It merely has an output rate so fast that the speaker becomes the filter which changes digital to analog. The old advertising about, "digital ready speakers" was absurd. All speakers are, "digital ready" because high speed switching amplifiers were designed to use the millions of already available speakers.
     
  13. crutschow

    Expert

    Mar 14, 2008
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    As #12 noted, amplification is done in the analog world, there is no amplification, as that term is normally defined, in the digital domain.
    You can, of course, scale a digital signal by multiplying the digital signal words by a fixed value, but that's not considered "amplification" in the normal sense of the term.

    Edit: My answer in post #4 was not a correct response to his question.
     
    Last edited: Jun 13, 2015
  14. sharanbr

    Thread Starter Active Member

    Apr 13, 2009
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    Dear AAC,

    I guess the operating voltage of digital circuit should not matter, as long as we have the right input to convert the digital input to analog output.
    I assume it is rather easy to multiply an input in digital domain when compared to Analog domain.
    The other advantage of digital domain would i guess is its resilience to PVT changes.

    But then, I guess some of the issues we see in Analog domain will continue to be there in digital domain.
    For example, different response at varying frequency would be present in digital domain too.
    Digital domain have to additionally deal with increased latency of the circuit, not to mention the complexity of handling feedback.

    Anyway, these are my thoughts ...
     
  15. sharanbr

    Thread Starter Active Member

    Apr 13, 2009
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    I would assume an ideal amplifier amplifies input signal at every instant by equal gain.
    Even if input is varying.
    For example, if input is 2 volt at instant t1 and 4 volt at instant t2 then after amplification by a gain factor of 2, these voltages would change to,
    t1 -> 2*2 t2 -> 4*2 ...

    Is this a correct assumption?
     
  16. #12

    Expert

    Nov 30, 2010
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    You have written the basic description of an amplifier with a gain of 2 volts per volt. A more perfect description would include time lag, frequency limitations, and accuracy limits.

    Analog designers check all those limitations with intent to make the finished product perform as if they (real world limitations) are insignificant. If you design, this will be part of your world. If you consume, you will assume these limitations have been well considered.
     
    Last edited: Jun 13, 2015
  17. WBahn

    Moderator

    Mar 31, 2012
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    That's only a valid assumption IF the amplifier in question is INTENDED to be linear. As I said, not all amplifiers are intended to be linear. For instance, you have an entire class of amplifiers known as logarithmic amplifiers.
     
  18. crutschow

    Expert

    Mar 14, 2008
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    Yes you can multiply a signal in the digital domain but, as I noted, that's not the necessarily the same as amplifying the signal in the analog domain.

    It really depends upon the purpose of this "amplification".
    If you define that purpose then we can more precisely discuss the issue.
     
  19. sharanbr

    Thread Starter Active Member

    Apr 13, 2009
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    Thanks. I intend to design but I am still learning Analog nuances and long way to go.
    As a side remark, long ago when we as students had to make a choice to specialize in analog or digital, most of us went on to go deeper in digital domain. How much of that decision I regret now.
     
  20. sharanbr

    Thread Starter Active Member

    Apr 13, 2009
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    Sure. I an new to and trying to understand amplifier concepts better.

    I have to ask this -- if I was amplifying an audio signal or signal for wireless communication, would such an amplifier be linear by design.
    Somehow, I tend to base my concept of amplifier using an audio signal. When I think about it, the audio sound from an amplifier would sound funny if part of input were amplified to one level and part amplified to another level.
     
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