Voltage amplifier with 2N3904

Discussion in 'The Projects Forum' started by yoannwyffels, Apr 12, 2016.

  1. yoannwyffels

    Thread Starter New Member

    Mar 23, 2016
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    Hello,

    I've read a lot about voltage (inverting) amplifier with 2N3904.
    I've found a pretty good example:
    http://www.instructables.com/id/Voltage-Amplifier/

    For what I'm trying to do, I'm on a +5V DC Power supply (not a +15V), and need to have a gain x3
    My input signal have a range between 0.5v and 1v

    Does this schematic will be working well ?

    Capture.JPG

    Thank you for your help !
     
  2. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Are you talking about a DC signal or an AC one. If AC, do you mean a) it has an amplitude within the 0.5v to 1V range or b) it has a positive peak of 1V and a negative peak of 0.5V? If AC, what frequency?
    Here's the result using your circuit with a 1V amplitude 1kHz AC signal. Do you see the problem and know the cure?
    SimpleAmp.PNG
     
  3. yoannwyffels

    Thread Starter New Member

    Mar 23, 2016
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    Hello Alec_t,

    I'm talking about a DC signal which go from +0.5v to +1v. Maybe this amplifier (with 2n3904) only work with AC signal ?
     
  4. dl324

    Distinguished Member

    Mar 30, 2015
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    Would C1 pass any DC component of the input signal?
     
  5. yoannwyffels

    Thread Starter New Member

    Mar 23, 2016
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    I think no, but even if I remove capacitor....
     
  6. yoannwyffels

    Thread Starter New Member

    Mar 23, 2016
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    Forget it, I will use any LM741 opamp to do what I want to do ! :)
     
    Dodgydave likes this.
  7. Alec_t

    AAC Fanatic!

    Sep 17, 2013
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    Even if you remove the capacitor the circuit shown won't work as you want with a ground-referenced 0.5V-1V input.
    a) 0.5V would switch the transistor off,
    b) taking the output at the transistor emitter gives no gain at all.
    c) if you take the output from the transistor collector you get gain, but there is inversion, i.e. as the input voltage rises the output voltage falls.
    When the input goes from 0.5V to 1V do you want an output going from 1.5V to 3V?
    Good luck with that :D
     
  8. dl324

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    With a 5V supply, you'll need a rail to rail opamp.
     
  9. Dodgydave

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    Jun 22, 2012
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    Use an opamp like lm358, lm324
     
  10. yoannwyffels

    Thread Starter New Member

    Mar 23, 2016
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  11. dl324

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    Read the datasheet. Input and output voltage are only guaranteed to within 3V of the supplies and minimum supply voltage is spec'ed at 10V. It will likely operate at a lower supply voltage and, if you're lucky, you'll be able to find a unit that gives better than typical voltage swings.
     
  12. yoannwyffels

    Thread Starter New Member

    Mar 23, 2016
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  13. dl324

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  14. Alec_t

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    Sep 17, 2013
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    Can't get your link to work. You can upload files direct to this site using the 'Upload a File' button next to the 'Post Reply' button.
     
  15. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    No, for two reasons. First, this is not a voltage amplifier, it is an emitter follower. This is a circuit configuration that has current gain but no voltage gain.

    Second, while R1 and R2 might be the correct ratio of values for biasing the circuit, their absolute values are way too small and will load down the input signal.

    ak
     
  16. AnalogKid

    Distinguished Member

    Aug 1, 2013
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    Well, that didn't take long.

    ak
     
  17. yoannwyffels

    Thread Starter New Member

    Mar 23, 2016
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    :) Yes AnalogKid, was trying to recycle some components I have in stock to do what I want, but when it's impossible,... it's impossible :)
     
  18. Bordodynov

    Active Member

    May 20, 2015
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    Maximum input signal (Vcc-Vce_sat)/2/Gain=(5V-0.4V)/2/3=0,766V

    Ampl7.png
     
    thomasbalshi likes this.
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