how can i adjust the Av gain of this amplifier from 12 to 10, the AC signal is 10mV, 1kHz,

Discussion in 'Electronics Homework Help' started by NYAME EPHRAIM, Jan 12, 2018 at 1:58 PM.

  1. NYAME EPHRAIM

    NYAME EPHRAIM

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    BJT GAIN-1.jpg


    [Mod Note -- image resized]
     
    Last edited by a moderator: Jan 12, 2018 at 9:45 PM
    NYAME EPHRAIM, Jan 12, 2018 at 1:58 PM
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  2. NYAME EPHRAIM

    duke37

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    10MV is quite a good voltage. Do not get within 20m of it.
    If the transistor has reasonable curret gain, then the collector current is similar to the emitter current.
    The voltages will be proportional to the resistors.
     
    duke37, Jan 12, 2018 at 2:32 PM
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    NYAME EPHRAIM

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    ok, so how can i achieve gain Av of 10, the

    the circuit is presently operating on Av gain of 12 ?
     
    NYAME EPHRAIM, Jan 12, 2018 at 2:38 PM
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  4. NYAME EPHRAIM

    NYAME EPHRAIM

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    Ok, so how can i achieve Av gain of 10, as the present circuit is operating at the Av gain of 12
     
    Last edited by a moderator: Jan 12, 2018 at 9:41 PM
    NYAME EPHRAIM, Jan 12, 2018 at 2:44 PM
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  5. NYAME EPHRAIM

    duke37

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    Is this a school question?

    I have given all the information you need. The gain is the ratio of the resistors at present 10k/830.

    What would you do to get a gain of 10?
     
    duke37, Jan 12, 2018 at 2:44 PM
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  6. NYAME EPHRAIM

    NYAME EPHRAIM

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    yes, my target is to adjust the components to achieve gain of 10 as the present gain is 12
    thanks
     
    NYAME EPHRAIM, Jan 12, 2018 at 3:09 PM
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  7. NYAME EPHRAIM

    Audioguru

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    The voltage gain is not 12 times, my simulation and calculation including the emitter resistance inside the transistor is a gain of about 11.5 times but the 830 ohms is not a standard resistor value.
    I used 1k ohms to replace the 830 ohms and got a voltage gain of about 9.5 times. Add 20k parallel with the 1k to get a gain of 10.
     

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    Last edited by a moderator: Jan 12, 2018 at 9:44 PM
    Audioguru, Jan 12, 2018 at 4:14 PM
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  8. NYAME EPHRAIM

    duke37

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    I do not think that a precise answer was required but an understanding of what gives the gain.
    As a rough calculation 10k/830 = 12
    For a gain of 10, use 10k/100

    We should not do school work without help from the student. An appreciation of the difference between 10MV and 10mV would be welcome.
     
    duke37, Jan 12, 2018 at 4:57 PM
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    NYAME EPHRAIM

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    thanks Sir,
    i did as ur said but did not get the gain of 10, please my hfe is 300, my transistor is BC 108, also Ac signal is 10 milil volts, 1KHz

    please will appreciate ur guidance
     
    NYAME EPHRAIM, Jan 12, 2018 at 5:22 PM
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  10. NYAME EPHRAIM

    Ratch

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    Why do you have your base voltage so low (1,.5 volts)? That makes your collector voltage 2.35 volts. You are only 0.85 volts from saturation. Try making the base voltage one-third of the voltage source, so as to be 4 volts. If you have a scope, see that no clipping is present.

    Ratch
     
    Ratch, Jan 12, 2018 at 7:35 PM
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  11. NYAME EPHRAIM

    duke37

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    I should have said 10k/1k
     
    duke37, Jan 12, 2018 at 8:02 PM
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  12. NYAME EPHRAIM

    Audioguru

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    hfe is AC current gain and has almost nothing to do with a transistor's voltage gain.
    The BC108 is very old and is not made anymore. A BC547 uses the same chip but in an epoxy package. A BC547B is a typical one and my new simulation uses it.

    The voltage gain of a transistor with no additional load is the ratio of its collector resistor to its emitter resistor in series with the transistor's internal emitter resistance. Since the emitter resistor value is much higher than the internal emitter resistance then the voltage gain is roughly 10k/1k= 10.0 times. I added a 22k resistor parallel with the 1k emitter resistor to make the total emitter resistor 957 ohms since the internal emitter resistance of the transistor is 1k - 957 ohms= 43 ohms at this collector current.
    My simulation shows the output is not clipping and the voltage gain is exactly 10.0 times.
     

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    Audioguru, Jan 12, 2018 at 8:36 PM
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  13. NYAME EPHRAIM

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    @NYAME EPHRAIM, please be careful that you understand @Audioguru's answer.

    If you use it literally, you might get the answer right in practice, but wrong in your assignment.

    I suggest that you make sure you understand the relationship between the resistors and the (appropriate) gain of the stage before you submit your answer.
     
    (*steve*), Jan 13, 2018 at 11:38 PM
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