Help with a common Emitter Amplifier

Discussion in 'Homework Help' started by janner, Nov 16, 2009.

  1. janner

    Thread Starter New Member

    Nov 15, 2009
    6
    0
    Hi Guys,

    This is my first post on here so first of all "Hello!"
    Right down to business. I have to design a two stage common emitter amplifier with a gain of 30, I have been given loads of notes but Im needing someone to point me in the right direction with it. Firstly im trying to just design a single stage amplifier get it working then basically just join two together. I know that then I can get the gain by just using 1/beta in the feedback.

    I have been told that I can choose the supply and then choose the Ic. I used a supply of 15V and then an Ic of 4mA. The formulas I have been given to get the rest of the circuit are as follows
    1, Choose Ic I used: 4mA
    2, Make Ve 10% of Vcc I calculated 1.5V
    3, Re = Ve/Ic I calculated 375 ohms
    4, Rc = Vcc/2Ic I calculated 1.875K ohms
    5, R2 = 10 x Re I calculated 3.75K ohms
    6, R1 = (Vcc-(Ve+0.7))x R2 I calculated 21.8K ohms
    Ve+0.7

    Ok so I have done all my calculations and then built the circuit on multisim. The circuit inverts the output but does not ampilfy it? :confused: The output voltage is extately the same magnitude as the input :confused:
    I have attached a copy of my circuit with the wave forms on it but if any other info is needed please just leave a message.

    This does seem a monster first post but any help anyone could give me would be much appreciated. Thanks :)
     
  2. hobbyist

    Distinguished Member

    Aug 10, 2008
    764
    56
    I must say this calculation is a new one to me.

    But if it works that's good. (the equation that is)

    VB / R2 = ID

    and

    (VCC - VB) / ID = R1

    So some how I'll need to rearange these equations to see if I can get that #6, equation.

    Be an interesting algebra problem.
     
  3. janner

    Thread Starter New Member

    Nov 15, 2009
    6
    0
    Aye the equation is divided by (Ve+0.7) it just came out abit wrong when typing sorry.

    I got it to work I just removed the RL resistor I had in the circuit. so thanks anyways guys.
     
  4. hobbyist

    Distinguished Member

    Aug 10, 2008
    764
    56
    (VE + 0.7v.) = VB

    so (VB / R2) = divider Curent.

    The R1 value is {(VCC - VB) / divider current.}

    Are you sure you got your R1 value right?

    Maybe I need to rearange and substitute the equations to get what they gave you for R1 equation.
     
  5. janner

    Thread Starter New Member

    Nov 15, 2009
    6
    0
    no sorry the calculation for R1 is

    R1=

    (Vcc-(Ve+0.7)) xR2
    Ve+0.7

    so I got =

    (15-(1.5+0.7)) x 3750 = 21.8KΩ
    1.5+0.7

    Like i said I have the circuit amplifiying with a gain of 5 but I want to make it much bigger so I need to add in a Capacitor (Ce) so Im off to do more reading on how to do this beacuse as soon as I put one in the circuit my output goes very wrong!
     
  6. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Do you need an AC gain of 30 or a DC gain of 30?

    The Gain of 5 that you are getting currently, is that an AC measurement?
     
  7. janner

    Thread Starter New Member

    Nov 15, 2009
    6
    0
    AC I think?? its a AC signal source. I was just told a gain of 30.

    I have measured my gain of 5 as AC
     
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
    6,357
    718
    Can you post your schematic as it currently is?

    A .png screenshot works the best.
     
  9. janner

    Thread Starter New Member

    Nov 15, 2009
    6
    0
    That's it i think? :confused:
     
  10. Audioguru

    New Member

    Dec 20, 2007
    9,411
    896
    The gain of the transistor without a load is slightly less than Rc/Re.
    Increase the gain by increasing Rc or reducing Re then re-bias it.

    You can also increase the gain without re-biasing it by connecting a resistor in series with a capacitor from the emitter to ground. This reduces the effective value of Re at AC.
     
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