First Circuit Design Project

Discussion in 'Homework Help' started by captivated, Apr 4, 2010.

  1. captivated

    Thread Starter New Member

    Apr 4, 2010
    This is my first time designing an actual circuit. I'm a sophomore in college and was wondering if i could get some help.
    I'm trying to design a BJT Amplifier (npn) that has the following specifications:
    -9v battery operation
    -Rin>5000 Ω (100-7KHz)
    -rout<500 Ω(100-7KHz)
    -Midband voltage gain 10V/V (from source to output a with a 1k load capacitively coupled connected
    -upper 3db frequency cutoff 7KHz
    -lower 3db frequency cutoff<100KHz
    -circuit must have bias stability β values between 50 and 200
    (nominal value 100)

    Source may be modeled as a thevenin equivalent having a sinusodial output of 50mV, 100-20KHz and Rs-50 Ω. Minimize power consumption in your design.

    I don't know where to start nor do I have an idea what I'm doing. I'm supposed to use SPICE amongst all this to figure it out. What is the Midband voltage gain? How do I do this?
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
  3. hobbyist

    Distinguished Member

    Aug 10, 2008
    see if this helps any.

    Here are a couple equations I came up with to determine RE and RC in terms of total input resistance required.

    RT is the total specified INPUT resistance, this case > 5K.
    RB is the parrallel combination of the 2 base resistors, this needs to be larger than RT.
    Bmin is the specified min. beta.
    Av is the chosen voltage gain for a stage.


    1.) RE = RT*RB / {Bmin.*(RB-RT) }

    2.) RC = Av.*RT*RB / {Bmin.*(RB-RT)}

    Using #1,
    is good for determining what value of RE could be chosen to obtain the specified overall input resistance. Using this equation, different values of RB could be substituted to acquire a RE value close to what you would need to have the proper bias voltage at the collector for linear operation, per given input signal voltage.

    Using #2.
    is good for making a quick determination, if the input resistance needed and the output resistance specified, are going to work for one stage, or would you need another stage.

    Also by pluging different values for Av. you can determine what gain would be possible with the specified resistance out (RC) and resistance in (RT).

    These may help ,at least getting a start on your project.

    If I come up with more equations I'll post them as well.

    Have fun...

    Last edited: Apr 7, 2010