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Need help on DC Biasing

Discussion in 'Electronics Homework Help' started by BlackMelon, Jun 6, 2016.

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  1. BlackMelon

    BlackMelon

    188
    5
    Aug 7, 2012
    Hello,

    The problem statement is on the file "Bias Ask". After I've seen this, I quickly draw a small signal model of the whole circuit in the file "small signal ask", which allows me to consider this amplifier as "Series-Shunt" or "voltage to voltage" amplifier.

    The problem is not with the small signal circuit but the DC biasing. I need to know the DC collector current(IC) first so I can determine the "early resistance, ro". I set the vs=0V... And so what's next? How can I determine the IC since the 200uA is not emitter current? Can we consider the op-amp as an ideal one (V+ = V-) in DC biasing?

    Thank you
    BlackMelon
     

    Attached Files:

  2. Ratch

    Ratch

    1,092
    333
    Mar 10, 2013
    What are you going to bias, the transistor?. The op-amp is taking care of the base, and the emitter is 0.7 V less than the base. What more is there to do?

    The collector current will vary depending on what resistance you connect to Vo. And who cares, as long as the BJT can handle the current. If you calculate the voltages, you can calculate the current.

    Derive the transfer function assuming a load RL on Vo. Once you know the voltages, you can determine the current.
    The open loop gain is high, but not infinite. I believe the gain was 100000. Use it in your transfer function. Do you know how to do it? Once you know the transfer function, you can get the input and output impedance by the PIT (Port Immittance Theorem).

    Ratch
     
    BlackMelon likes this.
  3. BlackMelon

    BlackMelon

    188
    5
    Aug 7, 2012
    I'm going to find the bias point of the transistor (ICQ, VCEQ)


    You might have to look at this: https://en.wikipedia.org/wiki/Early_effect. My ro doesn't mean the "output resistance", but it means the resistance resulting from an Early Effect.
    It says in the topic "small signal model" that the ro = VA/IC. We know the early voltage(VA) from the question but we don't know the DC collector current.

    Ok, I'll look into it carefully. Thank you for the advise :)

    BlackMelon
     
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