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CE Design question

Discussion in 'General Electronics Discussion' started by Battleship, Mar 27, 2013.

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  1. Battleship


    Mar 27, 2013
    Good day

    I`m fairly new to Electronics and still learning as i go on. I want to design a CE amplifier with the following requirements

    Voltage gain: 100v/v
    Input resistance: 30KOhm
    Output resistance: 2.2KOhm

    With a split supply +-9v

    I`m able to get all of these except a high input resistance of 30 k. Rin=(B+1)re?.My Rin = 2k.

    I also tried using a split emitter but then my voltage gain drops to 18v/v.

    Any help will be appreciated!

    Attached is a schematic

    Attached Files:

    Last edited: Mar 27, 2013
  2. Lyker


    Mar 27, 2013
    Hello Battleship,
    Because you want a gain of 100 and an output impedance of 2.2kΩ means you will need to shunt the emitter with around 22 ohms. Given a base impedance of Zb = (hfe+1)*RE means that a typical transistor with an hfe of 100 around will present a base impedance of around 2k2 and that’s without base biasing.
    C1 in your circuit will shunt the emitter with an impedance of 3.3Ω at 1kHz, the hfe of a BC547B is approximately 300 so this will give you input impedance of 1303Ω at 1kHz, and as I say you don’t have base biasing yet.
    Also it’s worth considering the intrinsic emitter resistance (re). For example 100µA quiescent collector current at room temperature will result in a re value of 258Ω, so most of your input signal will be lost there (picture this as a resistor in series with the base-emitter junction).
    Your gain also suggests low input signal levels where again re will be a nuisance. You would do better cascading a couple of stages to achieve your requirement.
    Hope this helps.
  3. Battleship


    Mar 27, 2013
    Thank you for the reply Lyker.

    Tried the intrinsic emitter and got the results i was looking for. re=11ohm, Rout = 2.2k,Rin= >30k, and Av=100v/v.

    Attached Files:

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