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KCC's quizzes AQQ235 about a bipolar common emitter amplifier - a kind proposal from our colleague Martin Walker

Apologize for our non-technical audience since this quiz is more for our FAEs...

A kind proposal from our colleague Martin Walker, ADI Product Marketing Engineer, UK:

Most of our electronic engineers have seen this sort of circuit in their first year study time using a bipolar transistor in its 3 famous basic configurations: common emitter, common collector and common base.

Here above is a common emitter configuration.

Conditions:

  • Vcc = 6V
  • Vout = 3V
  • Tc = 25°C

Q1 is a BJT NPN with current gain β of several hundreds.

RB1 and RB2 are large compared to RC and RS

 Questions :

  1. Is there enough information to work out the gain of this circuit?
  2. If so, what is the voltage gain of the circuit?
  3. Is it a good amplifier?
  4. If not, what would you do to mitigate for its limitations?

Again, many thanks Martin!



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[edited by: emassa at 2:19 PM (GMT -4) on 22 May 2023]
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  • OK, it's time to publish the official (and correct) answer (event though, here some subjectivity can be made! Again, many thanks to our colleague Martin Walker for having proposed this original topic

    1. Although we don’t have any component values, we can actually work out the circuit gain. Effectively, the collector load and biasing network have been set to give a half-rail output (3V/6V).
    2. The gain is -116 or 41dB

    In effect:

    Gain of grounded common emitter is -gmRc

    Transconductance gm=  IC/(kT⁄q)

    RL is set to give VCC / 2 so Rc=((VCC-VOUT))/IC

    So, voltage gain A :

     .

    As = 0.0258 at 25 °C, A=-116 (~41 dB)

    This leads to the rule of thumb that the internal resistance of the emitter re = 25 / IC where IC is in mA

    1. No; the gain is highly temperature dependent because VT (the thermal voltage, kT/q changes with temperature and. if the input signal is large enough, it will substantially modulate the collector current Ic, making the gain non-linear.

     4. Adding some negative feedback, usually achieved by adding an emitter resistor Re (much larger than the internal resistance of the emitter terminal, re) will improve the amplifier. This will reduce the gain to -Rc/Re but it will be more stable with temperature, more linear with applied signal input and will increase the bandwidth of the amplifier.

    For a full explanation, see The Art of Electronics (Third Edition) Chapter 2 – 2.3.4. The Common Emitter Revisited.

     You can even simulate it to see the gain is 41dB in the ac analysis and see the non-linearity in the transient simulation.  Martin had attached the LTspice file but it should show the same results in EESim.

    Big applause to our 4 winners (based also on reasoning and explanation provided):

     ,  ,  and  

    And be ready for the next coming challenge!

  • I am indebted to Paul Brokaw for the question.  Here's his talk on ADITube:

    Sheerluck Ohms and the 33dB Solution

     http://aditube.analog.com/video/W165XOB5A7S4/Sheerluck-Ohms-and-the-33dB-Solution-

    Hope you had fun with this.

    Martin.

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