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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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  • 1. There is sufficient information regarding the gain of the circuit. 

    2. Voltage gain Av = (-3B)/(26mV) where B is the Beta of the circuit. 

    3. Seems like the amplifier is highly dependent on Vbe of the circuit, which limits the design on the specified temperature ( which is where I got the 26mV ) - a parameter that varies as soon as power is drawn from the supply. 

    4. One thing to add is to simply put an emitter resistor to act as a voltage feedback in its input, thus reducing the design's dependency on Vbe. 

  • Woops, something went wrong with my model, forgot that the input impedance is Bre and not just re. So B cancels out.

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