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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. No, the amplification depends on the current gain of the Transistor an on its Base-Emitter diode characteristics. 

    2. -

    3. No, its amplification is very dependent on the actual transistor, and its temperature. The operating point will not be stable and thus the available voltage range at the output is not guaranteed 

    4. The amplifier should be enhanced by an Emitter resistor to stabilize operating point of the amplifier. If a high gain is desired, an additional R-C in parallel to this emitter resistor will enhance gain. 

  • Thanks Bernhard for your feedback.

    Not sure about your statement on question 1: indeed amplification is somehow influenced by the gain beta of the BJT, but when it is large, the AC voltage gain is less dependent ! It is like the open gain of an opamp: when approaching infinite, the close loop gains become totally indendent. 

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  • Thanks Bernhard for your feedback.

    Not sure about your statement on question 1: indeed amplification is somehow influenced by the gain beta of the BJT, but when it is large, the AC voltage gain is less dependent ! It is like the open gain of an opamp: when approaching infinite, the close loop gains become totally indendent. 

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