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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]
Parents
• 1.  Is there enough information?  I assume that we have the values of the resistors, within some range like Rb1 = 460k and Rb2 = 100k, and something reasonable for Vs, like between 1 and 2 volt for example. In fact, with these values, I found that we can get the beta value of the transistor by reading Vout:  beta = 1200 - 200 Vout.  (Not bad, I was impressed of the linearity from beta between 200 and 700 ! ) So, I would answer yes to this question, given "reasonable" assumptions. As counter-example, we can easily enter in deep saturation of the transistor too with Rb2 >= Rb1, that is why I assumed among other things that we are able to get a reading of 3 volt for Vout, which implies that the NPN is not blocking, not in deep saturation, but in active mode). And clearly, beta cannot become negative, so the linearity is definitively limited to some "range".
2. Within what I consider reasonable variations, the formula for beta that I gave up here is independent of Vs ( at least, for values of Vs between 1 and 2 volt). I don't think that we can speak of (constant) gain in this case, if the gain is computed as Vout /  Vs.
3. I won't describe this circuit as an amplifier, but as a beta-reader (given the linearity).
4. It is surely very dependent of temperature since it seems controlled by the Vbe diode behavior. I am not sure on what direction to go in. Do we wish to keep the linearity of the beta value versus Vout, do we wish an amplifier, or something else.

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Additional: You can experiment with the CircuitLab simulation of the circuit (we obtain a lightly different equation, with a less ideal model for the NPN): /https://www.circuitlab.com/circuit/sd2453n4byu6/betareader/

• Thanks Michel for your extensive  explanation on this circuit! Answer to question 1 is correct! A lot can be said when you know the Vout is set at the middle of VCC.

Concerning questions 2, 3 and 4, some different interpretations / appreciations can be made; we can wait for other feedback to compare their arguments. But your remark related to the temperature effect is true.