Electronic – BJT amplifier negative feedback question

amplifierbjtfeedback

This is a BJT amplifier we have learned

I read from the book that the feedback type of it is voltage series negative feedback.
I have 2 questions :

  1. If this is a negative feedback circuit how can we compute its open loop voltage gain?
  2. Is this thing(learn how to analyse bjt/mos negative feedback)important?

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Best Answer

  • My answer to 2.: Yes it is very important because the principle of negative feedback is most important for all electronic amplifiers, filters, oscillators,.. More than that it plays a dominant role for all stability analyses (remember: Amplifiers with feedback can become unstable).

  • My answer to 1.: I think, it is not possible to calculate the "open-loop voltage gain" because the transistor is NOT a voltage amplifier. Instead, it acts as a "voltage-controlled current source" [Ic=f(Vbe)]. That means: The open-loop transfer characteristic is the transconductance gm=Ic/Vt.

(As another example, the same situation exists for the integrated operational transconductance amplifier (OTA), having an "open-loop gain" which also is identical to the transconductance gm of the device).