89-npn-bjt-common-emitter-amplifier-with-negative-feedback

Common emitter amplifier realized with NPN bjt with feedback is given in figure bellow.
Simulate NPN BJT common emitter amplifier with negative feedback.

common-emitter-amplifier-with-negative-feedback

Common emitter amplifier with feedback is class A amplifier, same as common emitter amplifier without feedback. Even though both amplifiers are in the same class, negative feedback makes big difference. DC biasing of both amplifiers is the same, since both amplifiers operates in the same class. By adding resistor into emitter branch, amplifier is far less sensitive (smaller gain) and more stable comparing to amplifier without negative feedback. How come that only one resistor putted into appropriate branch have such impact on overall voltage gain? As we mentioned earlier, bipolar junction transistor is highly nonlinear element with exponential dependence of collector (and emitter) current from VBE. If emitter is grounded, small changes of electric potential of the transistor base will exponentially influence emitter current. On the other hand, if there is the resistor in emitter branch, increase of electric potential of the transistor base will increase emitter current, and higher emitter current will (by KVL) increase voltage drop on RE i.e. electric potential of he transistor emitter will also increase, and voltage VBE will be smaller comparing to the situation when emitter was grounded. NPN BJT operates in active mode, so overall voltage gain can be calculated from small AC signal equivalent circuit, given in figure bellow.

common-emitter-amplifier-with-negative-feedback-small-signals

From given equivalent model, it can be calculated that voltage gain of common emitter amplifier with bipolar NPN transistor with feedback applied is:

npn_bjt_cea_w_fdbck_eq1_ispr

Now, if β=300 and h11=1kΩ, RC=5kΩ, and RE=1kΩ, overall gain is only -4,97. Without negative feedback, i.e. without RE, overall gain would be -1500 for the same common emitter amplifier! In many practical applications, for β>100 and h11 and RE in range of several kΩ, voltage gain can be approximated with:

npn_bjt_cea_w_fdbck_eq2

Go to NPN BJT common emitter amplifier with negative feedback.

External Links:

NPN BJT Common Emitter Amplifier with negative feedback on Wikipedia
NPN BJT Common Emitter Amplifier with negative feedback on HyperPhysics
NPN BJT Common Emitter Amplifier with negative feedback on AllAboutCircuits
NPN BJT Common Emitter Amplifier on Electronics-Tutorials

 

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