Common emitter amplifier realized with npn bjt is given in figure bellow.
Purpose of R1–R2 voltage divider is to bias NPN BJT into active mode and make possible amplification of any small AC signal applied to its base terminal. Choice of the resistors R1 and R2 should be such that voltage divided from power supply VCC is close to 0.6-0.7V. However, with emitter grounded, divided voltage (and also DC voltage potential of transistor base) could easily exceed 0.7V, and that would put transistor into deep saturation and possibly burn the transistor. By adding resistor RE, electrical potential of the emitter can increase, and so, if for example divided voltage on the base is let’s say 1V, electrical potential of the emitter is 0.3V, so VBE=0.7V! On the other hand, if we are talking about NPN BJT common emitter amplifier without feedback, capacitor CE should be added in order to suppress high frequency changes on the emitter, and so to act as short connection for AC signals. Common emitter amplifier is the single stage amplifier configuration with largest voltage gain, so expected input signal is with very small voltage amplitude. Further, it means that input impedance should be large, i.e. resistors R1 and R2 should have values higher then 100kÎ©. Another criterion for determining R1 and R2 is that DC component of the base current is such that quiescent point is positioned exactly in the middle of the static or DC load line. In that case, common emitter amplifier operates as an class A amplifier.
Maximum collector current corresponds to saturation of the bipolar transistor. So, IC(MAX) is for VCE=0.2V and then
Minimum collector current is when bipolar transistor is Cut-Off. Then collector current is zero and output voltage is VCC. By putting quiescent point in the middle of the load line, when input voltage is zero, output voltage is one half of the VCC. Amplifier operating in Class A, amplifies input signal in both positive and negative half period. This is different from amplifier operating in class B, when amplifier amplifies input signal exclusively during positive or negative half period. In order to calculate common emitter amplifier parameters, such as input impedance, output impedance, voltage and current gain, it is practical to represent amplifier with its small AC signal equivalent. For small AC signal, branch between base and emitter is represented with equivalent resistor
where VT is threshold voltage, that is typically 0.5V and IC0 is quiescent collector current, typically 10 to 100mA, and Î² is current gain of given npn bipolar transistor. Parameter h11 usually vary from 1kÎ© to 10kÎ©.
Bipolar NPN transistor has to be in active regime, i.e. PN junction base-emitter has to be forward biased. It means that for silicon NPN transistor VBE has to be in range from 0.5V to 0.7V. Further, AC input signal has to be, rail to rail 100mV. Let’s assume that in quiescent point, VBE=0.6V. If input AC is on the minimum of -50mV, VBE=0.55V. If input is in maximum of +50mV, VBE=0.65V. Polarization of bipolar NPN transistor prepare transistor to operate in active regime, and then, for small AC input signals equivalent model is given in figure bellow.
From given equivalent model, it can be calculated that voltage gain of common emitter amplifier with bipolar NPN transistor is:
For example, if Î²=300 and h11=1kÎ©, resistor in collector branch of only 1kÎ© will be sufficient for amplifier to have amplification of -300 times. Replacing RC of 1kÎ© with another one of 10kÎ©, amplifier would increase voltage gain to -3000 times! However, we shouldn’t exaggerate with RC value, because too large resistor can easily put transistor in saturation, and then there would be no amplification of input signal, because output voltage would be equal to VCES=0.2V and would remain fixed on that value. Why the output voltage is with opposite phase from the input signal, with this kind of amplifier? Simply, if input AC voltage increases, electric potential of transistor base increases to potential of transistor emitter. This raise of VBE voltage will produce higher collector current. Higher collector current will produce higher voltage drop on RC resistor, so output voltage that is VOUT=VCC-ICRC will decrease, and vice versa.
NPN BJT Common Emitter Amplifier on Wikipedia
NPN BJT Common Emitter Amplifier on HyperPhysics
NPN BJT Common Emitter Amplifier on AllAboutCircuits
NPN BJT Common Emitter Amplifier on Electronics-Tutorials