Bipolar Junction Transistor Circuits Voltage and Power Amplifier Circuits Common Emitter Amplifier The circuit shown on Figure 1 is called the common emitter amplifier circuit The important subsystems of this circuit are 1 The biasing resistor network made up of resistor R1 and R2 and the voltage supply VCC 2 The coupling capacitor C1 3 The balance of the circuit with the transistor and collector and emitter resistors VCC R1 RC C1 vi vo R2 RE Figure 1 Common Emitter Amplifier Circuit The common emitter amplifier circuit is the most often used transistor amplifier configuration The procedure to follow for the analysis of any amplifier circuit is as follows 1 Perform the DC analysis and determine the conditions for the desired operating point the Q point 2 Develop the AC analysis of the circuit Obtain the voltage gain 22 071 6 071 Spring 2006 Chaniotakis and Cory 1 DC Circuit Analysis The biasing network R1 and R2 provides the Q point of the circuit The DC equivalent circuit is shown on Figure 2 VCC I CQ RC RTH V0Q VB I BQ VTH I EQ RE Figure 2 DC equivalent circuit for the common emitter amplifier The parameters I CQ I BQ I EQ and VOQ correspond to the values at the DC operating pointthe Q point We may further simplify the circuit representation by considering the BJT model under DC conditions This is shown on Figure 3 We are assuming that the BJT is properly biased and it is operating in the forward active region The voltage VBE on corresponds to the forward drop of the diode junction the 0 7 volts C ICQ I BQ B I BQ re V BE on E Figure 3 DC model of an npn BJT 22 071 6 071 Spring 2006 Chaniotakis and Cory 2 For the B E junction we are using the offset model shown on Figure 4 The resistance re is equal to re VT IE 1 1 kT which at room temperature is VT 26 mV re q is in general a small resistance in the range of a few Ohms Where VT is the thermal voltage VT IE 1 re VBE Figure 4 By incorporating the BJT DC model Figure 3 the DC equivalent circuit of the common emitter amplifier becomes VCC RC I CQ C V0Q I BQ RTH VB B I BQ VTH re E V BE on RE I EQ Figure 5 22 071 6 071 Spring 2006 Chaniotakis and Cory 3 Recall that the transistor operates in the active linear region and the Q point is determined by applying KVL to the B E and C E loops The resulting expressions are B E Loop VTH I BQ RTH VBE on I EQ RE 1 2 C E Loop VCEQ VCC I CQ RC I EQ RE 1 3 Equations 1 2 and 1 3 define the Q point AC Circuit Analysis If a small signal vi is superimposed on the input of the circuit the output signal is now a superposition of the Q point and the signal due to vi as shown on Figure 6 VCC RC C1 ICQ i c ICQ i c V0Q vo VB RTH vi RE VTH IEQ i e Figure 6 Using superposition the voltage VB is found by 1 Set VTH 0 and calculate the contribution due to vi VB1 In this case the capacitor C1 along with resistor RTH form a high pass filter and for a very high value of C1 the filter will pass all values of vi and VB1 vi 2 Set vi 0 and calculate the contribution due to VTH VB 2 In this case the VB 2 VTH And therefore superposition gives VB vi VTH 22 071 6 071 Spring 2006 Chaniotakis and Cory 1 4 4 The AC equivalent circuit may now be obtained by setting all DC voltage sources to zero The resulting circuit is shown on Figure 7 a and b Next by considering the AC model of the BJT Figure 8 the AC equivalent circuit of the common emitter amplifier is shown on Figure 9 ic RC ib vi RTH v ce v be ic Ri vo ib v be vi RTH ie v ce RE Ro RC vo RE ie b a Figure 7 AC equivalent circuit of common emitter amplifier C ic ib B ib re E Figure 8 AC model of a npn BJT the T model ic C ib RC vo ib B re vi RTH E ie RE Figure 9 AC equivalent circuit model of common emitter amplifier using the npn BJT AC model 22 071 6 071 Spring 2006 Chaniotakis and Cory 5 The gain of the amplifier of the circuit on Figure 9 is Av ic RC ib RC RC vo 1 re RE vi ie re RE 1 ib re RE 1 5 For 1 and re RE the gain reduces to Av RC RE 1 6 Let s now consider the effect of removing the emitter resistor RE First we see that the gain will dramatically increase since in general re is small a few Ohms This might appear to be advantageous until we realize the importance of RE in generating a stable Q point By eliminating RE the Q point is dependent solely on the small resistance re which fluctuates with temperature resulting in an imprecise DC operating point It is possible with a simple circuit modification to address both of these issues increase the AC gain of the amplifier by eliminating RE in AC and stabilize the Q point by incorporating RE when under DC conditions This solution is implemented by adding capacitor C2 as shown on the circuit of Figure 10 Capacitor C2 is called a bypass capacitor VCC R1 RC vo C1 vi R2 RE C2 Figure 10 Common emitter amplifier with bypass capacitor C2 Under DC conditions capacitor C2 acts as an open circuit and thus it does not affect the DC analysis and behavior of the circuit Under AC conditions and for large values of C2 its effective resistance to AC signals is negligible and thus it presents a short to ground This condition implies that the impedance magnitude of C2 is much less than the resistance re for all frequencies of interest 1 re C 2 22 071 6 071 Spring 2006 Chaniotakis and Cory 1 7 6 Input Impedance Besides the gain the input Ri and the output Ro impedance seen by the source and the load respectively are the other two important parameters characterizing an amplifier The general two port amplifier model is shown on Figure 11 Ro Ri vi vo Av i Figure 11 General two port model of an amplifier For the common emitter amplifier the input impedance is calculated by calculating the ratio Ri vi ii 1 8 Where the relevant parameters are shown on Figure 12 C ic ie ii vo ib B Ri vi RC re RTH E ie RE Figure 12 22 071 6 071 Spring 2006 Chaniotakis and Cory 7 The input resistance is given by the parallel combination of RTH and the resistance seen at the base of the …