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 1Rand 2R and the voltage supply . CCV2. The coupling capacitor . 1C3. The balance of the circuit with the transistor and collector and emitter resistors. RCR1VCCvoC1R2viRE 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 1DC Circuit Analysis The biasing network (1Rand 2R) provides the Q-point of the circuit. The DC equivalent circuit is shown on Figure 2. V0QRTHREVBIBQVCCICQIEQRCVTH Figure 2. DC equivalent circuit for the common emitter amplifier. The parameters CQI, BQI, EQI and correspond to the values at the DC operating point- the Q-point OQV 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 corresponds to the forward drop of the diode junction, the 0.7 volts. ()BE onVIBQICQVBE(on)IBQβBCEre Figure 3. DC model of an npn BJT 22.071/6.071 Spring 2006, Chaniotakis and Cory 2For the B-E junction we are using the offset model shown on Figure 4. The resistance is equal to er TeEVrI= (1.1) Where is the thermal voltage, TVTkTVq≡, which at room temperature is 26 mVTV = . er is in general a small resistance in the range of a few Ohms. VBE1/reIE Figure 4 By incorporating the BJT DC model (Figure 3) the DC equivalent circuit of the common emitter amplifier becomes V0Q RTHREVBVCCIEQRCV TH IBQICQVBE(on)IBQβreBEC Figure 5 22.071/6.071 Spring 2006, Chaniotakis and Cory 3Recall 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: (1.2) ()B-E Loop:TH BQ TH BE on EQ EVIRV IR⇒= + + (1.3) C-E Loop:CEQ CC CQ C EQ EV V IR IR⇒=−− 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. RTHRE+ icRCvi+ voVTHC1VCCV0QICQ+ ieIEQ+ icICQVB Figure 6 Using superposition, the voltage is found by: BV 1. Set and calculate the contribution due to vi ( ). In this case the capacitor C1 along with resistor 0THV =1BVTHR form a high pass filter and for a very high value of C1 the filter will pass all values of vi and 1BVvi= 2. Set vi=0 and calculate the contribution due to ( ). In this case the THV2BV2BTVVH= And therefore superposition gives BVviVTH=+ (1.4) 22.071/6.071 Spring 2006, Chaniotakis and Cory 4The 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. R TH R E ib ic i e R C vi vov be v ce + + - - (a) RTHRE ibi c ie R C viv be v ce ++ --v o+- RiRo (b) Figure 7. AC equivalent circuit of common emitter amplifier ibicibβBCEre Figure 8. AC model of a npn BJT (the T model) RTHREβreBEieRCibibCvi ic+ vo - 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 5The gain of the amplifier of the circuit on Figure 9 is ()(1)() 1cC bC CveeE beE eEiR iR RvoAvi i r R i r R r Rββββ−−== = =−+++ ++ (1.5) For 1β>> and the gain reduces to erR<<E CvERAR≅− (1.6) Let’s now consider the effect of removing the emitter resistor ER. First we see that the gain will dramatically increase since in general is small (a few Ohms). This might appear to be advantageous until we realize the importance of erER in generating a stable Q-point. By eliminatingER the Q-point is dependent solely on the small resistance 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 erER in AC and stabilize the Q-point by incorporating ER 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. RCR1VCCvoC1R2viRE+-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 for all frequencies of interest. er 12erCω<< (1.7) 22.071/6.071 Spring 2006, Chaniotakis and Cory 6Input Impedance Besides the gain, the input, iR, and the output, oR, 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. vi+-vo+-RiRoAvi Figure 11. General two port model of an amplifier For the common emitter amplifier the input impedance is calculated by calculating the ratio iiivRi= (1.8) Where the relevant parameters are shown on Figure 12 . RTHREβreBEieRCibieCvi + vo -icRi/βii Figure 12 22.071/6.071 Spring 2006, Chaniotakis and Cory 7The input resistance is given by the parallel combination of THR and the resistance seen at the base of the BJT which is equal to (1 )( )eErRβ++ //(1 )( )iTH eERRrRβ=++ (1.9) Output Impedance It is trivial to see that the output impedance of the amplifier is oCRR= (1.10) 22.071/6.071 Spring 2006, Chaniotakis and Cory 8Common Collector Amplifier: (Emitter Follower) The common collector amplifier circuit is shown on