Bipolar Junction Transistor Circuits Biasing BJT Operating Regimes Let s start by reviewing the operating regimes of the BJT They are graphically shown on Figure 1 along with the device schematic and relevant parameters IC Saturation Breakdown IB4 Active C IB3 IC IB2 V CE B IB1 IB 0 Cutoff IB V BE IE E VCE Figure 1 BJT characteristic curve The characteristics of each region of operation are summarized below 1 cutoff region B E junction is reverse biased No current flow 2 saturation region B E and C B junctions are forward biased Ic reaches a maximum which is independent of IB and VCE VBE No control 3 active region B E junction is forward biased C B junction is reverse biased VBE VCE VCC I C I B Control 4 breakdown region I C and VCE exceed specifications damage to the transistor 22 071 6 071 Spring 2006 Chaniotakis and Cory 1 We will focus on operation in the active region In this region of operation the model of the BJT is shown on Figure 2 C IC IB B IB V BE E Figure 2 Large signal model of the BJT operating in the active region The large signal model represents a simple state machine The two states of interest are 1 B E junction is forward biased VBE 0 7 Volts current flows and the BJT is on 2 B E junction is off no current flows and the BJT is off We are interested in using the transistor as an amplifier with amplification A as shown on Figure 3 for which V0 AVI VI A V0 Figure 3 Amplifier symbol For the generic BJT circuit the voltage transfer characteristic curve output voltage versus input voltage is shown on Figure 4 For amplification the transistor must operate in the active or linear region 22 071 6 071 Spring 2006 Chaniotakis and Cory 2 V0 Cutoff Active regime large slope amplification Saturation VCE sat VBE on VI Figure 4 Voltage transfer curve for BJT circuit This presents a challenge since we normally have a signal that is carried by for example a time dependent voltage which is permitted to go to or through zero Now we can not simply apply this voltage to the base since the transistor would be moving in and out of the linear operation region Consider the amplifier circuit of Figure 5 We will qualitatively investigate the voltage transfer characteristics of this circuit for two cases of the input signal VI These two cases are graphically illustrated on Figure 6 a and b VCC RC IC Vo V1 RB IB Figure 5 Amplifier circuit The fluctuations of the input signal on Figure 6 a result in excursions outside the active region of operation As shown on the plot a portion of the signal is clipped Compare this to the case shown on Figure 6 b Here the input signal has been shifted to the middle of the 22 071 6 071 Spring 2006 Chaniotakis and Cory 3 active region and as a result the fluctuations of the input are within the range of operation and the complete amplified signal appears at the output Clipped signal VBE on V0 Active region t VI t a V0 VBE on Active region t t VI b Figure 6 Therefore the simple solution is to offset the input voltage to the middle of the linear response region In this way the system may accept input voltage fluctuations and still 22 071 6 071 Spring 2006 Chaniotakis and Cory 4 remain in the active region of operation This offsetting of base voltage is called biasing and we will next investigate various circuit configurations that accomplish this task The Q Point The Quiescent Q point corresponds to a specific point on the load line The amplifier operates at the Q point when the amplitude of the time varying signal is zero This is shown on the characteristic curve shown on Figure 7 V0 VBE on Q Point VI Figure 7 The Q point may also be represented on the I C versus VCE characteristic plot as shown on Figure 8 Here the Q point is at the intersection of the load line with one of the operating curves of the transistor IC Load line Q point IC sat Ib4 IB3 IB2 IB1 VCC VCE Figure 8 22 071 6 071 Spring 2006 Chaniotakis and Cory 5 In fact the Q point could be at any of the intersection points between the load line and the transistor curves We have chosen the Q point corresponding to I B 2 on the plot of Figure 8 since that point is at about the midpoint of the load line In amplifier design applications the Q point corresponds to DC values for I C and VCE that are about half their maximum possible values as illustrated on Figure 9 This is called midpoint biasing and it represents the most efficient use of the amplifiers range for operation with AC signals The range that the input signal can assume is the maximum and no clipping of the signal can occur assuming that the signal range is less than the maximum available IC Q point IC sat ICQ VCEQ VCC VCE Figure 9 Q point for midpoint biasing BJT Bias Types Base Bias Consider the circuit shown on Figure 10 VCC RC RB V0 vi Figure 10 Base bias circuit 22 071 6 071 Spring 2006 Chaniotakis and Cory 6 The DC operating point Q point for this circuit is determined independent of the signal source vi We have indicated the AC signal by the lower case v in order to distinguish it from the DC operating point indicated by the capital V The DC current into the base may be determined by voltage around the base emitter loop which gives VCC I B RB VBE 1 1 and the DC base current becomes IB VCC VBE RB 1 2 For operation in the active region IC I B VCC VBE RB 1 3 Consideration of the collector emitter loop gives VCC I C RC VCE 1 4 And the collector current is IC VCC VCE RC 1 5 Equation 1 5 is the load line equation for this circuit and it along with Equation 1 3 define the Q point We see that the Q point for the Base bias configuration depends on the value of the transistor The value is an imprecise quantity and it is also a quantity whose value changes with temperature Therefore the Q point will shift around as changes which is not a desirable characteristic of the design DO NOT DESIGN WITH 22 071 6 071 Spring 2006 Chaniotakis and Cory 7 Voltage Divider Bias Next let s consider the circuit shown on Figure 11 Here we have used a voltage divider network at the base of the transistor and we have also added resistor RE at the emitter The capacitor C is called the coupling capacitor and it provides DC isolation between the amplifier and the signal source vi Therefore the role of the coupling capacitor is to suppress all DC components that might be present in …