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. VCEICIB4IB3IB2IB1I=0BSaturationActiveBreakdownCutoffCBEIEICIBVBEVCE++-- 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 β. . No control. CE BEVV<3. active region: B-E junction is forward biased, C-B junction is reverse biased , BE CE CCVVV<<CBIIβ= . Control 4. breakdown region: CI and exceed specifications damage to the transistor CEV22.071/6.071 Spring 2006, Chaniotakis and Cory 1We will focus on operation in the active region. In this region of operation the model of the BJT is shown on Figure 2 IBICVBEIBβBCE 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, 0.7BEV= 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 0 IVAV= VIV0A 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 2VIV0 Active regime(large slope) amplificationSaturationCutoffV(sat)CEV(on)BE 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 . These two cases are graphically illustrated on Figure 6 (a) and (b). IV ICIBVCCV 1VoRBRC 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 3active 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. VIV0VBE(on) Clipped signal Active region tt (a) VIV0VBE(on) Active region tt (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 4remain 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. VIV0VBE(on) Q-Point Figure 7 The Q-point may also be represented on the CI versus 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. CEVVCEICIB3IB2IB1VCCIC(sat)Ib4Q-pointLoad line Figure 8 22.071/6.071 Spring 2006, Chaniotakis and Cory 5In 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 2BI 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 CI and 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) CEV VCEICVCCIC(sat)Q-pointICQVCEQ Figure 9. Q-point for midpoint biasing BJT Bias Types Base Bias. Consider the circuit shown on Figure 10. RCRBviVCCV0 Figure 10. Base bias circuit 22.071/6.071 Spring 2006, Chaniotakis and Cory 6The 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 CC B B BEVIRV=+ (1.1) and the DC base current becomes CC BEBBVVIR−= (1.2) For operation in the active region, CC BECBBVVIIRββ−== (1.3) Consideration of the collector-emitter loop gives CC C C CEVIRV=+ (1.4) And the collector current is CC CECCVVIR−= (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 β value of the transistor. The β value is an imprecise quantity and it is also a quantity whose value changes with temperature. Therefore, the Q-point will shift around as β changes which is not a desirable characteristic of the design. DO NOT DESIGN WITH β 22.071/6.071 Spring 2006, Chaniotakis and Cory 7Voltage 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 ER 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