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3 Static Characteristics III 3.1 Inverter Design VCC IC RC IB F C B RB T VO E Vi IC Maximum gain Fig. 3.1 Bipolar Transistor Inverter Circuit Generally choose IC mid range for a high value of F. Consider a supply VCC = 5V. A typical mid-range value of IC would be 3 - 5 mA. For IC = 5mA, RC VCC 5V 1kΩ ICMAX 5mA A typical F = 50 for an integrated transistor. If we use a base overdrive factor of σ u 5 Then, σu βFR C RB 5 50 1kΩ RB RB 50kΩ 10kΩ 5 The idea is simply to make sure that RB provides sufficient current to overdrive the transistor. Rewriting the relationship, the choice is really: RB F RC . σu 1 3.2 Operation and Output Characteristic IC IC The collector current that flows through the transistor depends on the bias applied to the base-emitter junction according to the normal exponential law for the p-n junction. MAX Typically: VBE cut - in = 0.6V VBE on = 0.7V IC Forward active mode VBE sat = 0.8V VBE CUT - IN VBE Figure 3.2 VBE SAT ON Collector Current as a Function of Base-Emitter Bias Referring to the output characteristic of Fig. 3.3, which shows an operating load line, the following can be seen. At very low input voltages, VBE is small and no current flows in the collector of the transistor, apart from some small leakage current. Hence, the output voltage is at VO VCE VCC . If the input voltage to the inverter is slowly increased, eventually the base-emitter junction reaches the point of cut-in when Vi VBE CUT IN at point A on the characteristic in Fig. 3.3. Here, IC 0 so that VO VCE VCC still. As the input voltage is further increased, base current starts to flow and the transistor enters the forward active mode where I C F IB and the operating point travels upward along the load line. Eventually, the point B is reached where IB ICMAX /F and the transistor arrives at the edge of saturation. In this case, the collector current reaches its maximum value and the output voltage falls to its minimum of VO VCE SAT 0.1 0.2V . Further increase in the input voltage overdrives the transistor deeper into saturation but has little effect on the collector current or the output voltage. 2 saturation VO VCE cut-off forward active mode IC F IB SAT Vi VBE CUT - IN IC mA IC MAX 5 IB edge of saturation 90 A B 80 A 4 load line 70 A RC 1k 60 A 3 50 A V i increasing 40 A 2 30 A 20 A 1 10 A A IB edge of conduction =0 ICE0 VCE 0 VO VCE Fig. 3.3 1 3 2 VCE VCC - ICRC SAT 4 5 VO VCC Output IC vs VCE Characteristic for Single Transistor Inverter 3 3.3 Transfer Characteristic The transfer characteristic of a logic gate is simply a plot of the steady-state output voltage vs input voltage over its range of operation such as that shown in Fig. 3.4. Initially if Vi = 0, T is OFF so that IC = 0 and VO = VCC. As Vi is slowly increased, a point is eventually reached at A, where the transistor begins to turn ON. This is referred to as the cut-in point for the transistor or the “edge of conduction”. As Vi is further increased, the transistor becomes fully conducting and enters the forward active mode between the points A and B on the characteristic. As the collector current increases, the output voltage falls from VCC towards ground until the transistor reaches the edge of saturation at point B. The slope of the characteristic between points A and B is essentially determined by the gain of the circuit. Finally, as the transistor is driven well into saturation, the output voltage levels off at VO VCE SAT . The critical points on the characteristic are points A and B as these are the points which define the transition region in the output between a high and low logic level. They also define the boundaries of operation between, the Cut-off, Forward Active and Saturation modes of operation of the transistor. 3.4 Logic Voltages (a) Output Levels From the transfer characteristic for the single transistor inverter, it can be seen that the output LO and HI logic voltages are well defined. VOL VCE SAT ; VOH VCC (b) Input Levels The input logic voltages are not as clearly defined and can occupy ranges where the output remains at the correct logic level. The critical points, A and B, on the characteristic can be used as critical points to define the limiting values for these ranges. Point A defines the input voltage which just begins to turn on the transistor and is, hence, the maximum input LO voltage. Hence: ViL MAX VBECUT-IN 4 typically 0.6V Point B defines the minimum input voltage which will just keep the transistor at the edge of the saturation region and hence also at the edge of the linear forward active region. At this point: Vi ViHMIN ICMAX FIB V VBE SAT VCC VCE SAT F iHMIN RC RB (VCC VCE SAT )R B ViHMIN VBE SAT FR C ViHMIN VBE SAT RB 1 (VCC VCE SAT ) VBE SAT (VCC VCE SAT ) FR C σu For the component values established above: ViHMIN 0.8 1 5 0.2 0.8 0.2 4.8 0.8 0.96 1.76 V 5 5 VO CUTOFF VOH 5 SATURATION FORWARD ACTIVE A 4 3 2 1 B VOL Vi 0 Fig. 3.4 ViLMAX 1 ViHMIN 2 3 4 5 Transfer Characteristic of the Bipolar Transistor Inverter 6