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Transcript
2. Static Characteristics II 2.1 Simple Transistor Inverter VCC IC RC IB Logic Levels: HI Vout = VCC LO Vout = 0V ( VCE SAT) C B RB T VO E Vi Fig. 2.1 Simple Bipolar Transistor Inverter With V VL 0V, IB 0, i Input LO Output HI IC 0 VO VCC VH logic inverter action Under these conditions, the transistor is cut-off or simply “OFF”. This is equivalent to it acting like an open switch as shown in Fig. 2.2, where the state of the switch is controlled by the input to the base. Since the switch does not allow collector current to flow, then the output voltage is pulled up to the supply rail by the collector resistor, RC. In reality, the transistor is not an ideal switch and some leakage current flows through it making it look like a very large resistance >> 100k but which does not appreciably lower the HI output voltage. VCC Rc Ic = 0 Fig. 2.2 The Transistor Inverter in OFF Switch Mode Vi controls switch Vo = Vcc 1 On the other hand with, Vi VH VCC , Input HI IB VCC - VBE , RB Output LO : IC VCC - VCE SAT IC MAX RC and VO VCE SAT VL logic inverter action Under these conditions, with IB > IC max/βF, the transistor is driven into saturation and is referred to as “ON”. This is equivalent to it acting like a closed switch as shown in Fig. 2.3. In this case, the switch allows maximum collector current to flow so that all of the supply voltage is dropped across the resistor, RC, and the output voltage falls close to zero. In practice, there will be some small voltage drop between the collector and emitter of the transistor. This is usually less than 0.1V but at worst case is taken as 0.2V and is called VCE sat. V CC Rc IC IC MAX Figure 2.3 The Transistor Inverter in ON Switch Mode Vi controls switch V O 0V ( V CE ) SAT Note that, since VCE sat is close to zero in the saturation mode of operation then the power dissipated by the transistor, which is given as P = VCE x IC, is very small. The important thing is to ensure that sufficient base current is provided with the input voltage HI to guarantee and maintain the transistor operating in the saturation region. 2 2.2 Base Overdrive Factor The base overdrive factor is a measure of how far into saturation a transistor is driven i.e. how much the base current is above that which is necessary to bring the transistor to the edge of saturation (the boundary between saturation and the forward active region). Base Overdrive Factor σ u Actual Base Current Base Current Required to Reach Edge of Saturation The base current required to reach the edge of saturation is simply the base current required to initially bring the collector current to its maximum value. This is ICMAX/βF . Then: σu IB I C max β F (VCC VBE SAT )/R B (VCC VCE SAT )/β FR C If VCC VBE SAT , VCE SAT then this approximates to: σu VCC RB R F C VCC FR C RB Normally, a σ u of 5 is used to allow for temperature variations in F and also the fact that F itself is reduced when a transistor is operating in the saturation region. It also allows for manufacturing variations in F which can be as much as 3:1 for a discrete transistor. The degree of overdrive also becomes reduced when a load is connected to the circuit. 2.3 Base Charge in Saturation As the base current of the transistor is increased from zero, the collector current rises proportionately in the forward active region. The excess minority charge concentration present in the base region also increases proportionately. When the transistor enters the saturation region, however, the collector current remains constant at its maximum value, IC MAX , but the minority charge concentration continues to increase proportionately as the base current is further increased and the transistor is overdriven well into the saturation region as shown in Fig. 2.4. The excess minority charge stored in the base rises as the base current is increased in the forward active region. The slope of this profile reaches a maximum at the edge of 3 IC Forward active region Saturation region Q 'B IC IC MAX Q’ B EOS Slope F Edge of saturation IB IB IC IB u MAX F ICMAX F Fig. 2.4 Increase in Base Charge with Transistor Overdriven Edge of saturation Excess charge associated with base overdrive Rising charge profile as IB is increased in F.A. mode Fig. 2.5 Charge Profile in Base Region with Transistor in Saturation 4 saturation. On being overdriven, the charge continues to rise in the base but the slope of the profile remains constant at the value reached at the edge of saturation as can be seen in Fig. 2.5. Note that it is the slope of the minority carrier profile that determines the value of the diffusion current. 2.4 Physical Mechanism in Saturation In the forward active mode most of the electrons injected from the emitter into the base make their way to the collector. With a reasonable reverse bias on the base-collector junction, there is a substantial voltage drop across the transistor from collector to base. As the base current is increased, the collector current also rises and with the load resistor, RC, in the collector circuit, the voltage drop across the transistor, VCE, falls. Ultimately, with further increase in base current, VCE falls to allow the base-collector junction to become forward biased. This defines the onset of saturation of the transistor when both junctions become forward biased (see Fig. 2.6). When the base-collector junction becomes forward biased, it begins to conduct also. In this case, holes are injected from the base into the collector and also electrons are injected from the collector into the base. The holes crossing from the base into the collector region constitute the primary component of the base overdrive current. On reaching the collector, these holes can recombine with electrons reaching the collector region from the emitter. This also offsets any tendency for the collector current to rise with increasing bias on the base-emitter junction. This situation leads to a significant rise in the minority carrier concentration in the base as seen in the charge profiles in Fig. 2.6. The base can be considered in saturation as having a forward component of minority carrier charge flow and a reverse component of minority carrier charge flow due to the base overdrive. 5 VBE VBC + + Base-Emitter Forward Biased IB Base-Collector Forward Biased IC IE h h h e- e- e- ee- eE n B p C n- E nb pc pe forward component reverse component Fig. 2.6 Transistor Physical Operation in Saturation 6 B