Emitter voltage

The seeond seheme, shown in Figure 3-34, is ealled proportional base drive and always drives the transistor at or just below the transistor s saturation state. The eolleetor-emitter voltage is higher than with fixed base drive, but the transistor ean now switeh in about 100 to 200 nS. This is five to ten times faster than with fixed base drive. In praetiee, though, the fixed base drive seheme is used in the majority of low- to medium-power, low-eost applieations. Proportional base drive is used for the higher-power applieations. 

If the switch is a BJT, this is a clear no-no because a bipolar attempts to block reverse voltage, but is really not designed to operate with any reverse collector-emitter voltage. 

For a BJT, the Bias Point Detail gives the collector current, the collector-emitter voltage, and some small-signal parameters for the BJT at the bias point. For a jFET, the Bias Point Detail gives the drain current, the drain-source voltage, and some small-signal model parameters at the bias point. The results of the Bias Point Detail are contained in the output file. We will illustrate the Bias Point Detail analysis with the circuit below ... 

In this section we will investigate how the DC current gain (Hfe) of a bipolar junction transistor varies with DC bias collector current Icq, DC bias collector-emitter voltage Vceq, and temperature. We will use the basic circuit shown below for all simulations ... 

The next thing we would like to do is to see how the Hfe versus Ic curve is affected for different values of DC collector-emitter voltage. The curve in the previous example was generated at VCe = 5 V. We would now like to generate four curves at different values of Vce and plot them all on the same graph. We will generate curves at Vce = 2 V, 5 V, 10 V, and 15 V. We will use the same circuit and simulation profile as in the previous section ... 

The constant current source keeps the emitter current constant at 10 mA. Since the collector current is approximately equal to the emitter current, the constant current source keeps the collector current approximately constant at 10 mA. Since the base is grounded, the emitter voltage is a diode drop below ground ... 

EXERCISE 4-12 For a 2N3904 transistor, if Ic is held constant at 10 mA, how does the base-emitter voltage vary with temperature ... 

S0LUT10I1 Use the simulation of the previous example and plot the base-emitter voltage ... 

When biasing a BJT, we are also interested in the collector to emitter voltage, V g The minimum or maximum value of Vq can also be easily found using the Worst Case analysis. We can use the same setup that was used to find the collector current. All we have to do is modify the Monte CarlO/WorSt Case settings. Fill in the dialog boxes as shown below ... 

Note that the Output variable is VCE(Ql), the collector-emitter voltage of Ql. MIN and LOW are selected, so we are asking for the minimum value of Vqj. Run the analysis. The results are given at the end of the output file ... 

One problem, or should we say challenge, in designing this type of circuit is to ensure that the Darlington pair transistors are not overstressed by causing them to dissipate too much power. The maximum power the transistor can dissipate decreases with increasing collector-emitter voltage. 

Figure 9.21 shows that the total emitter current Ip depends exponentially on the emitter voltage VE and is displaced to the left, as the collector voltage Vc increases. Figure 9.22 shows that when the npn transistor is designed properly, the collector current Ic is almost independent of the collector bias Vc and increases linearly as the emitter current IE is increased. The collector current Ic is also very close to the emitter current IE that is, the dimensions and conductivities of the three regions are so adapted that the base current IB is kept small. The curves in Fig. 9.22 for the npn transistor resemble the curves in Fig. 9.11 for the vacuum-tube pentode. 

The total collector current Ic depends on the reverse saturation current IIS (as explained for pn junctions above also called Ico, or collector current at zero emitter current) and on the emitter voltage VE by the Ebers-Moll equation [already introduced in Eq. (9.6.1)] [14] ... 

One can define four coefficients or hybrid parameters hu, / 12, h2i, and I122 and assume a linear dependence of the emitter voltage vE and the collector current ic on the emitter current iE and the collector voltage vCr as follows ... 

On the other hand, for turn-off we do want to create a hard turn-off, yanking the base momentarily, several Volts below the emitter voltage. 

An enhancement to the efficiency of the Class B amplifier makes use of the addition of a third harmonic component of the right amount to the collector-emitter voltage waveform to cause near square-wave flattening when it is near zero (where the collector current is greatest). This modification alters the amplifier enough so that it enjoys a different classification called Class F. Flattening enhances efficiency by as much as one-eighth so that... 

It is not convenient to analyze the Class C amplifier by means of the loadline argument as in the cases of Classes A or B, but rather, a time- or phase-domain analysis proves to be more workable. The reason is that in the other classes conduction time is fixed at full time or half In Class C, the conduction time is variable so that a general analysis must be performed where time or phase is the independent variable, not collector-emitter voltage. A time or phase analysis for Classes A or B could have been done, but then only one method would be presented here and the intention is to present another way. The results are the same and comparisons presented at the end of this section. 

A phase domain diagram of the nonsaturated Class C amplifier is shown in Fig. 7.55 along with a representative schematic diagram. The term nonsaturated means that the active device is not driven to the point where the collector-emitter voltage is at the lowest possible value. The curve of V 0) in Fig. 7.55(a) does not quite touch the zero line. Another way of saying this is that the transistor is always active or never conducts like a switch. It is active in the same way as Classes A and B, and because of this similarity, it may be modeled in the same way, that is, as a dependent current source. 

The collector-emitter voltage VceiO), is sinusoidal, but it radians out of phase with the collector current. 

The collector-emitter voltage V ( ) swings from 2 Vcc to zero and differs from the voltage across the load R by the DC component Vcc- Stated before, /acH( ) harmonic currents, do not enter the equations because these currents do not appear in the load. 

Figure 7.59(a) illustrates the collector current vs. collector-emitter voltage behavior of the saturated transistor. Of course, the saturation angle is always contained within the conduction angle. The circuit diagram in Fig. 7.59(b) is the same as Fig. (7.55 b). 

The transistor collector current and collector-emitter voltage waveforms are sinusoidal, which they... 

The onset of saturation occurs when the collector-emitter voltage equals the saturation voltage. Ron is finite allowing Vsat to be greater than zero so that the saturated collector current is finite during saturation. 

FIGURE 11.47 (a) Schematic of measurement circuit using the emitter-switching-only method for measuring temperature of an npn bipolar transistor, (b) heating and measurement currents waveforms heating time tn> measurement time tM, (c) calibration plot of the TSEP base-emitter voltage vs. temperature. 

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