Fall time

Fig. 10. Speed (90—10% fall time) vs peak wavelength for commercial communication LED emitters. Output power levels in mW are given in parentheses.

We know from Chapter 2 that the harmonic amplitudes depend on the rise and fall times. That is one reason why engineers often try to slow down the Mosfet (increase its transition time), usually at the expense of some efficiency, though sometimes it can even help improve the efficiency, as we will see. 

As discussed in Section 15.2.1.3, the thermal coupling between A and T takes place through a contact thermal resistance Rc, which scales as l/r3 (see Section 4.4). The increase of Rc produces several drawbacks, as we shall see in this section, first of all an increase of the fall time of the detector pulses. 

Sample Shape Fall time (s) Broken/total %broken... 

For data transfer applications the modulation speed of an emitter is a decisive parameter. Here the long lifetime of the excited state in PS becomes problematic. The fall time of the EL is usually in the ps regime, while somewhat shorter values are reported for the rise time. Only for devices based on OPS has a significantly shorter fall time (0.03 ps) been reported [WalO]. For small signal modulations of the EL from PS, frequencies in the order of 1 MHz are reported Ts4, Co5]. Such modulation frequencies are sufficient for display applications. For data transfer via optical interconnects, however, they are much too low. 

Fall time, t/. the interval between the time the instrument indicates -100% of the step decrease in pollutant concentration (t oo) and the lag time (t,) ... 

The fall time does not necessarily equal the rise time. 

Figure 12.4 Bending response of a cantilever (as measured by the voltage output from a position-sensitive detector) to applied voltage pulse with and without TNT adsorbed on the surfaces. The bending of the uncoated cantilever follows the time profile of the applied voltage pulse (except the lengthening of the rise and fall times) and is presumably due to the difference between the thermal expansion coefficients of silicon and the doping material. The exothermic nature of the TNT deflagration event is clear due to the enhancement in bending of the cantilever.

The turn-on and turnoff measurements were performed at room temperature using a power supply voltage of 300V and a load resistance of approximately 20Q in common emitter mode. Typical turnoff and turn-on transients recorded at 25°C are shown in Figures 6.21 and 6.22. The device was turned off by simply removing the base current. A turn-on rise time of 160 ns and a turnoff fall time of 120 ns were observed at 25°C. In addition, a storage time of approximately 40 ns was observed. 

S0LUTI0I1 Use the pulsed voltage source. Set the rise and fall times to 1 ps so that the rise and fall times are much shorter than the pulse width and period of the square wave. Wire the circuit ... 

The pulsed voltage source can be used to create an arbitrary pulse-shaped waveform. We will use it to create a 1 kHz square wave. The rise and fall times of the square wave will be 1 ps. Double-click the LEFT mouse button on the pulsed voltage source graphic, <0, to obtain its spreadsheet and edit its attributes ... 

FALL TIME - The amount of time the voltage source takes to go from the pulsed voltage to the initial voltage, in seconds. 

We would like to create a 5 V square wave at a frequency of 1 kHz. The rise and fall times will be 1 ps. We will specify the following values for the attributes Period = lm, rise time = lu, fall time = lu, pulse width = 0.5m, initial voltage = 5, pulsed voltage = -5, delay time = 0. A few of these settings are shown in the spreadsheets below ... 

Click the OK button when you have made the changes to return to the schematic. The input to the inverter will be a short 1 ps pulse. The attributes of the pulsed voltage source are Period = 50u, rise time = In, fall time = In, Pulse width = lu, initial.voltage = 0, Pulsed voltage = 5, delay.time = lu. Double-click the LEFT mouse button on the pulsed voltage source graphic to obtain the spreadsheet for the source ... 

The results show that the base resistance affects the fall time and the rise time. See EXERCISE 6-19 to generate plots of fall time versus Rb. 

EYFnr.mE B-19 Continuing with EXERCISE B-1B, we would like to see how the base resistor Rt affects the rise and fall times. 50LUTI0I1 Rerun EXERCISE E-IB. When Probe runs, enable the Performance Analysis. First add the trace Falltim (V(VO)) ... 

When designing digital circuits we are usually concerned with the rise and fall times of the design, given device tolerances. The example given here is for a CMOS inverter, but the procedure used can be applied to any switching circuit with device tolerances. Wire the circuit below ... 

We would like to see how the rise and fall times vary with random device tolerances. We must set up the Transient Analysis to view waveforms versus time, and the Monte Carlo analysis to allow for device variations. First we will look at the input pulsed waveform. The property spreadsheet for Vi is ... 

II pulse width[pulsed voltage RISEJTIME INITIAL.VOLTAGE PERIOD DELAYTIME FALL TIME... 

The attributes specify a 0 to 5 V pulse with a 0.5 ps pulse width and 1 ps period. A delay time of 100 ns is specified so that the pulse does not start until 100 ns after the beginning of the simulation. The rise and fall times are 1 ns. We would like to set up a Transient analysis to simulate one cycle of die input. Select PSpice and then New Simulation Profile from the Capture menus, enter a name for the profile, and then click the Create button. By default the 77/776 Domain (Transient Analysis type is selected. Fill in the parameters as shown in the 77/776 Domain dialog box below ... 

We would like to find out what the minimum and maximum rise and fall times are from this data. We can view the results as a histogram. First, delete the trace Vfl/O to obtain an empty window ... 

The minimum and maximum fall times are approximately 17.5 ns and 86.6 ns, respectively. 

EXEHC1SE 9-0 Find the minimum and maximum rise and fall times for the BJT inverter studied in Section 6.K. Let the resistors have 20% Gaussian distributions and let have a uniform distribution from 50 to 350. Start with the circuit from Section 6.K, but use the 5% resistor model and the 2n3904 BJT model (R5pcnt and Q2n3904) ... 

The waveforms versus time for the rise and fall times are ... 

Reduce the rise and fall times of PULSE sources. Drastic changes in voltage can result in nonconvergence problems. Soften the edges of the pulse source by increasing the rise time and fall time of the pulse waveform. 

The parameters that will be measured in each of the software packages and the hardware are the minimum and maximum voltages, the rise and fall time of the output, and the effective pulse width. The response of the IsSpice model is shown as Fig. 3.32. Micro-Cap results are shown as Fig. 3.33. PSpice results are displayed in Fig. 3.34, and the hardware measurements are shown as Fig. 3.35. 

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