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UA ECE 304 - Temperature Dependence

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Changing the Temperature in PSPICE Simulations Overview Circuit behavior often has strong temperature dependence. For example, in bipolar circuits the current depends on ISexp(VBE/VT) where the thermal voltage VT = kBT/q (kB = Boltzmann constant, T= absolute temperature and q=electronic charge). When the temperature changes, so does VT. In addition, the scale current IS depends on temperature. In PSPICE, for example, IS is modeled as1 EQ. 1 ())V/E)(1T/T(XNOMSSTGNOMTIeTTI)T(I−= where IS is the scale current at the "nominal" temperature TNOM (set at 300K by default)2 and T is the operating temperature of the transistor (both in degrees Kelvin). Other parameters appear in EQ. 1 with values that are set in the •model statement for the particular type of transistor under consideration (for example, for the bipolar type Q2N2222, XTI = 3 and EG = 1.11V). Next is a discussion of how to control temperature in PSPICE, including setting individual temperatures for each device in the circuit. Setting TNOM The default value3 for parameter TNOM is set by selecting PSPICE/EDIT PROFILE/SIMULATION SETTINGS and using the OPTIONS tab to get the menu in Figure 1. FIGURE 1 Setting the value of TNOM Changing TNOM results in a notation in the OUTPUT file as shown below: *Analysis directives: •OP 1 See the on-line PSPICE REFERENCE GUIDE, p. 212 2 TNOM is the temperature at which the model parameters have been fitted to get good I-V curves. 3 This value can be overridden by specifying the variable T_MEASURED in the •model statement of the device. Copyright by John R Brews Page 1 7/8/2002•TEMP 27 •OPTIONS TNOM= 30.0 The entry •OPTIONS TNOM= 30.0 shows that TNOM has been set to 30ºC. The output file listing •TEMP 27 indicates that the circuit temperature is set at 27ºC. In other words, the menu of Figure 1 and the variable TNOM are about the selection of the model parameters, and have nothing to do with the circuit temperature. It is recommended that you leave this menu at the default value 27º C because this is the temperature where the model parameters were set up. Changing Circuit Temperature T FIGURE 2 Menu for setting the circuit temperature (the "global" temperature) in the TRANSIENT simulation; in this case a request is made for two runs at the temperatures T=27ºC and T=35ºC. The temperature of the entire circuit is set using the SIMULATION SETTINGS. For example, for a DC Bias simulation, we can follow the steps described by Herniter in his on-line supplement to his book. By selecting PSPICE/EDIT SIMULATION PROFILE and clicking on TEMPERATURE (SWEEP) we obtain the menu of Figure 2. In this menu, the temperature can be filled in, and checking the TEMPERATURE (SWEEP) box activates your choices.Changing the temperature leads to an entry in the OUTPUT FILE when the simulation is run like that below *Analysis directives: •OP •TEMP 150 The line •TEMP sets the so-called "global temperature specification" of the simulation. That is, all the devices in the circuit are set to this temperature. This approach works with other types of simulation as well. For example, for a TRANSIENT SIMULATION we can click on the ANALYSIS tab to obtain the menu shown in Figure 2. Here two temperatures have been selected, the default 27ºC and also 35ºC. Copyright by John R Brews Page 2 7/8/2002An example output waveform for a common emitter amplifier is shown in Figure 3. The corresponding added lines in the OUTPUT file are *Analysis directives: VPVNBVPVN•TRAN/OP 0 2m 0 1u •TEMP 27 35 In addition, before the listing for each simulation the OUTPUT file lists the temperature as shown below for the 35ºC run: **** INITIAL TRANSIENT SOLUTION TEMPERATURE = 35.000 DEG C Time0s 0.4ms 0.8ms 1.2ms 1.6ms 2.0msV(OUT)8V10V12V14V(35C,10.593V)(27C,11.550V) FIGURE 3 Example output waveform for two temperatures Temperature adjusted •model parameters If there are devices in your circuit, you can look at the •model parameters. For example, in the circuit of Figure 4 the temperature has been set using the menu of Figure 2 at T=127ºC. When the BIAS simulation is run, the output file shows the • model parameters of Figure 5. -5.08390V5.00000V+OUT1u19.8960mA+-VCC15V20.0168mASweep+-ACVAC1V+R2{RE}19.9519mA-4.51638V+R1{RR}20.0168mA0Q1Q2N222264.8858uA19.8960mA+-VEE-15V39.9128mA+-VDC{DC}0-5.07942V-+Transient AnalysisVSIN{VSW}1kHzQ2Q2N222273.1287uA19.8788mA0+R3{RE}19.9609mA FIGURE 4 Example current-mirror circuit where the temperature has been set at T = 127ºC Copyright by John R Brews Page 3 7/8/2002Q2N2222 NPN IS 1.43E-14 BF 255.9 NF 1.00E+00 VAF 74.03 IKF 0.2847 ISE 1.43E-14NE 1.307 BR 6.09E+00NR 1 RB 10 RC 1 CJE 2.20E-11MJE 0.377 CJC 7.31E-12MJC 0.3416 TF 4.11E-10XTF 3 VTF 1.70E+00ITF 0.6 TR 4.69E-08 XTB 1.5 CN 2.42E+00 D 0.87 FIGURE 5 Output file listing of •model parameters for Figure 4 at TNOM However, in addition, the output file shows the temperature-adjusted model parameters of Figure 6. Note, in particular, that the scale current has changed in accord with EQ. 1, and that BF has changed as well. Most other parameters also have changed somewhat, indicating that the detailed temperature dependence is too complex for hand analysis. Q2N2222 BF 393.9 ISE 6.602E-11 VJE 0.569 CJE 2.434E-11 RE 0 RB 10 BR 9.377 ISC 0 VJC 0.569 CJC 8.008E-12 RC 1 RBM 10 IS 1.545E-09 ISS 0 VJS 0.569 CJS 0 GAMMA 1E-11 RCO 0 VO 10 VTF 1.7 ITF 0.6 TR 4.691E-08XTB 1.5 CN 2.42 D 0.87 FIGURE 6 Output file listing for temperature-adjusted •model parameters at T=127ºC Individual device temperatures The above approach is fine if you want all the devices at the same temperature. However, in some circuits the operation is greatly affected when different devices have different temperatures. An example is the differential amplifier shown in Figure 74. This circuit depends on both transistors being matched, that is, both having the same model parameters. In Figure 7 the transistor temperatures relative to the global temperature are controlled by the parameters Q_TEMP1 and Q_TEMP2. As seen in Figure 7, both output voltages OUT1 and OUT2 are the same in this case where both transistors are at the same temperature. However, as shown in Figure 8, when the relative temperature of one transistor is increased relative to the global temperature by changing Q_TEMP2, the two outputs are no longer the same. How is this


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