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UA ECE 304 - Study Notes

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ECE 304: Making a Hybrid-? Sub-circuitSchematicUsing the hybrid-? sub-circuitMultiple instancesExamples of performance analysis plots using the hybrid-? sub-circuitUtility of the hybrid-? sub-circuitECE 304: Making a Hybrid-π Sub-circuit Schematic {gM}gM-++-E1GAIN = 1PARAMETERS:Tf = 500psVAF = 100VVTH = 25.85199mVCjc = 10pFCje = 10pFDOT_MODELrB = 10BASE0PARAMETERS:IC = 10mAQ_POINT+rX{rX}+Cpi{Cpi}+-G1GAIN = 1+Cu{Cu}+rO{rO}PARAMETERS:Cpi = {Cje+IC*Tf/VTH}Cu = {Cjc}rPI = {Bac/gM}rO = {VAF/IC}gM = {IC/VTH}rX = {rB}HYBRID_PIBac = 200EMITTER+rPI{rPI}COLLECTOR iCvPIINFIGURE 1 The hybrid-π model of the bipolar transistor with gM usable as a variable Figure 1 is the PSPICE hybrid-π model of the bipolar transistor valid for high frequencies. The GAIN part allows use of a variable small-signal transconductance gM1. Three parameter lists are added. The list HYBRID PI PARAMETERS provides the hybrid-π parameters needed to specify the circuit. Numbers can be used here, taken from the PSPICE OUTPUT file. In Figure 1, however, the hybrid-π parameters are calculated using •model parameters from the list DOT MODEL PARAMETERS. These simple formulas can be changed. The •model list and the Q-point current IC can be omitted if numbers are used for the hybrid-π parameters. The circuit of Figure 1 is complex, and in most circuits several transistors are used. If we make a small-signal equivalent circuit with many transistors, it is handy to convert the hybrid-π model to a PSPICE SUB-CIRCUIT. The steps outlined below are described in detail in Herniter's book2, §7.G, pp. 458-472 with the needed menus. 1. Create the sub-circuit and make its connections attach via PORTS 2. NAME THE SUB-CIRCUIT → here it is called HYBRIDPI 3. Create a netlist as shown in Figure 2. In Figure 1, named nodes make the netlist readable. * source HYBRIDPI •SUBCKT HybridPI BASE COLLECTOR EMITTER * Use a parameter list to set parameter values E_gM IC 0 VALUE {{gM} * V(VPI)} E_E1 VPI 0 IN EMITTER 1 G_G1 COLLECTOR EMITTER IC 0 1 R_rPI EMITTER IN {rPI} C_Cu IN COLLECTOR {Cu} R_rO EMITTER COLLECTOR {rO} C_Cpi EMITTER IN {Cpi} R_rX IN BASE {rX} •ENDS FIGURE 2 Sub-circuit netlist; a comment line (*) has been added using the NOTEPAD text editor to remind the user to add a parameter listing when this sub-circuit is used 1 Parts E and G cannot use variables for their gain parameters, only numerical values. 2 Marc E. Herniter, Schematic Capture with Cadence PSPICE, Second Edition, Prentice Hall, '03 Copyright by John R Brews Page 1 10/19/20024. Establish a sub-circuits library 5. Edit the standard symbol for the sub-circuit 6. Add the sub-circuit library to the list of GLOBAL libraries Using the hybrid-π sub-circuit The sub-circuit contains the hybrid-π parameters Cµ, Cπ and so forth that have to be specified using a parameter box. The required parameter box is labeled HYBRID PI in Figure 1 and, if numerical values from the PROBE output file are used, this is the only parameter box needed. However, if you want to look at the parametric dependence on Q_point current IC, or if you want to check the circuit dependence on •model parameters based upon formulas for the hybrid-π components, the extra parameter boxes are needed. Multiple instances It often happens that several transistors with different hybrid-π parameters are needed. This can be accomplished by pasting the sub-circuit into the schematic, highlighting it by left-clicking with the mouse, right-clicking, and selecting EDIT PSPICE MODEL. The PSPICE MODEL EDITOR menu of Figure 3 appears. FIGURE 3 Creating a second instance of the hybrid π model called HybridPI2 with parameter names different from those in the original model called HybridPI. The name of the model can be changed to HybridPI2, for instance, and the parameter names can be changed to gM2, rPI2 and so forth. Then the new model can be saved. To convert the pasted sub-circuit to refer to this new parameter list, right-click on the symbol of the sub-circuit to be changed and choose EDIT PROPERITES. Figure 4 shows the PROPERTY EDITOR with the VALUE and IMPLEMENTATION values changed from HYBRIDPI to HYBRIDPI2 to invoke the HYBRIDPI2 model. FIGURE 4 Changing the IMPLEMENTATION and VALUE entries of a second part to HYBRIDPI2 to make this part refer to model HYBRIDPI2 instead of the part HYBRIDPI An example applying this procedure to a diff amp with active load is shown in Figure 5. Copyright by John R Brews Page 2 10/19/20020NPN2HybridPI2BASECOLLECTOREMITTER+R1{RL}PNP1HybridPIBASECOLLECTOREMITTERNPN1HybridPI2BASECOLLECTOREMITTERPARAMETERS:Cpi = {Cje+IC*Tf/VTH}Cu = {Cjc}rPI = {Bac/gM}rO = {VAF/IC}gM = {IC/VTH}rX = {rB}HYBRID_PIBac = 200PARAMETERS:RL = 100PARAMETERS:rPI2 = {Bac2/gM}gM2 = {IC/VTH}rO2 = {VAF/IC}Cpi2 = {Cje+IC*Tf/VTH}Cu2 = {Cjc}HYBRID_PI2Bac2 = 200rX2 = {rB}-++-E1GAIN = -1PARAMETERS:Tf = 500psVAF = 100VVTH = 25.85199mVCjc = 10pFCje = 10pFDOT_MODELrB = 10PNP2HybridPIBASECOLLECTOREMITTER+R2{RL}Sweep+-ACVAC0.5VPARAMETERS:IC = 10mAQ_POINT00 INOUTFIGURE 5 Small-signal implementation of a differential amplifier with active load and difference-mode excitation; the NPN transistors have model parameters with different names than the PNP's, which allows the NPN parameters to be varied independently from the PNP-parameters. The parameter βAC can be filled in from the OUTPUT Q-point data. It is used only to calculate rπ, and is not a parameter of the sub-circuit HYBRIDPI. Examples of performance analysis plots using the hybrid-π sub-circuit gM0 100m 200m 300m 400m 500m 600m 700mupper3dB(V(OUT))200K250K300K(616.40m,237.07K)(400.00m,237.53K)(200.00m,241.32K)(50.00m,265.58K) FIGURE 6 Variation of 3dB corner frequency with transconductance of mirror transistors, gM; this plot is difficult to generate without your own hybrid-π sub-circuit Copyright by John R Brews Page 3 10/19/2002gM20 100m 200m 300m 400m 500m 600m 700mupper3dB(V(OUT))0250K500K(600.00m,182.80K)(400.00m,233.45K)(200.00m,323.94K)(50.00m,455.98K) FIGURE 7 Variation of 3dB corner frequency with the transconductance gM2 of the lower (output) transistors; this plot is hard to obtain without your own hybrid-π sub-circuit Cpi2100p 1.0n 10n 100n 1.0u 10uupper3dB(V(OUT))0200K400K(1.000u,15.62K)(100.00n,107.78K)(10.00n,218.43K)(100.00p,237.80K) FIGURE 8 Direct sweep of the hybrid-π model parameter Cπ; until Cπ > 10nF, it has little effect on the corner


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