1 of 7Experiment 7Current Sources and Voltage SourcesW.T. Yeung and R.T. HoweUC Berkeley EE 105Spring 20051.0 ObjectiveThis experiment will introduce techniques for current source biasing. Several differentcurrent sources will be considered. Some requirements for current sources include highoutput resistance with a wide range of voltage drops and independence from externalfactors such as supply variation or temperature variation. The second kind of sourcewe’ll be considering is a voltage source. MOS current sources often are biased from avoltage source. An independent voltage source is important to keep a current sourceproperly biased without any variations.To show your understanding of the lab, your write-up should contain:• A discussion on the different types of current sources• A discussion on the choosing the right type of current source• A discussion on the valid range of operation for various current sources2.0 Prelab• H & S: Chapter 9.4• For the current sources in Figs. 1 and 2, what is IREF, IOUT, the current supply’s inter-nal resistance (in terms of small signal parameters) and the minimum output voltage required to have the circuit act as a current source. Let RREF=1 kΩ.• For the circuit in Fig. 4, determine the current through RREF if all the devices have W/L=1. Use your measured values for Kn and Kp. Let RREF=1 kΩ and ignore the back-gate effect.Procedure2 of 7 Experiment 7 Current Sources and Voltage Sources3.0 Procedure3.1 Simple Current Source1. Construct the circuit in Fig. 1. Let RREF=5 kΩ. Find and record the current IREF. FIGURE 1. Simple Current Source (SBSOURCE, Lab Chip 4)2.Vary the output voltage from 0 to 5 V and measure the current ISUP from the voltage drop across the 1 kΩ resistor. You should record several points below 0.5 V in order to observe saturation effects. Use a 100 Ω resistor if the supply voltage is unsteady.3. Plot ISUP vs. VOUT. and ISUP vs. VCC - VOUT = VSUP. Compare the results with SPICE.4. From the plot, find the output resistance.3.2 Cascode Current Source1. Load the default FET - program.2. On the first page, change the definition of SUM4 to constant voltage (instead of common). Delete the definition for SMU3.3. Press Next Page twice to go to the Measurement page. Here, set SMU2 to sweep from 0 to 5V. Set SMU4 to be 5V.4. Connect SMU1 to pin 14, SMU2 to pin 25, and SMU4 to pin 28. Also, connect the between pin 28 and 24. The sweeping voltage on SMU2 provides , so you don’t need an external voltage source.RREFVOUTPUTIREFISUPBIASvOUTPIN 27PIN 26PIN 141 kΩPIN 28VCC = 5 VIdVdsVsub()Vd()Vsub()Rref1kΩ()VoutputProcedureExperiment 7 Current Sources and Voltage Sources 3 of 75. Press Single to take a measurement. You are plotting vs , both at SMU2, which in this case are and .6. Execute the program to obtain the plot of the cascode’s I-V characteristics.7. Using the Marker and Cursor, find the output resistance. (refer to Exp. 1 if you have forgotten how to find the slope of a line.)8. Note the minimum operating voltage for this current source.9. How does the cascode compare with the simple current source?10. Obtain a hardcopy of your data.FIGURE 2. Cascode Current Sink (CASBSINK, Lab Chip 5)IdVdsIsupVoutputVCC = 5 VAVOUTPUTIREFISUPBIASVOUTPIN 25PIN 24PIN 28PIN 14RREFProcedure4 of 7 Experiment 7 Current Sources and Voltage SourcesFIGURE 3. Extrapolated line to find the output resistance of the cascode current source3.3 Totem Pole Voltage SourceThe following schematic shows a totem pole voltage source.FIGURE 4. Totem Pole Voltage Source on Lab Chip 5VREF1VREF2VREF3 VDD = 5VRREFW/L = 46.5/1.5W/L = 46.5/1.5W/L = 46.5/1.5PIN 14PIN 23 PIN 21PIN22M2M3M1 PIN 28ProcedureExperiment 7 Current Sources and Voltage Sources 5 of 71. Construct the circuit by placing a 1 kΩ resistor for RREF between VREF1 and VREF2. Measure the drain current and the reference voltages.2. How do the reference voltages compare with theoretical values? How can you account for the difference?3. The reference voltages act like batteries. Their values remain constant as long as there are no leakage currents at that node. For the NMOS transistor shown in figure 5, use VREF2 to generate a reference current, IOUT. Vary VOUT and determine the mini-mum output voltage of this NMOS current source. What is the output resistance?4. Replace the NMOS with one with a different W/L ratio on Lab Chip 1 (Drain = PIN 6, Gate = PIN 7, Source = PIN 8, and W/L=46.5/3) and repeat procedure 3. How do the results compare?FIGURE 5. NMOS Transistor as a Current Source (Lab Chip 1)AVOUTISUPDRAINGATESOURCEPIN 14 PIN 4PIN 3PIN 5VREF2 L=1.5uW=46.5uPIN 28 5 VOptional Experiments6 of 7 Experiment 7 Current Sources and Voltage Sources4.0 Optional Experiments4.1 Resistor Ratioed Current Source1. Construct the current mirror shown below (devices on Lab Chip 2). FIGURE 6. Resistor Ratioed Current Source1. Let R1 = R2 = 100Ω and RREF=5 kΩ.2. Record values for ISUP, VBE1 and VBE2.3. Change the value of resistor R2 to 1 kΩ. What is IOUT?4. Now switch the resistors. What is ISUP now?5. Derive an approximate relationship between ISUP and IREF. Does your data follow this relationship?6. Let R1 = 1kΩ and R2 be 100Ω, 3kΩ, followed by 5kΩ. This should give you betterinsight into how this mirror works. You need not take a detailed sweep here.4.2 Totem Pole Voltage Source1. The reference voltages act like batteries. Their values remain constant as long as there are no leakage currents at that node. For the NMOS transistor shown in figure 5, use VREF2 to generate a reference current, IOUT. Vary VOUT and determine the minimum out-put voltage of this NMOS current source. What is the output resistance?VCC = 5 VRREFAVOUTIREFISUPR1R2COLLBASEEMITPIN 28PIN 14PIN 19BASEPIN 16PIN 20COLLPIN 17PIN 18EMITPIN 15Q1Q2Optional ExperimentsExperiment 7 Current Sources and Voltage Sources 7 of 72. Replace the NMOS with one with a different W/L ratio on Lab Chip 1 (Drain = PIN 6, Gate = PIN 7, Source = PIN 8, and W/L=46.5/3) and repeat procedure 3. How do the results
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