Experiment 7 Current Sources and Voltage Sources W T Yeung and R T Howe UC Berkeley EE 105 Spring 2005 1 0 Objective This experiment will introduce techniques for current source biasing Several different current sources will be considered Some requirements for current sources include high output resistance with a wide range of voltage drops and independence from external factors such as supply variation or temperature variation The second kind of source we ll be considering is a voltage source MOS current sources often are biased from a voltage source An independent voltage source is important to keep a current source properly 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 sources 2 0 Prelab H S Chapter 9 4 For the current sources in Figs 1 and 2 what is IREF IOUT the current supply s internal 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 k 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 k and ignore the backgate effect 1 of 7 Procedure 3 0 Procedure 3 1 Simple Current Source 1 Construct the circuit in Fig 1 Let RREF 5 k Find and record the current IREF FIGURE 1 Simple Current Source SBSOURCE Lab Chip 4 VCC 5 V PIN 28 VOUTPUT PIN 27 BIAS RREF IREF 1 k PIN 14 PIN 26 vOUT ISUP 2 Vary the output voltage from 0 to 5 V and measure the current ISUP from the voltage drop across the 1 k resistor You should record several points below 0 5 V in order to observe saturation effects Use a 100 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 Source 1 Load the default FET I d V ds program 2 On the first page change the definition of SUM4 V sub 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 V d to sweep from 0 to 5V Set SMU4 V sub to be 5V 4 Connect SMU1 to pin 14 SMU2 to pin 25 and SMU4 to pin 28 Also connect the R ref 1k between pin 28 and 24 The sweeping voltage on SMU2 provides V output so you don t need an external voltage source 2 of 7 Experiment 7 Current Sources and Voltage Sources Procedure 5 Press Single to take a measurement You are plotting I d vs V ds both at SMU2 which in this case are I sup and V output 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 VCC 5 V PIN 28 RREF ISUP A BIAS PIN 24 IREF VOUT PIN 25 VOUTPUT PIN 14 Experiment 7 Current Sources and Voltage Sources 3 of 7 Procedure FIGURE 3 Extrapolated line to find the output resistance of the cascode current source 3 3 Totem Pole Voltage Source The following schematic shows a totem pole voltage source FIGURE 4 Totem Pole Voltage Source on Lab Chip 5 PIN 28 W L 46 5 1 5 VDD 5V M1 VREF1 PIN 23 RREF W L 46 5 1 5 M2 VREF2 PIN 21 W L 46 5 1 5 M3 VREF3 PIN22 PIN 14 4 of 7 Experiment 7 Current Sources and Voltage Sources Procedure 1 Construct the circuit by placing a 1 k 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 minimum 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 ISUP PIN 28 5 V A DRAIN PIN 3 GATE PIN 4 W 46 5u L 1 5u VREF2 VOUT SOURCE PIN 5 PIN 14 Experiment 7 Current Sources and Voltage Sources 5 of 7 Optional Experiments 4 0 Optional Experiments 4 1 Resistor Ratioed Current Source 1 Construct the current mirror shown below devices on Lab Chip 2 VCC 5 V PIN 28 RREF ISUP COLL PIN 20 Q1 EMIT PIN 18 A IREF COLL PIN 17 Q2 BASE BASE PIN 19 PIN 16 VOUT EMIT PIN 15 R2 R1 PIN 14 FIGURE 6 Resistor Ratioed Current Source 1 Let R1 R2 100 and RREF 5 k 2 Record values for ISUP VBE1 and VBE2 3 Change the value of resistor R2 to 1 k 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 1k and R2 be 100 3k followed by 5k This should give you better insight into how this mirror works You need not take a detailed sweep here 4 2 Totem Pole Voltage Source 1 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 output voltage of this NMOS current source What is the output resistance 6 of 7 Experiment 7 Current Sources and Voltage Sources Optional Experiments 2 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 Experiment 7 Current Sources and Voltage Sources 7 of 7
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