Experiment 11 - Current Sources and Voltage Sources

W.T. Yeung and R.T. Howe

UC Berkeley EE 105

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

H & S: Chapter 9.4

For the current sources in figures 1 and 2, what is I_REF, I_OUT, 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 R_REF=1 k.

For the circuit in figure 4, determine the current through R_REF if all the devices have W/L=1. Use your measured values for K_n and K_p. Let R_REF=1 k and ignore the backgate effect.

1. Construct the circuit in figure 1. Let R_REF=5 k. Find and record the current I_REF.

FIGURE 1.

2. Vary the output voltage from 0 to 5 V and measure the current I_SUP 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 I_SUP vs. V_OUT. and I_SUP vs. V_CC - V_OUT = V_SUP. Compare the results with SPICE.

4. From the plot, find the output resistance.

1. Figure 2 shows a cascode current sink from Lab Chip 5.

2. Place the Lab Chip 5 into the test fixture of the HP-4145.

3. Load the program PCS using the keystrokes: [GET] PCS [EXE].

4. Connect SMU1 to the BIAS pin and SMU2 to the OUT pin.

5. Enter for the constant-slope current the nominal current you found in procedure 3.1.

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.

FIGURE 3.

The following schematic shows a totem pole voltage source.

FIGURE 4.

1. Construct the circuit by placing a 1 k resistor for R_REF between V_REF1 and V_REF2. 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 V_REF2 to generate a reference current, I_OUT. Vary V_OUT 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.

1. Construct the current mirror shown below (devices on Lab Chip 2).

FIGURE 6. Resistor Ratioed Current Source

Let R₁ = R₂ = 100 and R_REF=5 k.

2. Record values for I_SUP, V_BE1 and V_BE2.

3. Change the value of resistor R₂ to 1 k. What is I_OUT?

4. Now switch the resistors. What is I_SUP now?

5. Derive an approximate relationship between I_SUP and I_REF. 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.