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EE330 Laboratory Experiment No. 7 N-MOSFET Current Mirrors

[Reference: Chapter 8 of Sedra & Smith, 7th ed.; pp. 512-517]

Objective: 1. To assemble and characterize MOSFET current mirrors.

Materials: 1. Breadboard 2. NMOS transistors – ALD 1106 N-MOS FET transistors (see attached Data Sheet) 3. A reference resistor Rref to set the reference current magnitude Iref 5. Jumper wires for interconnection on your breadboard 6. Multimeter for measuring voltages and currents

7. Power supply (for VDD)

Circuit Schematics: (Note: there are two different current mirrors in this experiment.)

A. Simple Current Mirror (SCM):

Simple current mirror:

Circuit Components:

The circuit parameters are listed in the table below.

Component Value

Reference current

resistor Rref

TBD

N-MOS transistors as required (2)

ALD 1106 NMOS array of 4 N-MOSFET transistors

VDD + 6 volts (nominal) for SCM

Rref

Q1 Q

2

I0

Iref

VDD

2

The above table lists components needed for this experiment. Parameter values for the ALD 1106 N-channel MOSFET transistor are provided in the Data Sheet in the appendix at the end of this document.

Theory of the Simple Current Mirror:

Focus initially on N-MOS transistor Q1 which is connected in the so-called “diode connection.” That is, the drain and gate are shorted together so that the drain node is at the same potential as the gate. Hence, the drain-to-source voltage is equal to the gate-to-source voltage (i.e., VDS = VGS). From the theory of the MOSFET, we know that VGS must exceed the threshold voltage Vt of the FET for drain current to flow. The characteristic curve for the “diode connection” is shown below. [Reference: Figure 5.14 on page 268 of Sedra & Smith.]

One way to know the value of VGSf at Iref is to measure it. Otherwise, one can estimate (guess) its value and iterate upon measuring it on the breadboard. Knowing the value of VGSf at Iref allows for determining resistor Rref to establish current Iref. The setting of the voltage supply is VDD = 6 volts. The goal for a current mirror is to establish a stable Iref value and then to mirror (or replicate) current Iref in other branches of the circuit. In other words, we want I0 to equal Iref regardless of the applied value of VDS of the mirroring transistor. This assumes identical transistor geometries of course. What causes a current mirror to deviate from Iref = I0? An error or deviation can result from (1) the mirroring transistor’s finite output drain-to-source resistance r0, (2) a parametric mismatch between transistors Q1 and Q2, and (3) a temperature difference between transistors Q1 and Q2. In integrated circuits the transistors are physically close together for thermal matching and they are fabricated simultaneously on the same wafer. So, they should be well matched and thermally coupled as well as physically possible. Let us analyze how a finite output resistance causes I0 to deviate from Iref. To do this we write the equations for the drain currents of transistors Q1 & Q2. These equations are

Vt V

GS

ID

Iref

VGSf

3

2

1 1 1

1

2

0 2

2

20 2

1

11 ; where in

2

11

2

1

1

ref n GS t DS DS GS

n GS t DS

DS

ref GS

WI V V V V V Q

L

WI V V V

L

W L VI

I W L V

You should recognize these equations from Sedra & Smith, or from the class handout on

“Output Resistance.” Note: The output resistance can be modelled using the Early voltage

parameter VA or parameter ; where VA = 1/. The simple current mirror transistor Q1 has a fixed drain-to-source voltage VDS1 equal to its gate-to-source voltage VGS. Hence, current Iref is

proportional to the factor (1 + VGS1), but that will be fixed in value because VDS1 = VGS1. But the

mirrored current I0 is proportional to the factor (1 + VDS2), which increases in value as drain-to-source voltage VDS2 increases. This can give substantial of deviation of I0 if output resistance r0 is low. We do not have a precise value for r0 – the data sheet does say the output conductance

(the parameter denoted by Gos) is “typically” 200 micromhos (or S) at IDS = 10 mA & VDS = 5 V. Operating at IDS = 5 mA means r0 must be increased by a factor of two. (Why a factor of two? Clue: think about how we model r0 – refer to the handout on “Output Resistance.”) This implies a nominal value for output resistance r0 = 10,000. Finally, the ratio I0 / Iref indicates we can scale this current ratio by just changing the width-to-length ratios of transistors Q1 and Q2. The output resistance r0 presented at the drain node of transistor Q2, the branch in which I0 is flowing into, is simply the transistor’s(Q2) output resistance r02.

Select Value of Resistor Rref:

To set the value of resistor Rref requires knowing the reference current Iref – for our experiment we choose 5 mA. Thus, Rref can be determined from 5 mA (Rref) = VDD – VGSf. Different n-channel MOSFET devices will vary in VGSf value. Thus, you will want to compensate for this in adjusting the reference current to 5 mA (assume we want to set it to better than a 2% error from the 5 mA target value).

Measurement of the Simple Current Mirror: Construct the simple current mirror shown in the schematic diagram appearing on the first page – use your standard breadboard to build it. CAUTION: Be very careful in handling the ALD 1106 NMOS transistors. They are susceptible to electrostatic discharge and to applied terminal overvoltage. Your instructor can give you guidelines for handling MOSFET devices

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safely. Always be careful in applying terminal voltages to a MOSFET terminals; a good practice is to begin from 0 volts and slowly turn up the voltage being careful to not exceed the “Absolute Maximum Ratings” for the MOSFETs as given on the data sheet.

(1) Measure the current Iref (you are targeting for a nominal current of 5 mA)

Iref = ________ mA, with Rref = _________ ohms (this is the value you determined)

(2) Measure the mirrored current I0 with several different values of drain-to-source voltage VDS2 (transistor Q2). For example, you might select VDS2 = 1.0, 2.0, 3.0, 4.0 and 5.0 volts, recording the current I0 at each voltage level. Record your measurements in the table.

VDS2 (volts) Current I0 (mA)

(3) Next, Using the data from the above table, calculate the value of and the output resistance r02 of transistor Q2.

= ________ (units?) [Remember VA = 1/] r02 = _______ ohms

B. Modified Wilson Current Mirror Next, we are going to try to improve current mirror performance beyond the simple

current mirror. Using a cascode connection will allow for the output resistance ro2 to be boosted in value – one such a design is the “modified Wilson current source.” The “modified Wilson current mirror” is an attempt to improve performance by increasing the effective output resistance at the drain of the mirror MOSFET. To understand the modified Wilson approach start with the circuit schematic drawing shown below.

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Modified Wilson current mirror: Four MOSFETs are used in the “modified Wilson current mirror.” An advantage comes from a much higher output resistance at the drain node of transistor Q3. The disadvantage is that the voltage headroom is reduced because two MOSFETs are used which increases the voltage across the current mirror circuitry. The increase in output resistance is approximately given by

3 03 02out mR g r r

where the factor (gm3r03) is generally much greater than unity, thereby giving a large increase in output resistance. This is an example of a “cascode connection” often used to increase the output resistance at a terminal. [See Section 8.6 in Sedra & Smith (pp. 559-567) for a discussion on using cascoding to improve current mirror performance.]

Measurement of the “Modified Wilson Current Mirror”: The next part of this laboratory experiment is to build the “modified Wilson current mirror” and compare its performance to the simple current mirror. In this case choose VDD = 7 volts (rather than 6 volts for the simple current mirror) because of the requirement for more headroom due to the stacking of two MOSFETs vertically.

Rref

Q4 Q

3

I0

Iref

VDD

Q2 Q

1

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The circuit parameters for the modified Wilson current mirror are listed in the table below.

Component Value

Reference current

resistor Rref

TBD (note the value will most likely change)

N-MOS transistors as required (4)

ALD 1106 NMOS array of 4 N-MOSFET transistors

VDD + 7 volts (nominal)

Construct the simple current mirror shown in the schematic diagram appearing above – use your standard breadboard to build it. CAUTION: Be very careful in handling the ALD 1106 NMOS transistors. They are susceptible to electrostatic discharge and to applied terminal overvoltage.

(4) Measure the current Iref (you are still targeting for a nominal current of 5 milliamperes but the supply voltage is now 7 volts instead of 6 volts)

Iref = ________ mA, with Rref = _________ ohms

(5) What is the total voltage V0 from the drain of transistor Q3 to ground? V0 = ________ volt(s)

(6) Measure the mirrored current I0 for several different values for the output voltage V0

(combined voltage across transistors Q2 and Q3). For example, you might select V0 = 2.0, 3.0, 4.0, 5.0 and 6.0 volts; recording the current I0 at each voltage level. [Note: V0 = VDS2 + VDS3]

VDS2 (volts) Current I0 (mA)

(7) Next, Using the data from the table above, calculate the value of and the output resistance R0 of the drain node of transistor Q3.

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= ________ (units?) [Remember VA = 1/] R0 = _______ ohms

(8) How does the output resistance r02 of transistor Q2 in the simple current mirror compare to the output resistance R0 in the modified Wilson current mirror?

Conclusions:

Write a paragraph on what you learned in doing this experiment. (For example, if you encountered problems during the experiment, how did you solve the problem and what did you learn in solving the problem?)

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APPENDIX – ALD 1106 DATA SHEET http://www.aldinc.com/pdf/ALD1116.pdf

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Note: This plot is insufficient to estimate the output resistance.

Laboratory created by Dr. Donald Estreich (October 25, 2017)