4. OPERATIONAL AMPLIFIERS

CIRCUITS by Ulaby & Maharbiz

Overview

Tech Brief 7: IC FabricationWafer: Thin slice of semiconductor material with highly polished surface

Processed wafer is cut into many dies or chips.

Lithography: Defining spatial pattern

Photoresist: Polymer material that does not allow etching or deposition of areas underneath it.

Tech Brief 7: IC Fabrication

Lithography: Defining spatial pattern

Photoresist: Polymer material that does not allow etching or deposition of areas underneath it.

Tech Brief 7: IC Fabrication

Tech Brief 7: IC Fabrication

Tech Brief 7: IC Fabrication

Tech Brief 7: IC Fabrication

Operational Amplifier “Op Amp”

Two input terminals, positive (non- inverting) and negative (inverting)

One output Power supply V+ , and

Op Amp showing power supply

Op Amp with power supply not shown (which is how we usually display op amp circuits)

Inside The Op-Amp (741)

Gain Key important aspect of op amp: high

voltage gain Output , A is op-amp gain (or

open-loop gain) – different from circuit gain G

Linear response

Equivalent Circuit

Example 4-1: Op Amp Amplifier

KCL at Node a:

KCL at Node b:

2

210

RRR

vvGs

For infinite A:

= 4.999975

= 5

Node a

Node b

Negative Feedback Feedback: return some of the output to the

input Negative feedback decreases input signal Achieves desired circuit gain, with wide

range for inputNegative Feedback No Feedback

5CC

sVv sAvv 0

AVv CC

s Range of Range of5Gain = 5 Range of : ‒2 V to +2 VGain = 1millionRange of : ‒10 mV to +10 mV

Circuit Analysis With Ideal Op Amps

Use nodal analysis as before, but with “golden rules” N Do not apply KCL at op amp output

No current into op amp

No voltage drop across op amp input

Noninverting Amplifier

021

Rv

Rvv non

so vRRRv

2

21

spn vvv

(max) = Vcc

At node

Inverting Amplifier

0 pn vv

Example 4-2: Input Current Source

Relate output voltage to input current source

Summing Amplifier

Example 4-4:

Solution:

Difference Amplifier

Note negative gain of channel 1

Voltage Follower“Buffers” Sections of Circuit

What is the op amp doing?

depends on both input and load resistors

is immune to input and load resistors

Example 4-5: Elevation Sensor

Sensor ResponseDesired Output

h = elevation, inversely proportional to air pressure

Example 4-6: Multiple Op-Amp Circuit

Measurement Uncertainty

(T = 21°C)

v2V0 = V2 ± 1% of V2

21°C ± 0.21°CG = 1± 1%

G = 1 1%

v2(T = 21°C)

Thermistor

Thermistor

v1Fixed Reference Temp = 20°C

V0 = (V2 ‒ V1) ± 1% of (V2 ‒ V1)

1°C ± 0.01°C

Direct Measurement

Differential Measurement

Much better measurement uncertainty

Instrumentation AmplifierHighly sensitive differential amplifier

122

321

5

4 vvR

RRRRRvo

Digital to Analog ConverterConverts digital value into analog

voltage

4-digit example

Digital to Analog ConverterRepresent digital value with analog

voltage

MOSFET (Field Effect Transistor)

Active Device: Voltage Controlled Current Source

Gate voltage controls drain/source current

MOSFET Equivalent Circuit

Characteristic curves Idealized response

Example 4-9: MOSFET Amplifier

Given:

Determine

Load Line You can use a “load line” to graphically determine Vout = VDS for a given Vin = VGS

RL

VDD

VDD/RD

Digital Circuit: MOSFET Inverter

VDD = 15 V

RL

G

S

D ID

DSout VV GSin VV

Output“High”Logic 1

Output“Low”Logic 0

In Out0 11 0 Input

“Low”

In Out

VDD

0 1 2 3 4 50

5

10

15

VGS=V in

VD

S=V

out

Output “Low”Logic 0

Output “High”Logic 1

Input “High”

Read-Only Memory (ROM) Circuits

VREAD = 1VBIT = 0100

Another Digital Circuit Element: NAND

A B Out0 0 10 1 11 0 11 1 0

AB

OutVDD

AVout

B

No current flows through resistor, unless both A and B inputs turn their transistors on

to “pull down” Vout

NAND gates can be used to build any binary logic function

Another Digital Circuit Element: NOR

Current will flow if either A or B inputs turn their transistors on to “pull down” Vout

A B Out0 0 10 1 01 0 01 1 0

AB

Out

A

VDD

Vout

B

NOR gates can be used to build any binary logic function

Tech Brief 8: Electromagnetic Spectrum

Example: Multisim Instruments

Multisim Table

Multisim: MOSFET I-V Analyzer

Summary