semiconductor devices - legh richardson electrical...

Jul-14 Legh Richardson

Semiconductor Devices

Part 3Characteristics of an Operational

Amplifier Part 1


Operational Amplifiers 1What is an Op Amp?A three terminal device; two inputs and on output, ignoring power lines and internal adjustment control


Operational Amplifiers 1What is an Op Amp?Designed for the military and NASAMonolithic (All on one Silicon Chip)Advantages:1. Temperature variations and are shared with all the transistors2. Thermal runaway and instability is unlikely with single

component transistors (gates) on one silicon chip3. Considerably smaller and lighter than discrete components4. Transistor pairs can be easily matched for gain and

temperature response (very important for accuracy)5. Circuit design is made easy (technician level of understanding)6. Cheap to design, build, manufacture


Operational Amplifiers 1

Practical pin outs of the basic Op Amp


Operational Amplifiers 1inside the 741 Op Amp

(2)

(3)

(1)

(5)

(6)

(4)

(7)

(A)

(B)

(C)

(D)(E)


Operational Amplifiers

(A) – Long Tailed Pair (current sink)(B) - Current source(C) - Darlington Pair (high amplification)(D) - Push Pull Power Output (Class B )(E) - Current Source

These we covered when dealing with Transistors applications


Operational Amplifiers 1A simplification of the Op Amp can be shown in 3 main stages of operation.

The difference amplifierAmplifies the difference between the inverting and non-inverting signals placed on the inputs of the amplifier

The high gain amplifierThe voltage signal difference is passed onto the high gain section, as much as 10^6. It will never be greater than the supply rails

The buffer amplifierBoosts the current output allowing to be used to supply various loads. High impedance input low impedance output


Operational Amplifiers 1The ideal Op Amp.1. Infinite gain2. Infinite input impedance3. Infinite Bandwidth4. Zero output impedance5. Zero current and voltage

offset


Operational Amplifiers 1Practical limitations:1. Infinite gain and Bandwidth, Trade off (compromise)

between infinite gain and infinite frequency bandwidth

2. Offset, requires calibration to offset any stray amplification, basic nulling any unwanted changes

3. Slew Rate, reaction to change is not immediate so the slew rate is a response to the applied signal SR = dVout/ dt

4. CMRR, common mode rejection ratio is how well a signal common to both inputs is rejected and is measured in decibels


Operational Amplifiers 1Open Loop (no feedback) and High Gain will have a minimum signalbandwidthLow Gain or Unity gain will have a large signal bandwidth or frequency range


Operational Amplifiers 1

OffsetWhen the inputs are held together and grounded and the output shows a voltage then an offset is required

The negative feedback amplifier shown here with Offset using a 10k potentiometer connected across pins 1 and 5

-

+

R2

R1

1

5

10k

-Vcc

+Vcc

-

+

-Vcc

+Vcc

Vout


Operational Amplifiers 1Slew RateInput change

Output change

1V

0 time

1V

0 time


Operational Amplifiers 1Common mode rejection ratio

CMRR is the ratio of wanted input signal connected to either inverted or non-inverted inputs to the collective noise attributed to both channels Mains hum is an example of an unwanted signal.The op amp amplifies the wnted signal and attenuates the common signal. The ability to differentiate is measured in decibels (dB)


Operational AmplifiersBasic operation



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