CHAPTER 20 OPERATIONAL AMPLIFIERS OPAMPS 1 Introduction to

CHAPTER 20 OPERATIONAL AMPLIFIERS (OP-AMPS) 1

Introduction to operational amplifiers n Symbol and Terminals 2

The Ideal Op-Amp n The ideal op-amp has ¨ Infinite voltage gain ¨ Infinite bandwidth ¨ Infinite input impedance(open) ¨ Zero output impedance 3

The Practical Op-Amp n The practical op-amp has: ¨ high voltage gain ¨ wide bandwidth ¨ high input impedance(open) ¨ low output impedance 4

The differential amplifier 5

The differential amplifier (continue) Single-Ended Input 6

The differential amplifier (con’t) Differential Input 7

The differential amplifier (continue) Common-Mode Input 8

Common-Mode Rejection Ratio n The measure of an amplifier’s ability to reject common-mode signals is a parameter called the common-mode rejection ratio (CMRR) is the differential gain is the common-mode 9 gain

A simple op-amp arrangement 10

OP-AMP PARAMETERS n Input offset voltage (Vos) is the differential dc voltage required between the inputs to force the differential output to zero volt 11

OP-AMP PARAMETERS n Input bias current is the average of both input currents and is calculated as follow: 12

OP-AMP PARAMETERS n Input impedance ¨ The differential input impedance ¨ Common-mode input impedance 13

OP-AMP PARAMETERS n Input Offset Current is the difference of the input bias currents, expressed as an absolute value 14

OP-AMP PARAMETERS n The output impedance is the resistance viewed from the output terminal of the opamp 15

OP-AMP PARAMETERS The common-mode input voltage range is the range of input voltages which will not cause clipping or other output distortion n Open-Loop Voltage Gain, Aol n Open-loop voltage gain is the internal voltage gain of the device and represents the ratio of output voltage to input voltage when there are no external components, and it can range up to 200, 000 and is not a well controlled parameters 16

OP-AMP PARAMETERS n Common-Mode Rejection Ratio (CMRR) ¨ The accepted definition of CMRR for an opamp is the open-loop voltage gain Aol divided by the common-mode gain 17

Slew rate n n The maximum rate of change of the output voltage in response to a step input voltage is the slew rate of an op-amp The slew rate is dependent upon the frequency response of the amplifier stages within the opamp Slew rate =∆Vout /∆t 18

NEGATIVE FEEDBACK n n When negative feedback is present, the noninverting and inverting inputs are nearly identical Without negative feedback, an extremely small difference in the two input voltage drives the op-amp to its output limits and it becomes onlinear 19

OP-AMP configurations with negative feedback n Closed-Loop Voltage Gain, Acl , is the voltage gain of an op-amp with negative feedback 20

Noninverting amplifier 21

Op-amp voltage-follower Acl(VF)=1 22

Inveting Amplifier 23