CHAPTER 20 OPERATIONAL AMPLIFIERS (OP-AMPS)

Introduction to operational amplifiers Symbol and Terminals

Op-Amp Characteristics

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

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

Comparison between ideal and non-ideal op-Amp Ideal Op-AMP Infinite Gain Infinite Bandwidth Infinite Input Resistance Zero Output Resistance Zero Noise No Temperature Sensitivity Zero Power Consumption Perfectly Linear … Non-Ideal Op-AMP 100,000 0-MHz 100 M Ohm 50 Ohm Microvolt microV/C mWatt 0.1% nonlinearity …

A simple op-amp arrangement

The differential amplifier 2 sets of Common Emitter Amplifier (mirror)

Operational Mode Single-Ended Input Differential Input Common Mode Input

Single-Ended Input

 Either input is shorted to ground  Output of non-zero input  Amplified  different phase from input  Output of zero input  Mirror of output of non-zero input  But same phase as input

Differential Input

The differential amplifier (con’t) Differential Input

Both inputs  Same Magnitude and frequency  But different phase To analyze output  Use superposition Consider each input at a time Output Voltage  Each output is summed of the results of both inputs

Common Mode Input

The differential amplifier (continue) Common-Mode Input

Common Mode Input The most widely used Inputs  Same Magnitude and frequency  Same phase To analyze output  Use superposition Consider each input at a time Output Voltage  Zero output Result from input 1 is canceled by the result from input 2

Common-Mode Rejection Ratio 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 gain

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

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

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

OP-AMP PARAMETERS Input impedance  The differential input impedance  Common-mode input impedance

OP-AMP PARAMETERS The output impedance is the resistance viewed from the output terminal of the op- amp

OP-AMP PARAMETERS The common-mode input voltage range is the range of input voltages which will not cause clipping or other output distortion Open-Loop Voltage Gain, A ol 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

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 op-amp Slew rate =∆Vout/∆t Slew rate

NEGATIVE FEEDBACK 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 nonlinear

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

Op-Amp Applications Comparator Non-inverting Amp. Inverting Amp. Summing Amp.

Comparator Output depends on both input voltages  if V+ > V- Vout = Vs+ -> +5V  Else if V+ < V- Vout = Vs- -> -5V

Noninverting amplifier

Inverting Amplifier

Summing Amp.