10-07-2012, 01:12 PM
Operational Amplifiers and Applications
Op Amps.ppt (Size: 1.72 MB / Downloads: 136)
Chapter Goals
Understand the “magic” of negative feedback and the characteristics of ideal op amps.
Understand the conditions for non-ideal op amp behavior so they can be avoided in circuit design.
Demonstrate circuit analysis techniques for ideal op amps.
Characterize inverting, non-inverting, summing and instrumentation amplifiers, voltage follower and first order filters.
Learn the factors involved in circuit design using op amps.
Find the gain characteristics of cascaded amplifiers.
Special Applications: The inverted ladder DAC and successive approximation ADC
Differential Amplifier Model: Basic
Represented by:
A = open-circuit voltage gain
vid = (v+-v-) = differential input signal voltage
Rid = amplifier input resistance
Ro = amplifier output resistance
Ideal Operational Amplifier
The “ideal” op amp is a special case of the ideal differential amplifier with infinite gain, infinite Rid and zero Ro .
If A is infinite, vid is zero for any finite output voltage.
Infinite input resistance Rid forces input currents i+ and i- to be zero.
The ideal op amp operates with the following assumptions:
It has infinite common-mode rejection, power supply rejection, open-loop bandwidth, output voltage range, output current capability and slew rate
It also has zero output resistance, input-bias currents, input-offset current, and input-offset voltage.
The Inverting Amplifier: Configuration
The positive input is grounded.
A “feedback network” composed of resistors R1 and R2 is connected between the inverting input, signal source and amplifier output node, respectively.
Inverting Amplifier:Voltage Gain
The negative voltage gain implies that there is a 1800 phase shift between both dc and sinusoidal input and output signals.
The gain magnitude can be greater than 1 if R2 > R1
The gain magnitude can be less than 1 if R1 > R2
The inverting input of the op amp is at ground potential (although it is not connected directly to ground) and is said to be at virtual ground.
Inverting Amplifier: Example
Problem: Design an inverting amplifier
Given Data: Av= 20 dB, Rin = 20kW,
Assumptions: Ideal op amp
Analysis: Input resistance is controlled by R1 and voltage gain is set by R2 / R1.
and Av = -100
A minus sign is added since the amplifier is inverting.
Non-inverting Amplifier: Example
Problem: Determine the output voltage and current for the given non-inverting amplifier.
Given Data: R1= 3kW, R2 = 43kW, vs= +0.1 V
Assumptions: Ideal op amp
Analysis:
Since i-=0,
Gain Error
Gain Error is given by
GE = (ideal gain) - (actual gain)
For the non-inverting amplifier,
Gain error is also expressed as a fractional or percentage error.
Gain Error: Example
Problem: Find ideal and actual gain and gain error in percent
Given data: Closed-loop gain of 100,000, open-loop gain of 1,000,000.
Approach: The amplifier is designed to give ideal gain and deviations from the ideal case have to be determined. Hence,
. Note: R1 and R2 aren’t designed to compensate for the finite open-loop gain of the amplifier.
Analysis:
The Unity-gain Amplifier or “Buffer”
This is a special case of the non-inverting amplifier, which is also called a voltage follower, with infinite R1 and zero R2. Hence Av = 1.
It provides an excellent impedance-level transformation while maintaining the signal voltage level.
The “ideal” buffer does not require any input current and can drive any desired load resistance without loss of signal voltage.
Such a buffer is used in many sensor and data acquisition system applications.
The Summing Amplifier
Scale factors for the 2 inputs can be independently adjusted by the proper choice of R2 and R1.
Any number of inputs can be connected to a summing junction through extra resistors.
This circuit can be used as a simple digital-to-analog converter. This will be illustrated in more detail, later.
The Difference Amplifier
This circuit is also called a differential amplifier, since it amplifies the difference between the input signals.
Rin2 is series combination of R1 and R2 because i+ is zero.
For v2=0, Rin1= R1, as the circuit reduces to an inverting amplifier.
For general case, i1 is a function of both v1 and v2.