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Operational Amplifiers (4.1-4.3). Dr. Holbert April 3, 2006. Op Amps. Op Amp is short for operational amplifier. An operational amplifier is modeled as a voltage controlled voltage source. An operational amplifier has a very high input impedance and a very high gain. Use of Op Amps.

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operational amplifiers 4 1 4 3

Operational Amplifiers (4.1-4.3)

Dr. Holbert

April 3, 2006

ECE201 Lect-16

op amps
Op Amps
  • Op Amp is short for operational amplifier.
  • An operational amplifier is modeled as a voltage controlled voltage source.
  • An operational amplifier has a very high input impedance and a very high gain.

ECE201 Lect-16

use of op amps
Use of Op Amps
  • Op amps can be configured in many different ways using resistors and other components.
  • Most configurations use feedback.

ECE201 Lect-16

applications of op amps
Applications of Op Amps
  • Amplifiers provide gains in voltage or current.
  • Op amps can convert current to voltage.
  • Op amps can provide a buffer between two circuits.
  • Op amps can be used to implement integrators and differentiators.
  • Lowpass and bandpass filters.

ECE201 Lect-16

the op amp symbol

+

-

The Op Amp Symbol

High Supply

Non-inverting input

Output

Inverting input

Ground

Low Supply

ECE201 Lect-16

the op amp model
The Op Amp Model

v+

Non-inverting input

+

vo

Rin

+

Inverting input

A(v+ -v- )

v-

ECE201 Lect-16

typical op amp
Typical Op Amp
  • The input resistance Rin is very large (practically infinite).
  • The voltage gain A is very large (practically infinite).

ECE201 Lect-16

ideal op amp
“Ideal” Op Amp
  • The input resistance is infinite.
  • The gain is infinite.
  • The op amp is in a negative feedback configuration.

ECE201 Lect-16

the basic inverting amplifier
The Basic Inverting Amplifier

R2

R1

+

+

+

Vin

Vout

ECE201 Lect-16

consequences of the ideal
Consequences of the Ideal
  • Infinite input resistance means the current into the inverting input is zero:

i- = 0

  • Infinite gain means the difference between v+ and v- is zero:

v+ - v- = 0

ECE201 Lect-16

solving the amplifier circuit
Solving the Amplifier Circuit

Apply KCL at the inverting input:

i1 + i2 + i-=0

R2

i2

R1

i1

i-

ECE201 Lect-16

slide12
KCL

ECE201 Lect-16

solve for v out
Solve for vout

Amplifier gain:

ECE201 Lect-16

recap
Recap
  • The ideal op-amp model leads to the following conditions:

i- = 0 = i+

v+ = v-

  • These conditions are used, along with KCL and other analysis techniques, to solve for the output voltage in terms of the input(s).

ECE201 Lect-16

where is the feedback
Where is the Feedback?

R2

R1

+

+

+

Vin

Vout

ECE201 Lect-16

review
Review
  • To solve an op-amp circuit, we usually apply KCL at one or both of the inputs.
  • We then invoke the consequences of the ideal model.
    • The op amp will provide whatever output voltage is necessary to make both input voltages equal.
  • We solve for the op-amp output voltage.

ECE201 Lect-16

the non inverting amplifier
The Non-Inverting Amplifier

+

+

+

vin

vout

R2

R1

ECE201 Lect-16

kcl at the inverting input
KCL at the Inverting Input

+

+

+

i-

vin

vout

i1

i2

R2

R1

ECE201 Lect-16

slide19
KCL

ECE201 Lect-16

solve for v out20
Solve for Vout

ECE201 Lect-16

a mixer circuit
A Mixer Circuit

R1

Rf

+

R2

v1

+

+

+

v2

vout

ECE201 Lect-16

kcl at the inverting input22
KCL at the Inverting Input

R1

Rf

i1

if

+

R2

v1

i2

i-

+

+

+

v2

vout

ECE201 Lect-16

slide23
KCL

ECE201 Lect-16

slide24
KCL

ECE201 Lect-16

solve for v out25
Solve for Vout

ECE201 Lect-16

slide26

Class Example

  • Learning Extension E4.1
  • Learning Extension E4.2
  • Learning Extension E4.3

ECE201 Lect-16