CHAPTER 1: INTRODUCTION TO OPERATIONAL AMPLIFIERS

1 / 40

# CHAPTER 1: INTRODUCTION TO OPERATIONAL AMPLIFIERS - PowerPoint PPT Presentation

CHAPTER 1: INTRODUCTION TO OPERATIONAL AMPLIFIERS. Objectives. Describe basic op-amp characteristics. Discuss op-amp modes and parameters. Explain negative feedback. Analyze inverting, non-inverting, voltage follower and inverting op-amp configurations. BASIC OP-AMP. Symbol and Terminals.

I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.

## PowerPoint Slideshow about 'CHAPTER 1: INTRODUCTION TO OPERATIONAL AMPLIFIERS' - barny

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Objectives
• Describe basic op-amp characteristics.
• Discuss op-amp modes and parameters.
• Explain negative feedback.
• Analyze inverting, non-inverting, voltage follower and inverting op-amp configurations.
Symbol and Terminals
• A standard operational amplifier (op-amp) has;
• Vout is the output voltage,
• V+  is the non-inverting input voltage,
• V-  is the inverting input voltage.
• Typical op-amp operates with 2 dc supply voltages,
• +ve supply.
• –ve supply.

Figure 1a: Symbol

Figure 1b: Symbol with dc supply connections

An op amp is an active

circuit element designed

to perform mathematical

subtraction,

multiplication, division,

differentiation, and

integration.

741 general purpose op-amp made by Fairchild Semiconductor

Operational Amplifiers

The op amp is built using VLSI techniques.The circuit

diagram of an LM 741 from National Semiconductor is

shown below.

V+

Vin(-)

Vo

Vin(+)

Taken from National Semiconductor

data sheet as shown on the web.

Internal circuitry of LM741.

V-

_

Vin

AvVin

Zin=∞

Zout=0

Av=∞

+

The Ideal Op-Amp
• The ideal op-amp has;
• Infinite voltage gain.
• Infinite bandwidth.
• Infinite input impedance
• zero output impedance.
• The input voltage, Vin appears between the two input terminal.
• The output voltage is AvVinas indicated by the internal voltage source symbol.

Figure 2a: Ideal op-amp representation

_

Zin

Vin

AvVin

Zout

+

The Practical Op-Amp
• Characteristic of a practical op-amp are;
• Very high voltage gain.
• Very high input impedance.
• Very low output impedance.
• Wide bandwidth.

Figure 2b: Practical op-amp representation

Input Signal Modes

A) Single-Ended Input

• Operation mode;
• One input is grounded.
• The signal voltage is applied only to the other input.
• When the signal voltage is applied to the inverting input,
• an inverted amplified signal voltage appears at the output. (figure 3a)

_

+

Figure 3a

When the signal voltage is applied to the noninverting input with the inverting input grounded,
• a noninverted amplified signal voltage appears at the output. (figure 3b)

_

+

Figure 3b

B) Differential Input
• Operation mode;
• Two opposite-polarity (out-of-phase) signals are applied to the inputs
• This type of operation is also referred to as double-ended.
• The amplified difference between the two inputs appears on the output.

_

+

Figure 3c

C) Common-Mode Input
• Operation mode
• Two signal voltages of the same phase, frequency and amplitude are applied to the two inputs. (figure 3d)
• When equal input signals are applied to both inputs, they cancel, resulting in a zero output voltage.
• This action is called common-mode rejection.
• Means that this unwanted signal will not appear on the output and distort the desired signal.

_

+

Figure 3d

Common-Mode Rejection Ratio
• Desired signals can appear on only
• one input or
• with opposite polarities on both input lines.
• These desired signals are
• amplified and appear on the output.
• Unwanted signals (noise) appearing with the same polarity on both input lines are
• essentially cancelled by the op-amp and do not appear on the output.
• The measure of an amplifier’s ability to reject common-mode signal is called
• CMRR (common-mode rejection ration).
• Ideally, op-amp provides
• a very high gain for desired signal (single-ended or differential)
• zero gain for common-mode signal.
The higher the open-loop gain with respect to the common-mode gain,
• the better the performance of the op-amp in terms of rejection of common-mode signals.
• Therefore;

where Aol = open-loop voltage gain

Acm = common-mode gain

• The higher the CMRR, the better.
• A very high value of CMRR means that
• the open-loop gain, Aol is high and
• the common-mode gain, Acm is low.
• The CMRR expressed in decibels (dB) is
Open-Loop Voltage Gain
• Open-loop voltage gain, Aol of an op-amp
• is the internal voltage gain of the device
• represents the ration of output voltage to input voltage when there are no external components.
• The open-loop voltage gain is set entirely by the internal design.
• Open-loop voltage gain can range up to
• 200,000 and is not a well-controlled parameter.
• Data sheet often refer to the open-loop voltage gain as
• the large-signal voltage gain.
Example 1

A certain op-amp has an open-loop voltage gain of 100,000 and a common-mode gain of 0.2.

Determine the CMRR and express it in decibels.

Common-Mode Input Voltage Range
• All op-amp have limitation on the range of voltages over which they will operate.
• The common-mode input voltage range is
• the range of input voltages which when applied to both inputs will cause clipping or other output distortion.
• Many op-amp have common-mode input ranges of
• ±10V with dc supply voltages of ±15V.

I1

V1

_

I2

Vout

+

V2

Input Bias Current
• The input bias current is
• the dc current required by the inputs of the amplifier to properly operate the first stage.
• By definition, the input bias current is
• the average of both input currents and is calculated as;

Figure 4a: Input bias current is the average of the two op-amp input currents.

Input Impedance
• Two basic ways of specifying the input impedance of an op-amp are
• Differential.
• Common-mode.
• Differential input impedance is
• the total resistance between the inverting and the noninverting input.
• Measured by determining the change in bias current for a given change in differential input voltage.

ZIN(d)

Figure 4b: Differential input impedance

Common-mode input impedance is
• the resistance between each input and ground.
• Measured by determining the change in bias current for a given change in common-mode input voltage.

ZIN(cm)

Figure 4c: Common-mode impedance

Output Impedance
• The output impedance is
• the resistance viewed from the output terminal of the op-amp as indicated in figure 4d

Zout

Figure 4d: Op-amp output impedance

Slew Rate
• What is slew rate?
• The maximum rate of change of the output voltage in response to a step input voltage.
• Is dependent upon the high-frequency response of the amplifier stages within the op-amp.
• Is measured with an op-amp connected as shown in figure 4e

Figure 4e: Test circuit

Vin

0

+Vmax

Vout

-Vmax

∆t

• A pulse is applied to the input, the output voltage is measured as indicated in figure 4f.
• The width of the input pulse must be sufficient
• to allow the output to slew from its lower limit to its upper limit.
• A certain time interval ∆t, is required for the output voltage
• to go from its lower limit

–Vmax to its upper limit +Vmax, once the input step is applied.

Figure 4f: Step input voltage and the resulting output voltage

The slew rate is expressed as

Where ∆Vout = +Vmax-(-Vmax).

• The unit is volts per microsecond (V/μs).

Vout(V)

10

9

0

t

-9

-10

2μs

12μs

Example 2

The output voltage of a certain op-amp appears as shown in figure below in response to a step input.

Determine the slew rate.

OP-AMPS WITH NEGATIVE FEEDBACK
• Negative feedback is a process whereby a portion of the output voltage returned to the input with a phase angle opposed the input signal
• Higher input impedance
• More stable gain
• Improved frequency response
• Lower output impedance
• More linear operation
Closed-Loop Voltage Gain, Acl
• The closed-loop voltage gain is
• the voltage gain of an op-amp with external feedback.
• The amplifier configuration consists of
• the op-amp
• an external negative feedback circuit that connects the output to the inverting input.
• The closed-loop voltage gain is determined by
• the external component values and can be precisely controlled by them.
Noninverting Amplifier

Figure 5: Noninverting amplifier

• Noninverting amplifier is
• an op-amp connected in a closed-loop with a controlled amount of voltage gain is shown in figure 5.
• The input signal is applied to
• the noninverting (+) input.
• The output is applied back to
• the inverting (-) input through the feedback circuit (closed loop) formed by the input resistor Ri and the feedback resistor Rf.

Feedback network

This creates negative feedback as follows.
• Resistor Ri and Rfform a voltage divider circuit, which reduces Vout and connects the reduced voltage Vf to the inverting input.
• The feedback voltage is expressed as
• The closed-loop gain of the noninverting (NI) amplifier is
• Where
• Therefore;
Example 3

Determine the gain of the amplifier in figure below. The open-loop voltage gain of the op-amp is 100,000.

100kΩ

4.7kΩ

Voltage-Follower
• The voltage-follower configuration is a special case of the noninverting amplifier
• where all the output voltage is fed back to the inverting (-) input by a straight connection. (figure 6)
• The straight feedback connection has a voltage gain of 1 (no gain).
• The closed-loop voltage gain of a noninverting amplifier is 1/B.

Figure 6: Op-amp voltage-follower

Since B=1, for a voltage-follower,
• the closed-loop voltage gain of the voltage follower is

Acl(VF)=1

• The most important features of the voltage-follower configuration are
• very high input impedance
• very low output impedance.
• These features make it a nearly ideal buffer amplifier for the
• interfacing high-impedance sources
Inverting Amplifier
• Inverting amplifier
• An op-amp connected with a controlled amount of voltage gain. (figure 7)
• The input signal is applied through a series input resistor Ri to the inverting (-) input.
• The output is fed back through Rf to the same input.
• The noninverting (+) input is grounded.

Aol

Figure 7: Inverting Amplifier

For inverting amplifier
• The closed-loop voltage gain is the ratio of the feedback resistance (Rf) to the input resistance (Ri).
• This gain is independent of the op-amp’s internal open-loop gain.
• Thus, the negative feedback stabilizes the voltage gain.
Example 4

Given the op-amp configuration in figure below, determine the value of Rf required to produce a closed-loop voltage gain of -100.

2.2kΩ

Aol

Op-amp Impedances

Noninverting amplifier:

Where Zin is the open-loop input impedance (internal) of the op-amp (without feedback connection)

Inverting amplifier:

Generally, assumed to be Ri

Generally, assumed to be 0

Note that the output impedance has the same form for both amplifiers.

Example 5
• Determine the input and output impedances of the amplifier in Figure below. The op-amp datasheet gives Zin = 2MΩ, Zout = 75Ω, and Aol = 200,000.
• Find the closed-loop voltage gain.

Answer: (a) Zin(NI)=17.5GΩ, Zout(NI)=8.6mΩ, (b) Acl(NI) = 23.0

Example 6

Find the values of the input and output impedances in Figure below. Also, determine the closed-loop voltage gain. The op-amp has the following parameters: Aol = 50,000; Zin = 4MΩ; and Zout = 50 Ω