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

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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

Operations of addition,

subtraction,

multiplication, division,

differentiation, and

integration.

741 general purpose op-amp made by Fairchild Semiconductor

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.

Answer: a) 500,000 b) 114dB

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.

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

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

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
- Advantages:
- 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

Answer: 22.3

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
- low-impedance loads.

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.
- The negative sign indicates inversion. Therefore;

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

Answer: 220 kΩ

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

Answer: Zin(I)=1.0kΩ, Zout(I)=980mΩ, Acl(I)=-100

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