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Operational Amplifiers

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Operational Amplifiers

Benchmark Companies Inc

PO Box 473768

Aurora CO 80047

INTRODUCTION

What is an operational amplifier?

In this chapter, we will define what an operational amplifier is, and discuss the many parameters that distinguish one type of device from another.

- OBJECTIVES
- At the completion of this chapter, you will be able to define the following:

channel separationclosed-loop gaincommon-mode rejection ratiogain-bandwidth productinput bias Current offsetinput offset Currentinput offset voltageInput resistance

input voltage range

inverting input

loop gain

non-inverting input

open-loop gain

operational amplifier

output resistance

output voltage swing

slew rate

Objective cont.

- Interpret a typical op-amp data sheet.
- Measure some of the common op-amp parameters.

- THE IDEAL OP-AMP
- Before we start looking at actual operational amplifier circuits, we will briefly consider the operational amplifier, hereafter referred to Op amp, by itself.

- THE IDEAL OP-AMP
- The term Op-amp was originally used to describe a series of high-performance dc amplifiers that were used as the basis for analog computers.

- THE IDEAL OP-AMP
- Today’s integrated circuit op-amp is a very high-gain dc amplifier that uses external feedback networks to control its response.

Feedback

- Open Loop Mode
- The op-amp without any external feedback is described as being used in an open-loop mode.

- Open Loop Mode
- It is in this mode that we can describe the characteristics of the ideal op-amp:
- The open-loop gain is infinite.

- It is in this mode that we can describe the characteristics of the ideal op-amp:

- Open Loop Mode
- It is in this mode that we can describe the characteristics of the ideal op-amp:
- The input resistance is infinite.
- The output resistance is zero.

- It is in this mode that we can describe the characteristics of the ideal op-amp:

- Open Loop Mode
- It is in this mode that we can describe the characteristics of the ideal op-amp:
- The bandwidth is infinite.
- The output voltage is zero when the input voltage is zero (i.e., zero offset).

- It is in this mode that we can describe the characteristics of the ideal op-amp:

- Open Loop Mode
- In practice, however, no op-amp can meet these five ideal open-loop characteristics. However, as we shall see in the next few chapters, the world doesn’t come to an end because there is no such thing as the ideal op-amp.

- THE OP-AMP SCHEMATIC SYMBOL
- the op-amp has two inputs: one inverting, or - input, and one non-inverting, or + input.
- a single output

Inverting

Output

Non-Inverting

- THE OP-AMP SCHEMATIC SYMBOL
- the op-amp is powered normally by a dual-polarity power supply, typically in the range of ±5 to ± 15 volts.

Neg

Pos

- THE OP-AMP DATA SHEET
- Perhaps the best way to understand the many characteristics of an op-amp is to examine a manufacturer’s data sheet.

- THE OP-AMP DATA SHEET
- the data sheet usually contains the following information:
- A general description of the op-amp.
- An internal equivalent circuit schematic.
- Pin configuration of the device.
- The absolute maximum ratings.
- The electrical characteristics.
- Typical performance curves.

- the data sheet usually contains the following information:

- THE OP-AMP DATA SHEET
- we will cover most of the important parameters, using the type 741 op-amp as a representative example.

- THE OP-AMP DATA SHEET
- Important Parameters cont.
- Maximum Ratings:The maximum ratings given in the data sheet are the maximum the op-amp can safely tolerate without the possibility of destruction.

- Important Parameters cont.

- THE OP-AMP DATA SHEET
- Maximum Ratings:
- Supply Voltage (±Va )
This is the maximum positive and negative voltage that can be used to power the op-amp.

- Internal Power Dissipation (PD)
This is the maximum power that the op-amp is capable of dissipating, given a specified ambient temperature (i.e., 500 mW @ <75C).

- Differential Input Voltage (Vid )
This is the maximum voltage that can be applied across the + and - inputs.

- Input Voltage (Vicm )
This is the maximum input voltage that can be simultaneously applied between both inputs and ground, also referred to as the common-mode voltage. In general, this maximum voltage is equal to the supply voltage.

- Operating Temperature (Ta)
This is the ambient temperature range for which the op-amp will operate within the manufacturer’s specifications. Note that the military grade version (741) has a wider temperature range than the commercial, or hobbyist, grade version (741C).

- Output Short-Circuit Duration
This is the amount of time that the op-amp’s output can be short-circuited to ground or either supply voltage.

- Supply Voltage (±Va )

- Maximum Ratings:

- THE OP-AMP DATA SHEET
- Important Parameters cont.
- Electrical Characteristics:
- The Op amp’s electrical characteristics are usually specified for a supply voltage and ambient temperature. However, certain parameters may also have other conditions attached, such as a particular source resistance. Generally, each parameter will have a minimum typical, and/or maximum value. (See data sheet for examples)

- Electrical Characteristics:

- Important Parameters cont.

- THE OP-AMP DATA SHEET
- Electrical Characteristics: cont.
- Input Parameters:
- Input Offset Voltage (Voi)
This is the voltage that must be applied to one of the input terminals to give a zero output voltage. Remember, for an ideal op-amp, the output voltage offset is zero!

- Input Bias Current (Ib)
This is the average of the currents flowing into both inputs. Ideally, the two input bias currents are equal.

- Input Offset Current (los)
This is the difference of the two input bias currents when the output voltage is zero.

- Input Voltage Range (Vcm )
This is the range of the common-mode input voltage (i.e., the voltage common to both inputs and ground).

- Input Resistance (Zi)
This is the resistance “looking in” at either input with the remaining input with the remaining input grounded.

- Input Offset Voltage (Voi)

- Input Parameters:

- Electrical Characteristics: cont.

- THE OP-AMP DATA SHEET
- Electrical Characteristics: cont.
- Output Parameters:
- Output Resistance (Zoi)
This is the resistance seen “looking into” the op-amp’s output.

- Output Short-Circuit Current (Iosc )
This is the maximum output current that the op-amp can deliver to a load.

- Output Voltage Swing (±Vo max)
Depending on the load resistance, this is the maximum peak output voltage that the op-amp can supply without saturation or clipping.

- Output Resistance (Zoi)

- Output Parameters:

- Electrical Characteristics: cont.

- THE OP-AMP DATA SHEET
- Electrical Characteristics: cont.
- Dynamic Parameters:
- Open-Loop Voltage Gain (AOL)
This is the ratio of the output to input voltage of the op-amp without external feedback.

- Large-Signal Voltage Gain
This is the ratio of the maximum voltage swing to the change in the input voltage required to drive the output from zero to a specified voltage (e.g., ±10 volts).

- Slew Rate (SR)
This is the time rate of change of the output voltage with the op-amp circuit having a voltage gain of unity (1.0).

- Open-Loop Voltage Gain (AOL)

- Dynamic Parameters:

- Electrical Characteristics: cont.

- THE OP-AMP DATA SHEET
- Electrical Characteristics: cont.
- Other Parameters:
- Supply Current
This is the current that the op-amp will draw from the power supply

- Common-Mode Rejection Ratio (CMRR)
This is a measure of the ability of the op-amp to reject signals that are simultaneously present at both inputs. It is the ratio of the common-mode input voltage to the generated output voltage, usually expressed in decibels (dB).

- Channel Separation
Whenever there is more than one op-amp in a single package, such as a type 747 op-amp, a certain amount of “crosstalk” will be present. That is, a signal applied to the input of one section of a dual op-amp will produce a finite output signal in the remaining section, even though there is no input signal applied to the unused section.

- Supply Current

- Other Parameters:

- Electrical Characteristics: cont.

- GAIN AND FREQUENCY RESPONSE
- Unlike the ideal op-amp, the Op-amp that is used in various circuits does not have infinite gain and bandwidth. As shown in Fig.1, the open-loop gain AOL for a type 741 op-amp is graphed as a function of frequency.

- GAIN AND FREQUENCY RESPONSE
- At very low frequencies, the open-loop gain of op-amp is constant, but begins to “roll off” at approximately 6 Hz at a rate of -6 dB/octave or -20 dB/decade An octave is a doubling in frequency and a decade is a ten-fold increase in frequency.

- GAIN AND FREQUENCY RESPONSE
- This decrease continues until the gain is unity, or 0 dB. The frequency at which the gain is unity is called the unity gain frequency, fT.

- Open Loop and Closed Loop Gain
- When some of the output signal is fed back to the op-amp’s input, the ratio of the output to input voltage is termed the closed-loop gain, ACL, and is always less than the open-loop gain.

- Open Loop and Closed Loop Gain
- The difference in decibels between the open-loop and closed-loop gains is the loop gain, AL. When AOL and ACL are expressed as simple output-to-input ratios, the loop gain is expressed mathematically as:
AL = AOL/ACL

- The difference in decibels between the open-loop and closed-loop gains is the loop gain, AL. When AOL and ACL are expressed as simple output-to-input ratios, the loop gain is expressed mathematically as:

- Gain Bandwidth
- Perhaps the first factor in the consideration of a particular op-amp for a given application is its gain-bandwidth product, or GBP.

- Gain Bandwidth
- For the response curve below, the product of the open-loop gain and frequency is a constant at any point on the curve, so that:
GBP =AOLBW

- For the response curve below, the product of the open-loop gain and frequency is a constant at any point on the curve, so that:

- Gain Bandwidth
- Graphically, the bandwidth is the point at which the closed-loop gain curve intersects the open-loop gain curve, as shown in the figure below for a family of closed-loop gains.

- Gain Bandwidth
- Therefore, one obtains the bandwidth for any desired closed-loop gain by simply drawing a horizontal line from the desired value of gain to intersect the roll-off of the open-loop gain curve.

- Gain Bandwidth
- For a practical design situation, the actual design gain of an opamp circuit should be about a factor of 1/10 to 1/20 of the open-loop gain at a given frequency.

- Gain Bandwidth
- This ensures that the op-amp will function properly without distortion. As an example, using the response of Fig. 1-3, the closed-loop gain at 10 kHz should be about 5 to 10, since the open-loop gain is 100 (40 dB).

ACL

- Transient Response, (Rise Time)
- The time that it takes for the output signal to go from 10% to 90% of its final value when a step.

Input Signal

Output Signal

time

Timing Diagram

- Transient Response, (Rise Time)
- A function pulse is used as an input signal, and is specified under closed-loop conditions. From electronic circuit theory, the rise time is related to the bandwidth of the op-amp by the relation:
BW= 0.35/rise time

- A function pulse is used as an input signal, and is specified under closed-loop conditions. From electronic circuit theory, the rise time is related to the bandwidth of the op-amp by the relation:

- Summary
- Op-amps are designed to be powered from a dual, or bipolar, voltage supply which is typically in the range of ±5to ±15 volts.

- Summary
- That is, one supply is +5to +15 volts with respect to ground, and another supply voltage of -5to -15 volts with respect to ground.

- Summary
- However, in certain cases, an op-amp may be operated from a single supply voltage, which is explained in Chapters 8 and 9.

End of Lesson