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## Operational Amplifiers

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**Operational Amplifiers**Benchmark Companies Inc PO Box 473768 Aurora CO 80047**What is an Op-Amp?**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.**What is an Op-Amp?**• 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**What is an Op-Amp?**Objective cont. • Interpret a typical op-amp data sheet. • Measure some of the common op-amp parameters.**What is an Op-Amp?**• 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.**What is an Op-Amp?**• 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.**What is an Op-Amp?**• 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**What is an Op-Amp?**• Open Loop Mode • The op-amp without any external feedback is described as being used in an open-loop mode.**What is an Op-Amp?**• 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.**What is an 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.**What is an 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).**What is an 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.**What is an 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**What is an Op-Amp?**• 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**What is an Op-Amp?**• 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.**What is an Op-Amp?**• 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.**What is an Op-Amp?**• THE OP-AMP DATA SHEET • we will cover most of the important parameters, using the type 741 op-amp as a representative example.**What is an Op-Amp?**• 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.**What is an Op-Amp?**• 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.**What is an Op-Amp?**• 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)**What is an Op-Amp?**• 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.**What is an Op-Amp?**• 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.**What is an Op-Amp?**• 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).**What is an Op-Amp?**• 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.**What is an Op-Amp?**• 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.**What is an Op-Amp?**• 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.**What is an Op-Amp?**• 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.**What is an Op-Amp?**• 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.**What is an Op-Amp?**• 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**What is an Op-Amp?**• 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.**What is an Op-Amp?**• 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**What is an Op-Amp?**• 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.**What is an Op-Amp?**• 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.**What is an Op-Amp?**• 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.**What is an Op-Amp?**• 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**What is an Op-Amp?**• 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**What is an Op-Amp?**• 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**What is an Op-Amp?**• 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.**What is an Op-Amp?**• 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.**What is an Op-Amp?**• Summary • However, in certain cases, an op-amp may be operated from a single supply voltage, which is explained in Chapters 8 and 9.