CHARACTERISTICS OF OPERATIONAL AMPLIFIERS

INTRODUCTION

The characteristics of an ideal operational amplifier are described first, and thecharacteristics and performance limitations of a practical operational amplifier aredescribed next.

1.1 IDEAL OPERATIONAL AMPLIFIER

Properties of An Ideal Operational Amplifier

The characteristics or the properties of an ideal operational amplifier are:

i. Infinite Open Loop Gain,

ii. Infinite Input Impedance,

iii. Zero Output Impedance,

iv. Infinite Bandwidth,

v. Zero Output Offset, and

vi. Zero Noise Contribution.

The opamp, an abbreviation for the operational amplifier, is the most importantlinear IC.The three terminalsare: the non-inverting input terminal, the inverting input terminal and the outputterminal. The details of power supply are not shown in a circuit symbol.

1.1.2 Infinite Open Loop Gain

vo = – Ao × vi, where `Ao’ is known as the open-loopgain of the opamp. Let vo be -10 Volts, and Ao be 105. Then vi is 100 :V. Herethe input voltage is very small compared to the output voltage. If Ao is very large,vi is negligibly small for a finite vo. For the ideal opamp, Ao is taken to beinfinite in value. That means, for an ideal opamp vi = 0 for a finite vo. Typicalvalues of Ao range from 20,000 in low-grade consumer audio-range opamps tomore than 2,000,000 in premium grade opamps ( typically 200,000 to 300,000).

The first property of an ideal opamp: Open Loop Gain Ao = infinity.

1.1.3 Infinite Input Impedance and Zero Output Impedance

An ideal opamp has an infinite input impedance and zero output impedance. Thesketch in Fig. 1.2 is used to illustrate these properties. From Fig. 1.2, it can beseen that iin is zero if Rin is equal to infinity.

The second property of an ideal opamp: Rin = infinity or iin =0.

If the output resistance Ro is very small, there is no drop in output voltage due to the output resistance of an opamp.

The third property of an ideal opamp: Ro = 0.

1.1.4 Infinite Bandwidth

An ideal opamp has an infinite bandwidth. A practical opamp has a limited bandwidth, which falls far short of the ideal value. The variation of gain with frequency  which is obtained by modelling the opamp with a single dominant pole, whereas the practical opamp may have more than a single pole.

Zero Noise Contribution and Zero Output Offset

A practical opamp generates noise signals, like any other device, whereas an ideal opamp produces no noise. Premium opamps are available which contribute very low noise to the rest of circuits.  These devices are usually called as premium lownoise types. The output offset voltage of any amplifier is the output voltage that exists when it should be zero. In an ideal opamp, this offset voltage is zero.

PRACTICAL OPERATIONAL AMPLIFIERS

This section describes the properties of practical opamps and relates these characteristics to design of analog electronic circuits. A practical operational amplifier has limitations to its performance. It is necessary to understand these limitations in order to select the correct opamp for an application and design the circuit properly. Like any other semiconductor device, a practical opamp also has a code number. For example, let us take the code LM 741CP. The first two letters, LM here, denote the manufacturer. The next three digits, 741 here, is the type number. 741 is a general-purpose opamp. The letter following the type number, C here, indicates the temperature range. The temperature range codes are:

C commercial 0o C to 70C

I industrial -25o C to 85o C

M military -55o C to 125o C.

The last letter indicates the package. Package codes are:

D Plastic dual-in-line for surface mounting on a pc board

J Ceramic dual-in-line

N,P Plastic dual-in-line for insertion into sockets.

Standard Operational Amplifier Parameters

Understanding operational amplifier circuits requires knowledge of the parameters given in specification sheets. The list below represents the most commonly needed parameters.

1>Open-Loop Voltage Gain. Voltage gain is defined as the ratio of output voltage to an input

signal

2>Large Signal Voltage Gain. This is the ratio of the maximum allowable output voltage swing (usually one to several volts less than V- and V++) to the input signal required to produce a swing of ± 10 volts (or some other standard).

3>Slew rate. The slew rate is the maximum rate at which the output voltage of an opamp can change and is measured in terms of voltage change per unit of time. It varies from 0.5 V/:s to 35 V/:s. Slew rate is usually measured in the unity gain noninverting amplifier configuration.

4>Common Mode Rejection Ratio. A common mode voltage is one that is presented simultaneously to both inverting and noninverting inputs. In an ideal opamp, the output signal due to the common mode input voltage is zero, but it is nonzero in a practical device. The common mode rejection ratio (CMRR) is the measure of the device’s ability to reject common mode signals, and is expressed as the ratio of the differential gain to the common mode gain. The CMRR isv usually expressed in decibels, with common devices having ratings between 60 dB to 120 dB. The higher the CMRR is, the better the device is deemed to be.

5>Input Offset Voltage. The dc voltage that must be applied at the input terminal to force the quiescent dc output voltage to zero or other level, if specified, given that the input signal voltage is zero. The output of an ideal opamp is zero when there is no input signal applied to it.

6>Power-supply rejection ratio. The power-supply rejection ratio PSRR is the ratio of the change in input offset voltage to the corresponding change in one power-supply, with all remaining power voltages held constant. The PSRR is also called “power supply insensitivity”. Typical values are in :V / V or mV/V.

7>Input Bias Current. The average of the currents into the two input terminals with the output at zero volts.

8>Input Offset Current. The difference between the currents into the two input terminals with the output held at zero.

9>Differential Input Impedance. The resistance between the inverting and the noninverting inputs. This value is typically very high: 1 MS in low-cost bipolar opamps and over 1012 Ohms in premium BiMOS devices.

10>Common-mode Input Impedance

The impedance between the ground and the input terminals, with the input terminals tied together. This is a large value, of the order of several tens of MS or more.

11>Output Impedance. The output resistance is typically less than 100 Ohms.

Average Temperature Coefficient of Input Offset Current. The ratio of the change in input offset current to the change in free-air or ambient temperature. This is an average value for the specified range.

Average Temperature Coefficient of Input Offset Voltage. The ratio of the change in input offset voltage to the change in free-air or ambient temperature. This is an average value for the specified range.

Output offset voltage. The output offset voltage is the voltage at the output terminal with respect to ground when both the input terminals are grounded.

Output Short-Circuit Current. The current that flows in the output terminal when the output load resistance external to the amplifier is zero ohms (a short to the common terminal).

Channel Separation. This parameter is used on multiple opamp ICs (device in which two or more opamps sharing the same package with common supply terminals). The separation specification describes part of the isolation between the opamps inside the same package. It is measured in decibels. The 747 dual opamp, for example, offers 120 dB of channel separation. From this specification, we may state that a 1 :V change will occur in the output of one of the amplifiers, when the other amplifier output changes by 1 volt.

Minimum and Maximum Parameter Ratings

Operational amplifiers, like all electronic components, are subject to maximum ratings. If these ratings are exceeded, the device failure is the normal consequent result. The ratings described below are commonly used.

Maximum Supply Voltage. This is the maximum voltage that can be applied to the opamp without damaging it. The opamp uses a positive and a negative DC power supply, which are typically ± 18 V.

Maximum Differential Supply Voltage. This is the maximum difference signal that can be applied safely to the opamp power supply terminals. Often this is not the same as the sum of the maximum supply voltage ratings. For example, 741 has ± 18 V as the maximum power supply voltage, whereas the maximum

differential supply voltage is only 30 V. It means that if the positive supply is 18 V, the negative supply can

be only -12 V.

Power dissipation, Pd. This rating is the maximum power dissipation of the opamp in the normal ambient temperature range. A typical rating is 500 mW.

Maximum Power Consumption. The maximum power dissipation, usually under output short circuit conditions, that the device can survive. This rating includes both internal power dissipation as well as device output power requirements.

Maximum Input Voltage. This is the maximum voltage that can be applied simultaneously to both inputs. Thus, it is also the maximum common-mode voltage. In most bipolar opamps, the maximum input voltage is nearly equal to the power supply voltage. There is also a maximum input voltage that can be applied to either input when the other input is grounded.

Differential Input Voltage. This is the maximum differential-mode voltage that can be applied across the inverting and noninverting inputs.

Maximum Operating Temperature. The maximum temperature is the highest ambient temperature at which the device will operate according to specifications with a specified level of reliability.

Minimum Operating Temperature. The lowest temperature at which the device operates within specification.

Output Short-Circuit Duration. This is the length of time the opamp will safely sustain a short circuit of the output terminal. Many modern opamps can carry short circuit current indefinitely.

Maximum Output Voltage. The maximum output potential of the opamp is related to the DC power supply voltages. Typical for a bipolar opamp with ± 15 V power supply, the maximum output voltage is typically about 13 V and the minimum – 13 V.

Maximum Output Voltage Swing. This is the maximum output swing that can be obtained without significant distortion(at a given load resistance).

Full-power bandwidth. This is the maximum frequency at which a sinusoid whose size is the output voltage range is obtained.

Comparison of an ideal and a typical practical opamp

Property Ideal Practical(Typical)

Open-loop gain Infinite Very high (>10000 )

Open-loop bandwidth Infinite Dominant pole(–10

Hz)

CMRR Infinite High (> 60 dB)

Input Resistance Infinite High (>1 MS)

Output Resistance Zero Low(< 100 S)

Input Bias Currents Zero Low (< 50 nA)

Offset Voltages Zero Low (< 10 mV)

Offset Currents Zero Low (< 50 nA)

Slew Rate Infinite A few V/:s

Drift Zero Low