Classification and characteristics of operational amplifiers, classification and characteristics of operational amplifiers

1. Classification and characteristics of operational amplifiers

It has been more than 40 years since the birth of analog operational amplifiers. The earliest process was the silicon NPN process, which was later improved to the silicon NPN-PNP process (hereinafter referred to as the standard silicon process). After the junction field effect transistor (JFET) technology matured, the JFET process was further added. When the MOS tube technology matured. Especially after the CMOS technology matured, the analog operational amplifier has made a qualitative leap. On the one hand, it solved the problem of low power consumption; on the other hand, it solved the problem of direct processing of small DC signals by using mixed analog and digital circuit technology.

After years of development, analog operational amplifier technology has become very mature, with perfect performance and a wide variety of types. This makes beginners wonder how to choose the right one. In order to facilitate beginners to choose, this article uses two classification methods, process classification and function/performance classification, to classify integrated analog operational amplifiers, which is easy for readers to understand. This method may be different from the usual classification method.

1.1 Classification based on manufacturing process

According to the manufacturing process, the integrated analog operational amplifiers currently used can be divided into standard silicon process operational amplifiers, operational amplifiers that combine the standard silicon process with the JFET process, and operational amplifiers that combine the standard silicon process with the MOS process. The classification by process is to help beginners understand the impact of processing technology on the performance of integrated analog operational amplifiers and quickly grasp the characteristics of operational amplifiers.

The characteristics of integrated analog operational amplifiers based on standard silicon technology are low open-loop input impedance, low input noise, slightly low gain, low cost, low precision, and high power consumption. This is because the integrated analog operational amplifiers of standard silicon technology are all NPN-PNP tubes. Considering the frequency characteristics, there should not be too many intermediate gain levels, which will reduce the total gain, generally between 80~110dB. Standard silicon technology can be combined with laser correction technology to greatly improve the accuracy of integrated analog operational amplifiers. At present, the temperature drift parameter can reach 0.15ppm. By changing the standard silicon process, we can design general-purpose operational amplifiers and high-speed operational amplifiers. A typical example is LM324.

The operational amplifier using JFET process in standard silicon process mainly improves the input stage of the integrated analog operational amplifier of standard silicon process to JFET, which greatly improves the open-loop input impedance of the operational amplifier and incidentally improves the conversion speed of the general operational amplifier. Other integrated analog operational amplifiers are similar to standard silicon process operational amplifiers. The typical open-loop input impedance is about 1000m ohms. A typical example is TL084.

Operational amplifiers using MOS technology in standard silicon processes are divided into three categories:

(1) The first is to improve the input stage of the integrated analog operational amplifier in the standard silicon process to MOS

FETs, compared to JFETs, greatly increase the open-loop input impedance of op amps, increasing the switching speed of general-purpose op amps. Other integrated analog op amps are similar to standard silicon process op amps. Typical open-loop input impedance is about
10^12 ohms. A typical example is the CA3140.

(2) The second type is an analog operational amplifier using all-MOS FET technology, which greatly reduces power consumption, but the power supply voltage is reduced and the power consumption is greatly reduced. Its typical open-loop input impedance is about 10^12 ohms.

(3) The third category is to use full MOS

FET technology analog digital hybrid operational amplifier, mainly used to improve the processing accuracy of DC signals by using chopper stabilization technology. The input offset voltage can reach 0.01uV. The temperature drift parameter can reach 0.02 pages/minute. It is close to the ideal operational amplifier characteristics when processing DC signals. Its typical open-loop input impedance is about 10^12 ohms. A typical example is ICL7650.1.

1.2 Classification based on function/performance

According to the function/performance classification, analog operational amplifiers can generally be divided into general-purpose operational amplifiers, low-power operational amplifiers, precision operational amplifiers, high-input impedance operational amplifiers, high-speed operational amplifiers, broadband operational amplifiers, and high-voltage operational amplifiers. There are also some special operational amplifiers, such as programmable operational amplifiers, current operational amplifiers, voltage followers, etc. In fact, there are many kinds of operational amplifiers to meet the needs of the application. This article is based on the above simple classification method.

That is to say, as technology advances, the classification threshold has been changing. For example, the previous LM108 was originally classified as a precision op amp, but now it can only be classified as a general-purpose op amp. In addition, there are op amps with low power consumption and high input impedance, or similarly, it may also belong to more than one category.

General-purpose operational amplifiers are the cheapest operational amplifiers with the most basic functions. This type of amplifier is the most widely used.

On the basis of general-purpose operational amplifiers, the power consumption of low-power operational amplifiers is greatly reduced. It can be used in places where power consumption is limited, such as handheld devices (PDAs). It has low static power consumption and a low operating voltage close to the battery voltage. In addition, it can maintain good electrical performance at low voltage levels. With the development of MOS technology, low-power amplifiers are no longer a single phenomenon. The static power consumption of low-power operational amplifiers is generally less than 1MW.

A precision operational amplifier is an integrated operational amplifier with very low drift and noise and very high gain and common mode rejection ratio, also known as a low drift operational amplifier or a low noise operational amplifier. The temperature drift of this type of operational amplifier is generally less than 1uV/°C. Due to technological advances, the offset voltage of some early operational amplifiers was relatively high, possibly as high as 1mV; now the offset voltage of precision operational amplifiers can reach 0.1 mV; the offset voltage of precision operational amplifiers using chopper stabilization technology can reach 0.005mV. Precision operational amplifiers are mainly used in fields that require amplification processing accuracy, such as automatic instruments.

High input impedance operational amplifiers usually refer to integrated operational amplifiers with JFET or MOS tubes as input stages. This includes integrated operational amplifiers with full MOS tubes. The input impedance of high input impedance operational amplifiers is generally greater than 109 ohms. As a side feature of high input impedance operational amplifiers, the conversion speed is relatively high. High input impedance operational amplifiers are widely used. For example, sample and hold circuits, integrators, logarithmic amplifiers, instrumentation amplifiers, bandpass filters, etc.

A high-speed operational amplifier is an operational amplifier with a high conversion speed. Usually, the conversion speed is above 100V/us. High-speed operational amplifiers are used in high-speed AD/DA converters, high-speed filters, high-speed sample-and-hold, phase-locked loops, analog multipliers, security comparisons, and video circuits. Currently, the highest conversion speed can reach 6,000 V/us.

A broadband operational amplifier refers to an integrated operational amplifier with a -3dB bandwidth (BW), which is much wider than a general operational amplifier. Many high-speed operational amplifiers have wide bandwidths and can also be called high-speed broadband operational amplifiers. This classification is relative. The classification of the same operational amplifier under different working conditions may be different. Broadband operational amplifiers are mainly used in circuits that process input signals with a wider bandwidth.

High voltage operational amplifiers are designed to meet the requirements of high output voltage or high output power. It is mainly used to solve the problems of withstand voltage, dynamic range and power consumption. The power supply voltage of high voltage operational amplifiers can be higher than ±20VDC. The output voltage can be higher than ±20 VDC.

2. Main parameters of operational amplifier

There are many parameters for integrated operational amplifiers, among which the main parameters are DC parameters and AC parameters.

The main DC parameters include input offset voltage, input offset temperature drift (called input offset voltage drift), input bias current, input offset current, input offset current temperature drift (referred to as input offset current drift), differential open-loop DC voltage gain, common-mode rejection ratio, power supply voltage rejection ratio, output peak point voltage, maximum common-mode input voltage and maximum differential-mode input voltage.

The main AC parameters include open-loop bandwidth, unity-gain bandwidth, slew rate SR, full-power bandwidth, built-in time, equivalent input noise voltage, differential-mode input impedance, common-mode input impedance, and output impedance.

Let's take NE5532 as an example:

2.1 DC parameters

The following are the DC electrical characteristics of NE5532:

Input offset voltage VIO

Input offset voltage is defined as the compensation voltage between the two input terminals when the output voltage of the integrated operational amplifier is zero. The input offset voltage actually reflects the internal circuit symmetry of the operational amplifier. The better the symmetry, the smaller the input offset voltage. Input offset voltage is very important for operational amplifiers, especially for precision amplifiers or DC amplifiers. Therefore, it is an extremely important parameter of precision operational amplifiers.

Input offset voltage temperature drift αVIO (input offset voltage drift)

Input offset voltage temperature drift is defined as the ratio of input offset voltage change to temperature change within a given temperature range. This parameter is actually the complement of input offset voltage. It can easily calculate the drift of the amplifier due to temperature change within a given operating range. The input offset voltage of a general operational amplifier is in the range of ±10~20μV/°C. The input offset voltage temperature drift of a precision operational amplifier is less than ±1μV/°C.

Input bias current IIBS

The input bias current is defined as the average bias current between the two input terminals when the output DC voltage of the operational amplifier is zero. The input bias current is related to the manufacturing process. The input bias current of the bipolar process (i.e. the standard silicon process mentioned above) is between ±10nA and 1μA. The input bias current of an amplifier with FET as the input stage is generally less than 1nA.

Input offset current IIO

Input offset current is defined as the bias current difference between the two input terminals when the output DC voltage of the amplifier is zero. Input offset current also reflects the symmetry of the internal circuit of the amplifier. The better the symmetry, the smaller the input offset current. Input offset current is very important for operational amplifiers, especially for precision amplifiers or DC amplifiers.

Input offset current temperature drift αVIO (input offset current drift)

Input offset current temperature drift is defined as the ratio of the change in input offset current over a given temperature range to the change in temperature. This parameter is actually the complement of the input offset current. It can easily calculate the drift of an amplifier due to temperature changes over a given operating range.

Maximum common mode input voltage

The maximum common-mode input voltage is defined as the common-mode input voltage of the operational amplifier when the amplifier operates in the linear region and the common-mode rejection ratio (CMR) deteriorates significantly. The common-mode input voltage is generally defined as the maximum common-mode input voltage when the common-mode rejection ratio drops to 6dB. The maximum common-mode input voltage limits the maximum common-mode input voltage in the input signal. In the event of interference, this issue should be taken into account in circuit design.

Common Mode Rejection Ratio

Common-mode rejection ratio is defined as the ratio of differential-mode gain to common-mode gain when the operational amplifier is in the linear region. Common-mode rejection ratio is a very important parameter that can suppress differential-mode input. The common-mode rejection ratio in normal operation is between 80 and 120 dB.

Output peak voltage

The peak point voltage of the output is defined as the maximum voltage amplitude that the operational amplifier can output when it is operated in the linear region and under a specified load and is powered by a large power supply voltage.

2.2 AC parameters

The following are the AC electrical characteristics of NE5532:

Open loop bandwidth

The open loop bandwidth is defined as the frequency at which a small amplitude sinusoidal signal is fed to the op amp input, and the measured open loop voltage gain drops by 3 dB from the op amp’s DC gain (or equivalently 0.707 of the op amp’s DC gain). This is used for very small signal processing.

Unity gain bandwidth GB

The unity gain bandwidth is defined as the corresponding signal frequency when a small amplitude sinusoidal signal is input to the op amp input, and the closed-loop voltage gain from the output of the op amp drops by 3db (or equivalent to 0.707 of the op amp input signal). The unity gain bandwidth is a very important indicator. For sinusoidal small signal amplification, the unity gain bandwidth is equal to the product of the input signal frequency and the maximum gain at that frequency. In other words, when the signal frequency and signal to be processed are known, the unity gain bandwidth can be calculated and the appropriate op amp can be selected. This applies to the selection of op amps in small signal processing.

Slew rate SR

The slew rate of an operational amplifier is defined as the output rate of the amplifier measured at the output of the amplifier when a large signal (including a step signal) is input to the input of the operational amplifier when the operational amplifier is connected to a closed loop. It is a very important indicator for large signal processing. For general operational amplifiers, the slew rate is SR <= 10V/μs; for high-speed operational amplifiers, the slew rate is SR>

10V/μs. The maximum slew rate SR of current high-speed operational amplifiers has reached 6000V/μs. This is used in the selection of operational amplifiers in large signal processing.

Full power bandwidth FPBW

Full-power bandwidth is defined as the frequency range over which the op amp operates at full power. This frequency is limited by the slew rate of the op amp. Approximately, full-power bandwidth = SR/2πVop (Vop is the peak output amplitude of the op amp). Full-power bandwidth is a very important specification for op amp selection in large signal processing.

Differential mode input impedance (input impedance)

Differential mode input impedance (DMI) is defined as the ratio of the voltage change between the two input terminals to the corresponding input current when the operational amplifier operates in the linear region. The differential mode input impedance includes input resistance and input capacitance. At low frequencies, it only refers to input resistance. General products only provide input resistance. The input resistance of an operational amplifier using a bipolar transistor as the input stage does not exceed 10 megohms; the input resistance of a field effect transistor (FET) at the input stage is usually greater than 109 ohms.

Output Impedance

Output impedance is defined as the ratio of the change in voltage to the corresponding change in current when a signal voltage is added to the output of an op amp operating in the linear region. At low frequencies, it refers only to the output resistance of the op amp.