ideal opamp
The ideal operational amplifier is shown in figure 1. Negative feedback applied through a resistive element (or more generally, through a resistive element) can produce either of two classical closed-loop operational amplifier configurations: an inverting amplifier (Figure 2) and a noninverting amplifier (Figure 3). The classical formula of closed-loop gain in these configurations shows that the gain of the amplifier basically depends only on the feedback element. In addition, negative feedback can provide stable and distortion-free output voltage.
Voltage feedback (VFB) operational amplifier
Voltage feedback operational amplifiers are the same as the ideal operational amplifiers introduced above, and their output voltage is a function of the voltage difference between the two input terminals. For design purposes, the data table of voltage feedback operational amplifier defines five different gains: open-loop gain (AVOL), closed-loop gain, signal gain, noise gain and loop gain.
Negative feedback can change the size of AVOL. For a high-precision amplifier, the AVOL value of a feedback-free operational amplifier is very large, about 160dB or higher (the voltage gain is 10000 or higher).
Figure 1: ideal operational amplifier.
AVOL has a wide range and is usually given as a minimum/maximum value in the data table. AVOL also varies with voltage level, load and temperature, but these effects are very small and can usually be ignored.
When the feedback loop of the operational amplifier is closed, it can provide a closed-loop gain less than AVOL. There are two forms of closed-loop gain: signal gain and noise gain.
Operational amplifier (referred to as "operational amplifier") is used to adjust and amplify analog signals. Common applications include digital oscilloscopes and automatic test equipment, video and image computer boards, medical instruments, television broadcasting equipment, aircraft displays and air transport control systems, automobile sensors, computer workstations and wireless base stations.
ideal opamp
The ideal operational amplifier is shown in figure 1. Negative feedback applied through a resistive element (or more generally, through a resistive element) can produce either of two classical closed-loop operational amplifier configurations: an inverting amplifier (Figure 2) and a noninverting amplifier (Figure 3). The classical formula of closed-loop gain in these configurations shows that the gain of the amplifier basically depends only on the feedback element. In addition, negative feedback can provide stable and distortion-free output voltage.
Voltage feedback (VFB) operational amplifier
Voltage feedback operational amplifiers are the same as the ideal operational amplifiers introduced above, and their output voltage is a function of the voltage difference between the two input terminals. For design purposes, the data table of voltage feedback operational amplifier defines five different gains: open-loop gain (AVOL), closed-loop gain, signal gain, noise gain and loop gain.
Negative feedback can change the size of AVOL. For a high-precision amplifier, the AVOL value of a feedback-free operational amplifier is very large, about 160dB or higher (the voltage gain is 10000 or higher).
Figure 1: ideal operational amplifier.
AVOL has a wide range and is usually given as a minimum/maximum value in the data table. AVOL also varies with voltage level, load and temperature, but these effects are very small and can usually be ignored.
When the feedback loop of the operational amplifier is closed, it can provide a closed-loop gain less than AVOL. There are two forms of closed-loop gain: signal gain and noise gain.
Signal gain (A) refers to the gain generated by the input signal through the amplifier, which is the most important gain in circuit design. The following are the two most common expressions of signal gain in voltage feedback circuits, which are widely used in inverting and noninverting operational amplifier configurations.
Figure 2: Inverting amplifier (A) and noninverting amplifier (B) are two classic closed-loop operational amplifier configurations.
For inverting amplifier, a? =? -Rfb/Rin
For noninverting amplifier, a? =? 1? +? Rfb/Rin
Where Rfb is the feedback resistance and Rin is the input resistance.
Noise gain refers to the gain of noise source in operational amplifier, which reflects the influence of input offset voltage and voltage noise of amplifier on output. The noise gain formula is the same as the signal gain formula of the above noninverting amplifier. Noise gain is very important because it is used to determine the stability of the circuit. In addition, the noise gain is also the closed-loop gain used in the Porter diagram, which can provide the circuit design engineer with the maximum bandwidth and stability information of the amplifier. The loop gain is equal to the difference between the open-loop gain and the closed-loop gain, or equal to the total gain of the input signal that passes through the amplifier and is returned to the input terminal by the feedback network.
Fig. 3: (a) Open-loop gain and noise gain curves on the Bode diagram; (b) Frequency response of current feedback operational amplifier.
Gain bandwidth product of voltage feedback operational amplifier
The gain and bandwidth of an ideal operational amplifier are infinite. The most common real operational amplifier uses voltage feedback. The gain and frequency of this operational amplifier are related in a characteristic called "Gain Bandwidth Product (GBW)". This relationship in voltage feedback operational amplifier allows circuit design engineers to compromise between bandwidth and gain by controlling feedback resistance (or impedance).
What is the logarithmic response curve (Porter diagram)? The relationship between gain and frequency of voltage feedback operational amplifier is analyzed, which is helpful to explain GBW. The gain is constant from DC to the frequency determined by the main pole of the feedback loop. Above this frequency, the gain decays at the rate of 6dB/8 octave or 20dB/ 10 octave. This is called unipolar or first-order response. The attenuation rate of 6dB/ octave means that if the frequency is doubled, the gain is halved. This characteristic of voltage feedback operational amplifier enables circuit design engineers to compromise between bandwidth and gain.
Draw the open-loop gain and noise gain curves of the operational amplifier in the Porter diagram, and their intersections determine the maximum bandwidth or closed-loop frequency (fCL) of the amplifier (Figure 4). The intersection of these two curves is located at a position 3dB less than the maximum gain on the gain axis (vertical axis) of the Bode diagram. In fact, the noise gain asymptotically approaches the open-loop gain. In the upper and lower multiples of fCL, the difference between the asymptotic response and the true response will be 1dB.
Figure 4: (a) Input offset voltage of operational amplifier; (b) The input bias current of the operational amplifier.
Current feedback (CFB) operational amplifier
In current feedback operational amplifier, the open-loop response is the response of output voltage to input current. Therefore, unlike voltage feedback operational amplifier, the input-output relationship of current feedback operational amplifier is not expressed by gain, but by transimpedance, and the unit is ohm. But transimpedance is more common, so current feedback operational amplifier is also called transimpedance amplifier. The transimpedance of current feedback operational amplifier is 500k? ~ 1M？ amongst
Unlike voltage feedback operational amplifiers, current feedback operational amplifiers do not have a constant gain bandwidth product. That is, when the gain rolls off with the increase of frequency, the roll-off speed is not equal to 6dB/8 times. Current feedback operational amplifier can maintain high bandwidth in a wide gain range, but this is at the expense of limited selection of feedback impedance. For example, one of the limitations is that the capacitor is not allowed to be used in the feedback loop of the current feedback operational amplifier, because the capacitor will reduce the feedback impedance at high frequency, which will lead to oscillation. For the same reason, the stray capacitance around the inverting input of the operational amplifier must also be controlled. In addition, the slope characteristic of frequency response curve of current feedback operational amplifier is better than that of voltage feedback operational amplifier, although stray capacitance will weaken this advantage of current feedback operational amplifier.
The difference between current feedback operational amplifier and voltage feedback operational amplifier is also reflected in other aspects. For example, the current feedback operational amplifier has the best feedback resistance value to obtain the maximum bandwidth. Increasing the feedback resistance will reduce the bandwidth, while decreasing the resistance will reduce the phase margin, resulting in instability of the amplifier. The data table of current feedback operational amplifier provides the best feedback resistance value and power supply voltage value in a gain range, which makes the amplifier have the maximum bandwidth, which is very helpful to the design process. The optimal feedback resistance value is sensitive to many factors, even to the package type of operational amplifier. The data sheet may depend on whether the package is a small IC. (SOIC) package or double-row package (DIP), providing different resistance values.
Important characteristics of operational amplifier
If the voltage at both inputs of an operational amplifier is 0 V, then the voltage at the output should be 0 V. But in fact, there is always some voltage at the output, which is called offset voltage VOS. If the offset voltage at the output is divided by the noise gain of the circuit, the result is called input offset voltage or input reference offset voltage. This feature is usually given in the form of VOS in the data table. VOS is equivalent to a voltage source connected in series with the inverting input of the operational amplifier. A differential voltage must be applied to the two inputs of the amplifier to produce 0V output.
VOS varies with temperature. This phenomenon is called drift, and the magnitude of drift changes with time. The drift temperature coefficient TCVOS is usually given in the data table, but some operational amplifier data tables only provide the second or maximum VOS that can ensure the device to work safely in the working temperature range. The reliability of this specification is slightly poor, because TCVOS may be unstable or non-monotonic.
VOS drift or aging is usually defined as mV/ month or MV/ 1 1,000 hours. But this nonlinear function is proportional to the square root of the equipment service time. For example, the aging rate of 1mv/ 1000 hours can be converted to about 3mV/ year instead of 9mV/ year. The aging speed is not always given in the data table, even for high-precision operational amplifiers.
The input impedance of an ideal operational amplifier is infinite, so no current flows into the input terminal. However, the practical operational amplifier using bipolar junction transistor (BJT) at the input stage needs some working current, which is called bias current (IB). There are usually two bias currents: IB+ and IB-, which flow into two input terminals respectively. The range of IB value is very wide, and the bias current of special operational amplifier is as low as 60fA (about every 3? S passes through an electron), the bias current of some high-speed operational amplifiers can be as high as tens of milliamps.
The manufacturing process of monolithic operational amplifier tends to make the two bias currents of voltage feedback operational amplifier equal, but it cannot guarantee the two bias currents to be equal. In a current feedback operational amplifier, the asymmetry of the input means that the two bias currents are almost always unequal. The difference between these two bias currents is the input offset current IOS, which is usually very small.
Total Harmonic Distortion (THD) refers to the fundamental harmonic component generated by the nonlinearity of the amplifier. Usually, only the second and third harmonics need to be considered, because the amplitude of higher harmonics will be greatly reduced.
THD+N(THD+ noise) is the cause of noise generated by devices, which refers to the total signal power except the fundamental frequency. Most data tables give the value of THD+N, because most measurement systems can't distinguish signals related to harmonics and noise. THD and THD? +? N is used to measure the distortion caused by single-tone sine wave input signal.
A more useful and strict distortion measurement method is intermodulation distortion (IMD), which can measure the dynamic range caused by dual-tone interference instead of single-carrier interference. According to different applications, some second-order IMD components can be filtered, but the third-order components are more difficult to filter. Therefore, the data table usually gives the third-order cutoff point (IP3) of the device, which is the most basic method to measure the third-order IMD effect. This parameter is very important because in many applications (especially in radio receivers), the signal damage caused by third-order crosstalk products is very common and serious.
The compression point of 1dB represents the input signal level when the gain of the output signal is 1dB lower than the ideal input/output transfer function. This is the end of the dynamic range of the operational amplifier.
The signal-to-noise ratio (SNR) defines the dynamic range (dB) from the maximum signal level to the root mean square level of background noise.
Other characteristics become very important in radio frequency (RF) applications. For example, the dynamic range is the ratio of the maximum input level that the device can withstand to the minimum input level that the device can provide acceptable signal quality. If the input level of the device is between these two points, the device can provide relatively linear characteristics (under the limitation of the amplifier), and if the input level is not between these two points, the device will produce distortion.
Types of operational amplifiers
Power supply of operational amplifier
The power supply voltage range required for the normal operation of the first monolithic operational amplifier is 15V. Nowadays, due to the improvement of circuit speed and the use of low power supply (such as battery), the power supply of operational amplifier is developing towards low voltage.
Although the voltage specification of operational amplifier is usually specified as symmetrical bipolar voltage (such as 15? V), but these voltages are not necessarily required to be symmetrical or bipolar. For an operational amplifier, as long as the input terminal is biased in the active region (that is, in the * * * mode voltage range), the power supply of 15V is equivalent to the power supply of +30V/0V, or+20v/–10v. Operational amplifiers have no ground pins unless the negative voltage rail is grounded in single-supply applications. No device in the operational amplifier circuit needs to be grounded.
The input voltage swing of high-speed circuit is smaller than that of low-speed device. The higher the device speed, the smaller its geometric size, which means the lower the breakdown voltage. Because of the low breakdown voltage, the device must work at a low power supply voltage.
At present, the breakdown voltage of operational amplifier is generally around 7V, so the power supply voltage of high-speed operational amplifier is generally 5V, and it can also work at a single power supply voltage of +5V.
For a general-purpose operational amplifier, the power supply voltage can be as low as+1.8V. This operational amplifier is powered by a single power supply, but this does not necessarily mean that a low power supply voltage must be adopted. The terms single supply voltage and low voltage are two related but independent concepts.
Process technology of operational amplifier
The operational amplifier mainly adopts bipolar technology, but in applications that require analog and digital circuits to be integrated on the same chip, the operational amplifier using CMOS technology works well. JFET is sometimes used in the input stage to increase the input impedance and thus reduce the input bias current. FET input operational amplifiers (whether N-channel or P-channel communication) allow chip design engineers to design operational amplifiers whose input signal levels can be extended to negative voltage rails and positive voltage rails.
Because BJT is a current control device, the bipolar transistor of the input stage always generates some bias current (IB) (Figure 7). However, IB will flow through the impedance outside the operational amplifier, resulting in offset voltage, which will lead to system error. Manufacturers solve this problem by using super-beta transistors at the input stage or building an input architecture to compensate for bias. Super β transistor has an extremely narrow base region, and the current gain generated by this base region is much larger than that of standard BJT. This makes IB very low, but this is at the expense of frequency response performance degradation. In the bias compensation input, a small current source is added to the base of the input transistor, which can provide the bias current required by the input device, thus greatly reducing the net current of the external circuit.
Compared with BJT, the input impedance of CMOS operational amplifier is much higher, which makes the bias current and offset of current source output much smaller. On the other hand, compared with BJT, CMOS operational amplifier has higher inherent offset voltage and higher noise voltage, especially at lower frequency.
Operational amplifiers are classified according to their applications.
Chip manufacturers use different circuit designs and process technologies to emphasize certain operational amplifier characteristics of specific applications. The table above lists the commonly used terms of these operational amplifier types, as well as their characteristics and application scope. ?