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Hello everyone, I am Rose. Today I will introduce operational amplifier to you. An operational amplifier ("op-amp" for short) is a circuit unit with a very high amplification factor. This article includes its symbol, characteristics and parameters.
An operational amplifier (abbreviated "op-amp") is a circuit element having a high amplification factor. It is generally paired with the feedback network to generate a functional module in the actual circuit. It's a feedback amplifier with a unique coupling circuit. The output signal can be the outcome of mathematical operations on the input signal, such as addition, subtraction, differentiation, and integration. It was given the moniker "operational amplifier" because of its early use in analog computers to perform mathematical operations.
A three-terminal device is a simple, idealized op-amp.
Figure. 1
The left-hand terminals are inputs, whereas the right-hand terminals are outputs. The inputs are labeled differently: the plus sign denotes a non-inverting input, while the minus sign denotes an inverting input.
Two inputs, one output, and two power connections are all required for a real op-amp:
Figure. 2
The positive and negative supply voltages are used in the dual-supply op-amp circuit (on the left). The negative supply terminal is grounded in a single supply arrangement (right side).
We normally remove the power supply connections when drawing op amps because we believe the device is linked to a supply voltage to function effectively in a given application. It's vital to remember, too, that an op amp's output voltage range is restricted by its supply voltage.
The ideal input-output relationship of a typical op-amp is shown in the following figure:
Figure. 3
Despite the fact that true op-amps have complex circuitry, we can successfully complete many op amp-based design chores by assuming the op-amp is a voltage-controlled voltage source (VCVS). The control voltage is [(V IN+ - V IN-)], and the gain of the op-amp, denoted by A:
Figure. 4
is the proportional factor between the control voltage and the voltage generated by VCVS. (Volt I N+-Volt I N-) (Volt I N+-Volt I N-) (Volt I N+- The gain of the op-amp, indicated by A: (Volt I N+-Volt I N-), is the scaling factor between the control voltage and the voltage produced by VCVS.
Op-amps have an extremely high gain, often exceeding [105] or even [106]. This high (preferably infinite) gain is critical, as we'll see in subsequent films, not because we frequently need to enhance the signal's amplitude by five or six orders of magnitude, but because the amplifier will be large. The use of gain and differential input stages makes it easy to take use of the positive qualities of negative feedback. 105105 or perhaps 106106 are possibilities. This high (preferably infinite) gain is significant, as we'll see in subsequent videos.
Let's have a look at some of the additional characteristics mentioned by the VCVS model.
The difference between two input voltages is amplified by an op-amp, which is a differential amplifier.
Op-amps have common-mode rejection, according to the preceding assertion. Any voltage components present in the two input signals, such as noise or DC offset, will be rejected (i.e. ignored) by the op-amp.
An op-amp can be thought of as a differential-to-single-ended converter because it has a differential input stage and a single-ended output. However, it turns out that single-ended input signals are more closely connected to real-world op-amp applications. In reality, a device built for differential input signals is known as an instrumentation amplifier.
(1) The bipolar type, JFET type, and CMOS type op-amps are classified according to the process classification, which is the type of components picked by the op-amp.
Low input impedance, high speed, minimal noise, low offset, and high withstand voltage are all properties of the bipolar type. For example, the LM358 series, is widely utilized.
Medium input impedance, medium noise, and a big offset characterize the JFET type.
High input impedance, high noise, huge offset, and low power consumption are all characteristics of the CMOS type. The LMV358 series is widely used (2) It is classified into three categories based on accuracy: general-purpose (mV-level offset), high-precision (uV-level offset), and low noise (improving the bandwidth).
(3) It is split into three categories based on speed: high speed (100Hz or even above GHz), medium speed (tens of MHz), and low speed (less than tens of MHz) (KMz or MHz).
(4) It is classified into ultra-low power consumption (nA level IQ), micropower consumption (uA level IQ), and general type power consumption, according to the power consumption classification (more than 10uA).
Brief summary:
Low power consumption - small IQ - slow speed - high noise
High Accuracy - Small VOS - Large IQ - Low Noise (Structure or IQ Contribution)
High speed - high IQ - low noise (thermal noise)
1. Feedback
Because any op-amp can provide feedback, let's start with the standard negative feedback circuit shown in Figure 5.
Figure. 5 Typical Feedback Schematic
Negative feedback is the process of "feeding back" a portion of the output signal to the input, but to make the feedback negative, the output must be fed back to the negative (or "inverting input") side of the op-amp input using external circuitry and additional devices. The goal is to reduce the differential input voltage between the inputs to near zero, as expressed by the equation below (1)
Figure. 6
2. Open-loop gain
The gain of an amplifier without a closed feedback loop is referred to as open-loop gain (AVOL). This gain can be very high for precision op amps, on the order of 160dB or more. From dc to the dominating pole corner frequency, the gain is constant. After then, the gain drops by 6 decibels every octave (20 decibels per octave) (Note: 8-octave means double the frequency, 10-octave means ten times the frequency). The open-loop gain continues to fall at the same pace if the op-amp has only one pole, as shown in Figure 7. (single-pole response).
Figure. 7 Open Loop Gain (Bode Plot) - Single Pole Response
Typical op-amps, as shown in Figure 8, have more than one pole in practice.
Figure. 8 Open Loop Gain (Bode Plot) - Two Pole Response
The second pole increases the pace at which the open-loop gain drops to 12dB/octave (40dB/10octave), as seen in Figure 8. The op-amp is unconditionally stable at any gain if the open-loop gain falls below 0dB before reaching the frequency of the second pole. This circumstance is referred to as "Unity Gain Stable" in the standard. Refer to the graph of the open-loop gain frequency response in the ADI ADA4857 datasheet for more information (Figure 9).
Figure. 9 ADA4857 Open Loop Gain Frequency Response
Note: The open-loop gain is in an unstable state.
The amplifier may become unstable if the frequency of the second pole is reached and the closed-loop gain is larger than 1 (0db). Some op-amp designs, known as incompletely compensated op-amps, are only stable at greater closed-loop gains. At higher frequencies, however, op-amps may contain many additional parasitic poles, with the first two being the most essential.
The open-loop gain is not a parameter that can be carefully adjusted. Because the range is so wide, the parameters are usually presented as typical values rather than min/max values. This parameter may have a minimum value in some circumstances, usually related to high-precision operational amplifiers. In addition, the open-loop gain might change depending on the output voltage and load. Nonlinearity in the open-loop is referred to as open-loop gain nonlinearity. This parameter is similarly related to temperature in some way. In most circumstances, these impacts are modest and insignificant. In reality, open-loop gain nonlinearity is not often included in op-amp datasheets.
3. Closed-loop gain
Closed-loop gain refers to the gain of the amplifier when the feedback loop is closed. There are two forms of closed-loop gain: signal gain and noise gain.
Signal gain and noise gain
Open-loop gain is used in the traditional formulation for closed-loop amplifier gain. If G is the real closed-loop gain, NG is the noise gain, and AVOL is the amplifier's open-loop gain, then:、
Figure. 10
As a result, if the circuit's open-loop gain is strong, the closed-loop gain is mostly noise gain.
Figure. 11 Signal Gain and Noise Gain
It's worth noting that op-amp stability is determined by noise gain rather than signal gain. Although most current op-amps are stable at unity gain, some specialty amplifiers are not. Non-fully compensated op-amps provide unique advantages over ordinary unity-gain stable op-amps, such as reduced noise voltage and wider bandwidth.
4. Gain Bandwidth Product
The open-loop gain is lowered by 6dB/8 octaves for a single-pole response. That is to say, doubling the frequency reduces the gain by half. In contrast, halving the frequency doubles the open-loop gain. The result is a gain-bandwidth product, as seen in Figure 6. The product of multiplying the frequency by the open-loop gain is always a constant, but it must be within the entire curve's 6dB/8-octave drop. As a result, we have a figure of merit that can be utilized to see if an op-amp is suited for a given application. It's worth noting that the gain-bandwidth product applies exclusively to voltage feedback (VFB) op-amps.
Figure. 12 Signal Gain and Noise Gain
Do we need an op-amp with at least a 1MHz gain-bandwidth product if we have an application that demands a closed-loop gain of 10 and a bandwidth of 100kHz?
This explanation is a little too straightforward. In actuality, the response lowers by 3dB where the closed-loop gain intersects the open-loop gain due to the unpredictability of the gain-bandwidth product, so it's preferable to leave a little additional margin. An op-amp with a gain-bandwidth product of 1MHz is merely a minimum requirement in the above application.
5. Slew rate
Slew rate (SR) is the rate at which the amplifier output changes due to a sudden change in the amplifier input, and is usually measured in V/µs. The maximum operating frequency of large signals can be determined by the following formula:
f = SR/2πVp (where Vp is the peak voltage)
Some amplifiers feature extremely high slew rates in an attempt to win the engineer's favor with impressive figures, but they don't always succeed because distortion limits the maximum operating frequency. Looking at the distortion curve to discover which frequencies have unacceptable distortion for a given application is the simplest approach to tell. It's also critical to understand the system requirements in detail. After that, plug this frequency into the slew rate calculation to figure out how much slew rate you'll need.
Figure. 13 OP177 Maximum Output Amplitude and Frequency
6. Bandwidth
Some people believe that more bandwidth is better, but experienced analog engineers know that adequate bandwidth is better than too much bandwidth for a given application. The only way to truly comprehend the amplifier's features is to flip through the datasheet and look at the characteristic curves, which is the finest approach to analyze any parameter. Is the bandwidth curve prone to excessive peaking? This condition is described by some manufacturers as a large –3dB bandwidth, although it could also signal a device stability issue. Peaking may reduce the gain flatness of the amplifier, despite the –3dB bandwidth appearing to be large. As a result, the bandwidth is adequate for your requirements, while amplifiers with broader bandwidths necessitate greater stability and PCB layout.
7. Output type
The "output type" of an op-amp is generally categorized according to the output structure and application category of the amplifier, and the various outputs are mentioned in the "Linear Devices - Amplifiers - Instruments, Op Amps, Buffer Amplifiers" product category on the Digi-Key website.
"Output type" can be roughly divided into:
Differential op-amps have a positive and negative output, and they use a set gain to amplify the difference between the voltages at the two inputs.
Rail-to-Rail: The most common sort of op-amp (or rail-to-rail) has input and output voltage swings that are extremely close to or nearly equal to the supply voltage value (typically in the millivolt range).
Open Drain: The output of an open drain op-amp is connected to the transistor's base inside the IC. As a result, when the op-amp is turned on, the transistor's drain is turned on, and it can only sink current, as illustrated in Figure 8. Because this sort of op-amp is primarily utilized for current detection and has a limited number of applications, there are few options on the market.
Figure. 14 Open Drain Operational Amplifier Output Circuit Diagram
Push-Pull: A push-pull amplifier is one that uses both NPN and PNP transistors. The transistors are matched such that their gain, speed, and current parameters are about the same. In a push-pull power amplifier, the transistors alternate working in the positive and negative half cycles of the signal. However, unlike open-drain, the application range is limited, and few foreign manufacturers will create this output stage op-amp, leaving the market with few options.
Figure. 15 Class A, Class B, Class AB Output Stage Push-Pull Amplifier
Catalogue
FPGA / CPLD
Memory
MOS 
MCU 
DSP
OCEP
Secondary 
Other 