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Dec 28 2019

Operational Amplifier Basics in Electronics Overview

Ⅰ. Introduction

An operational amplifier, or op-amp for short, is fundamentally a voltage amplifying device designed to be used with external feedback components such as resistors and capacitors between its output and input terminals. 

Learn more about the most common characteristics, essential knowledge when selecting and using an op amp in electronics. We can conclude our section and look at Op Amp with the following properties and questions.

 

This video discusses some general rules easy to understand and analyze the operation of most opamp circuits, and it presents some ideal properties of op amps, and discusses how negative feedback, etc.

Ⅱ. Amplifier Figures of Merit


  • Negative Feedback

It is a important technique to improve bandwidth and distortion and control gain.

  • Open-loop Gain

It refers to the ratio of the voltage change at the output of the amplifier to the voltage change at the input when the amplifier input and output are open. 

  • Common-mode Rejection Ratio (dB)

It is the ratio of the amplifier's amplification factor of the differential voltage signal to the amplification factor of the common mode voltage signal.

  • Input Current Noise

It is the equivalent current noise applied in parallel with the input of the noiseless amplifier.

  • Output Current

It refers to the current driven by the load at the output of the op amp. It is usually a function: input overdrive, correlation between output voltage and power supply, temperature, source, and drain characteristics will differ.

  • Phase Margin

It is the phase shift between an output of the same frequency and an inverting input in an open-loop circuit.

  • Voltage Gain

It is the ratio of the change in output voltage to the change in input voltage.

  • Programmable Gain Buffer

It can set the gain resistance of the op amp (integrated on the template), and the gain can be set to +1, +2, or -1 through simple external connections.

  • Saturation Voltage

It is the voltage between the collector and emitter of the transistor under saturation conditions. In the saturated state, the emitter-base and collector-base are forward biased, so that the voltage between the collector-emitter is very low.

  • Rise Time

This refers to the time required for the output voltage to change from 10% of its final value to 90% of its final value.

  • Unity-gain Bandwidth

It refers to the frequency at which the amplifier's open-loop gain is equal to one. If the op amp frequency response has a single-pole roll-off, the unity-gain bandwidth is equal to 1UGBW.

  • Strobe “OFF” Voltage

The strobe OFF voltage is the minimum voltage at the strobe pulse and is guaranteed not to interfere with the comparator operation.

  • Input Current Index

It refers to the average of the current drawn from the two input pins. In addition, the input current is also commonly called bias current.

  • Gain Bandwidth Product

It refers to the product of the amplifier's bandwidth and the gain at which the bandwidth is measured.

  • Large Signal Voltage Gain

It refers to the ratio of the change in output voltage to the change in input voltage. This parameter is usually specified at a large output voltage, smaller than the maximum output voltage, which is the typical value under direct current conditions.

  • Offset Voltage Temperature Coefficient

It refers to the average rate of change in offset voltage due to changes in junction temperature within a specified temperature range.

  • Output High Voltage

It refers to the high DC output voltage of the comparator, which produces the high output current. And it is usually related to the totem pole or push-pull output of the comparator.

  • Input Source Current

It refers to the maximum output positive current produced under the comparator's push-pull output state.

Real Op Amp

  • Total Harmonic Distortion (THD)

When a pure sinusoidal signal is input to the op amp as Vin (w) = Vpsin (wt):

Input harmonic distortion: Vout(w)a1Vpsin(wt)+a2Vpsin(wt)+...+anVpsin(nwt)

The expression of THD is: THD(%)=[sqrt(a2xa2+a3xa3+...+anxan)/a1]x100

  • Common-Mode Input Impedance (RINCM)

It refers to the ratio of the change in the common-mode input voltage to the change in the input current at the inverting or non-inverting terminal.

  • Output Low Voltage

It refers to a low DC output voltage. The output drive is a low voltage sink current. This specification is usually related to the totem pole or push-pull output of the comparator.

Using a CMOS op amp as the output driver, although the circuit works well, but requiring a 1m shielded cable, and the oscillation of the operational amplifier is about 1MHz when there is no input signal. If shorten the cable to 10cm, the oscillation is stable.

Some op amps are not suitable for driving capacitive loads directly, such as long shielded cables, which is a capacitive load. In addition, coaxial cables have about 60-100pF capacitance per meter.

  • Harmonic Distortion

It refers to the unwanted spurious signals generated at the amplifier output due to the non-linearity of the signal line. When the input is a sinusoidal signal, these spurious signals will appear as integer times of the input frequency (for example, second harmonic, third harmonic).

  • Output Leakage Current (ILEAKAGE)

It means that the current enters the comparator output (the output is driven high). It often appears at the output of open collector and open drain.

  • Power Supply Rejection Ratio (PSRR)

It refers to the ratio of the change in the input offset voltage to the change in the power supply voltage, PSRR (dB) = 20log10 (DVOS / DVS)

  • Linear Phase Deviation

It refers to how a closed-loop phase response of an operational amplifier approaches and follows the linear relationship between phase change and frequency in a specific frequency band.

  • -3db

It refers to the frequency when the value of the small signal output amplitude of the closed-loop amplifier decreases to 3dB.

  • Common-mode Voltage Range

It refers to the typical value of the voltage range at the input, which determines the performance of the amplifier.

  • Specified Power Supply Range

It describes the power supply voltage required for the operational amplifier to operate.

  • Output Absorption Current

It refers to the highest output negative current of the comparator.

  • Output Voltage Swing

It refers to the maximum peak-to-peak swing of the output voltage under a specific load and power supply voltage.

  • Current Feedback

It refers to a technology used in current feedback amplifiers whose output signal reflects the value of the current input to the inverting input (transimpedance gain function). In some aspects, this topology has operational advantages over traditional voltage feedback.

  • Closed Loop Buffer

It refers to an amplifier with high input impedance and low output impedance and a fixed gain of +1. Its typical applications are used for isolation, increased output drive, capacitive load, etc., in addition, there is no need to set the gain resistance.

  • Closed-loop Gain 

It is the ratio of the change in the output voltage to the change in the input voltage after the feedback and input network added. Generally, this value is set using an external resistance.

  • Common-mode Range

The common-mode range, also known as the input voltage range, is a measure of the range of input voltages that the input pins of an op amp can accept. This specification is usually relative to the power supply amplitude.

  • Output Impedance

It refers to the ideal series output impedance of the ideal operational amplifier when there is no impedance, which is the approximate output impedance of the op amp measured under AC  conditions.

  • Transient Response

It refers to the step function response of the closed-loop system of the amplifier under the condition of small signal (usually less than 100mV).

  • Slew Rate

When given a transition or square wave input, the amount of change in the amplifier's output from one level to another. Typical values are averages of values measured based on a change in total output voltage from 10% to 90%.

  • Response Time

It is the time interval when the input step function makes the output from the initial value to the logic threshold voltage.

  • Unity Gain Frequency

It refers to the frequency at which the gain of the voltage feedback op amp is 1 (0 dB). For an ideal operational amplifier, its gain-bandwidth product is equal.

  • Intercept Point

It refers to the output power of the fundamental frequency, which is equal to the power value of the fundamental frequency in the specified distortion term (2nd, 3rd, or 3rd intermodulation).

  • Input Offset Current

It refers to the current difference between the two inputs.

  • Voltage Overdrive

It means that a certain amount of input step voltage exceeds the minimum drive input voltage required by the comparator to change from one logic level to the opposite logic level.

  • Differential Gain & Differential Phase

Differential gain refers to the change in the input and output of the gain, and differential phase refers to the phase change in the input stage. They are video measurements, and are a standard measurement in the broadcasting field to measure relative changes in the interpretation of video signal consistency.

  • Voltage Feedback

A technique used in traditional operational amplifiers, where part of the output voltage is fed back to the input, and the voltage difference between the two inputs is amplified by the operational amplifier.

  •  Avol Open-loop Voltage Gain

A is a sign of gain. The letter V written below indicates the gain of voltage, and the letter ol also written below is an abbreviation for open loop. Open-loop voltage gain refers to the gain (Vout / Vin) of the amplifier without feedback. Due to the existence of the bias voltage, these errors must be compensated.

  • Logic Threshold Voltage

It refers to the voltage that causes the comparator output state to change when the input offset voltage is exceeded.

  • Output Resistance

It refers to the value of the series resistance at the output of an ideal op amp with zero output resistance, which measured under DC conditions.

  • Gain Flatness

It refers to the volume of gains violently increasing and rapidly decreasing in a given bandwidth range measured in decibels (dB), which affects the most important parameter specifications such as phase margin, gain margin, and closed-loop gain.

  • Offset Current Temperature Coefficient

It refers to the average rate of change in deviation current due to changes in junction temperature within a specified temperature range.

  • Input Impedance

It is the ratio of input AC voltage to input AC current.

  • Input Voltage Noise

It refers to the equivalent voltage noise in series with a noiseless amplifier.

  • Input Offset Voltage

It is the product of the DC error voltage between the inputs and the closed-loop gain, because of the non-ideal balance between the input stage and the output is caused by the DC error voltage of the input terminals.

  • Gain Margin

Open loop gain when the phase between the inverting input and output crosses zero at a certain frequency.

  • Supply Current

It refers to the current required from the power supply to the unloaded amplifier and to the power supply at the output midpoint.

  • Settling Time

It refers to the time between the input step function initial value and the output voltage reaching the specified error band. The error band refers to the percentage of the total voltage change.

  • Differential Input Resistance

It is the ratio of the change in the input voltage to the change in the input current.

Simple Op Amp Circuit

Ⅲ. Q & A

Q1: What is the difference between a voltage feedback amplifier and a current feedback amplifier?

A: The internal circuits of these two op amps are different. The voltage feedback op amp is restricted by the internal design, and it only has a very low input bias current, but there is no internal limit on the differential input voltage, because it is limited only when external feedback is required. In contrast, for a current feedback amplifier, its differential input voltage is subject to internal design, but it does not limit its input bias current, so it is limited only when external feedback is required.

Q2: What is the difference between open and closed loops?

A: The open loop gain is actually the internal gain of the op amp without feedback, and usually takes any value between 1,000 and 10,000. Closed loop gain is the gain of the entire circuit, which is equal to the open loop gain divided by 1 plus the loop gain (the improvement coefficient). In fact, the gain of the op amp when there is no feedback is the open loop gain, and the gain when feedback is considered is closed-loop gain.

Q3: If the op amp has ideal AC characteristics, the Bode plot (gain-frequency response) is a unipolar system. What is the gain slip rate in dB / decade?

A: In a unipolar system, the gain drops (or decreases) at 20dB / decade, which is 6dB / octave. This is responsive to any single pole, and it is also suitable for a simple RC filter or an ideal operational amplifier. However, because op amps have more high-frequency poles, the phase shift will begin to increase as the frequency approaches the unity gain frequency of the op amp.

Q4: What is the difference between unity gain bandwidth, gain bandwidth product (GBP), and -3dB frequency?

A: Many op amps have an open-loop gain reduction rate of -20db / decade when the frequency is stable. At any point during this descent phase, the GBW is a constant. If the unity-gain operation of the op amp is stable, then the unity-gain bandwidth, or the frequency at which the open-loop gain is 1, is usually equal to GBP. In addition, GBP is not equal to (usually higher than) the unity gain bandwidth. The -3dB frequency is a measure of the bandwidth of an operational amplifier when it is operating in a closed loop. The -3dB point is the frequency at which the gain of the overall closed-loop system drops by 3dB. The unity gain frequency for closed loop applications can be calculated using BW=GBP/Av. The -3dB frequency and unity-gain bandwidth applied depend on the feedback gain setting, output swing, load, and circuit layout.

Q5: Why do some amplifiers oscillate with a capacitive load?

A: The output impedance of the op amp and the capacitance of the capacitive load may form a resistance-capacitance oscillation. Also they form an R-C oscillation at the output stage, which causes additional phase lag in the feedback signal. CMOS amplifiers have a high output impedance which will cause the electrodes to be approached or lower the unity gain frequency of the op amp. The additional phase lag of the electrodes will weaken the phase margin of the op amp The total phase lag of the amplifier causes the phase angle of the unity gain frequency to increase by more than 180 degrees to cause the total feedback phase shift in unity gain to exceed 180. degree. In addition, the output impedance of a CMOS amplifier is between 100 and 500, causing a relatively low pole frequency. And meanwhile, the output impedance of the high-speed bipolar operational amplifier is in the range of 1 to 100, which causes the pole frequency to be much higher than that of the CMOS operational amplifier, so that the pole is far from the unity gain frequency of the device. The drive of a CMOS amplifier to a capacitive load can be improved by placing an output resistor at the output and an external positive feedback capacitor.

Q6: If the output of the op amp stays close to the voltage rail, that is, the output rail, what is the reason?

A: There are many ways for operational amplifiers to rail. The difficulty is keeping it away from the "rail". If the input exceeds the input voltage range, the output is usually near to a supply voltage rail. In theory, if the output exceeds the actual supply voltage, and a higher supply voltage is given, the op amp will go to rail output again. If there is no feedback or the polarity of the feedback is wrong, the op amp goes to rail output again. At the same time, if the non-inverting input is higher than the negative inverting input, the op amp also goes to rail output. The application of the operational amplifier should be analyzed to ensure that the power supply voltage used has a proper input and gain, so that in normal operation, its input voltage is within the rated value and the output voltage is within the normal range.

Q7: What is the difference between the common-mode voltage and input voltage range of an op amp?

A: Common mode voltage means that one voltage is applied to both inputs at the same time. Input voltage range is the range of voltages that can be accepted by the input pins. It is necessary to remember that the op amp should suppress the common-mode voltage, and amplify the difference between the two input pins only.

Q8: The SPICE model of the bipolar operational amplifier works well, but the SPICE model of the CMOS operational amplifier does not work. Is there a need to set SPICE?

A: To input the appropriate bias current to the model, the SPICE model applied on CMOS operational amplifier needs to set the default GMIN option to the largest SPICE package value.

Q9: What is the difference between the amplifier's output current and short-circuit current?

A: Short circuit current refers to the current generated by the device if the output is connected directly to the power line. This indicates that the output current is limited depending on the design of the device. However, the short-circuit current does not represent the true output of the drive capability of the output. Due to the impedance characteristics of the output stage, the maximum output current is determined by the swing of the output voltage under load. In facet, the smaller the load, the larger the output swing; the larger the load, the smaller the output swing.

Q10: How to check the stability of an op amp circuit?

A: Check the stability of the control loop, such as the pulse load and related changes in output voltage. The pulse load may be a load current with a pulse or step change, so that the output of the op amp circuit should be connected to a series R-C circuit. The greater the circuit swing or vibration, the worse the stability of the circuit.

Q11: Are there any good ways to minimize noise when amplifying a low-level DC signal?

A: To obtain a high signal-to-noise ratio, the circuit must be well designed. This includes choosing the best amplifier bandwidth and knowing the impedance of the input signal. If the input signal source has a fairly high impedance, it makes no sense to choose a low voltage noise amplifier, which has high current noise.

Q12: How should design a low frequency (<1Hz) differentiator to minimize the output noise?

A: The only reason that the output of the differentiator contains noise is because there is a lot of gain and the input is noisy. The traditional differentiator uses Rs-Cs in series at the input and the Rf-Cf in parallel near the operational amplifier. It is not necessary to try more Rs or Cf to minimize noise. The noise of the output come from the differentiator does not mean that it is harmful, because it also amplifies useful signals. In addition, if disconnect a loop, the differential output noise may be beneficial and will stabilize the loop. If the output of the differentiator is quite noisy or has too much input noise, analyze which are the real sources of them.

Q13: How to protect the amplifier input from being higher or lower than the supply voltage?

A: What must be done is either to clamp the input of the device, or to limit the input current of the device, or ideally, do both. The easiest way is to choose a current limiting resistor to limit this current. The selection is based on the fact that the current generated by the circuit input at the maximum input voltage is less than the maximum current rating of the input pin. Usually, a 1K to 100K resistor in series with this input pin is effective. However, since the signal is usually connected directly to a non-inverting input pin, a non-inverting amplifier may need a protective resistor connected to this pin. For high impedance circuits, a large resistor and or low leakage current diode can be used.

Q14: What is the difference between a single-supply amplifier and a dual-supply amplifier?

A: There is no difference in the actual circuit, layout, and characteristics of the amplifier. When an operational amplifier is designated as dual power supply, the output load is usually referenced to ground (GND), while a single power supply operational amplifier is usually referenced to the midpoint voltage of a single supply, and it is usually specified to operate on lower voltages, but this is not a necessary requirement. Therefore, whether the op amp is powered by a single 5V power supply and ground (GND), or powered by +2.5 and -2.5V, these is no different.

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