Operational amplifier classification and selection

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Operational amplifier classification and selection [Copy link]

For large AC signals such as audio and video, it is more appropriate to choose an
op amp with a large SR (slew rate).

For circuits that process weak DC signals, it is more appropriate to use op amps with higher precision (i.e., smaller offset current, offset voltage, and temperature drift).

Operational amplifiers can be broadly divided into the following categories:

General purpose op amp
High impedance op amp
Low temperature drift op amp
High speed op amp
Low power op amp
High voltage and high power op amp
1. General purpose op amp

Its performance index is suitable for general use (low frequency and slow signal change), such as A741, LM358 (dual op amp), LM324 and LF356 with field effect tube as input stage.

2. High impedance op amp

The characteristics of this type of op amp are very high differential input impedance and very small input bias current. The main measure to achieve these indicators is to use the high input impedance of the field effect tube, but the input offset voltage of this type of op amp is relatively large.

This type of op amp includes LF356, LF355, LF347, CA3130, CA3140, etc.

3. Low temperature drift type op amp

In automatic control instruments such as precision instruments and weak signal detection, it is hoped that the offset voltage of the operational amplifier should be small and not change with temperature. Low temperature drift operational amplifiers are designed for this purpose.

Currently, the commonly used low-temperature drift op amps include OP07, OP27, OP37, AD508 and the chopper-stabilized low-temperature drift device ICL7650 composed of MOSFET.

4. High-speed op amp

In fast A/D and D/A as well as in video amplifiers, the operational amplifier conversion rate SR must be high and the unit gain bandwidth BWG must be large enough. The main features of high-speed operational amplifiers are high conversion rate and wide frequency response.

Common operational amplifiers include LM318, \mu A175, etc. Its SR = 50~70V/ms.

5. Low power op amp

As the application range of portable instruments expands, operational amplifiers with low power supply voltage and low power consumption must be used.

Commonly used low-power op amps include TL-022C, TL-160C, etc.

6. High voltage and high power op amp

The output voltage of the op amp is mainly limited by the power supply. In ordinary op amps, the maximum output voltage is generally only tens of volts, and the output current is only tens of milliamperes. If you want to increase the output voltage or output current, you must add an auxiliary circuit to the outside of the op amp.

High voltage and high power amplifiers can output high voltage and high current without any external circuits. The power supply voltage of the D41 amplifier can reach 150μm, and the output current of the A791 amplifier can reach 1A.

Note 1: Precision op amps are those with very low drift and noise, and very high gain and common-mode rejection ratio. The temperature drift of such op amps is generally less than 1\mu V/C.

Note 2: High input impedance op amp refers to an input stage integrated op amp made of junction field effect transistor or MOS tube. One of its additional characteristics is that the conversion speed is relatively high. High input impedance op amps are widely used, such as sample-and-hold circuits, integrators, logarithmic amplifiers, instrumentation amplifiers, bandpass filters, etc.

Note 3: High-speed op amps refer to op amps with a higher conversion rate, generally above 100V/\mu s. They are used in high-speed A/D, D/A, filters, phase-locked loop circuits, analog multipliers, etc.

Notes on Op-amp Selection-1
1. You should correctly understand and treat the various parameters of the op-amp, and do not blindly and one-sidedly pursue advanced indicators. For example, the input impedance of the op-amp at the input stage of the field effect tube is high, but the offset voltage is also large; the conversion rate of the low-power op-amp must be low. When using an op-amp to amplify weak signals, special attention should be paid to selecting an op-amp with very small offset and noise coefficient, such as ICL7650.

2. Ensure that the equivalent DC resistance of the op amp's in-phase and inverting terminals to ground is equal. In addition, in the printed circuit board wiring scheme composed of high-input, low-offset, low-temperature drift high-precision op amps, a guard ring should be added to its input terminal.

3. When the operational amplifier is used for DC amplification, it must be zeroed. The operational amplifier with a zeroing terminal should be connected to the zeroing circuit recommended in the relevant materials for zeroing.

4. In order to eliminate the high-frequency self-excitation of the op amp, the specified or recommended parameters should be referred to, and appropriate capacitors should be inserted between the specified vibration elimination pins to eliminate vibration. At the same time, cascading of more than two stages of op amps should be avoided to reduce the difficulty of vibration elimination.

5. In order to eliminate parasitic oscillations caused by internal resistance, a decoupling capacitor can be connected to the ground as close as possible to the power supply end of the op amp. Considering the inductance effect of the decoupling electrolytic capacitor, a ceramic capacitor with a capacity of 0.01uf~0.1uf is often connected in parallel at its two ends.

Things to note for single-supply op amps - 2
1. To perform single-supply amplification, the parameters that must be known at least are unity gain bandwidth, open-loop differential voltage gain, and maximum output swing. It should be noted that the greater the designed gain, the lower the corresponding bandwidth. For specific calculations, please refer to relevant materials.

2. In a single power supply, if the amplification factor is too large, self-excitation is very likely to occur. At this time, the capacitor on the feedback resistor should be selected based on the frequency of the signal to be amplified and the frequency of the self-excitation signal. The calculation method is f = \frac{1}{2\pi RC}, and C is generally 10PF to several hundred PF.

3. It is best to use a common-phase amplifier for the first stage of a single-power multi-stage op amp, so that the characteristics of the common-phase amplifier can be used to match the front and rear signals, and the second stage can use an inverting amplifier.

4. Op amps with offset voltage zeroing function should be used with caution. If the connection and wiring of the adjustment end are not careful, the offset will be even greater, especially the offset temperature drift.

5. The greater the gain, the greater the noise and the greater the gain error.

6. The larger the open-loop gain, the smaller the closed-loop gain error. The calculation of closed-loop gain is valid only when the open-loop gain is assumed to be infinite.

7. For signal sources with high internal resistance, op amps with low current noise should be selected.

8. The smaller the resistance around the op amp, the smaller the noise and offset. The lower limit of the resistance value selection is determined by the previous stage driving capability and power consumption.

9. For the same op amp, the greater the gain, the greater the output impedance.

Notes on Op Amp Selection-3
1. When amplifying small signals, consider the gain-bandwidth product of the op amp and leave enough open-loop gain;

When amplifying large signals, the conversion rate (slew rate) of the signal must be fully considered.

2. Accuracy: Although the offset voltage error can be corrected by software, you should try to use an op amp with a smaller offset voltage, which will reduce the design difficulty.

When the power supply impedance or the external resistor network resistance is large, the influence of input bias current should be considered. At the same time, the zero-drift amplifier can further reduce the difficulty of system zero adjustment in a wide temperature application range.

3. Zero adjustment problem of integrated operational amplifier: Due to the influence of input offset voltage and input offset current of integrated operational amplifier, when the input signal of the linear circuit composed of operational amplifier is zero, the output is often not zero. In order to improve the calculation accuracy of the circuit, it is required to compensate for the errors caused by offset voltage and offset current, which is the zero adjustment of the operational amplifier. Commonly used zero adjustment methods include internal zero adjustment (as shown in Figure a) and external zero adjustment (as shown in Figure b). For operational amplifiers without internal zero adjustment terminals, external zero adjustment methods should be used. Noise: Offset can be corrected at the back end, but noise cannot be corrected. The \frac{1}{f} of the operational amplifier should be fully considered:

Figure a and Figure b are not shown because of the messy layout.

4. Zero drift and temperature drift:

In DC applications, multi-stage DC amplifiers can only be directly coupled, and the Q point of the front stage is required to be stable to avoid affecting the back stage. However, the zero drift and temperature drift of the front stage hinder this. Therefore, it is necessary to select an op amp with a zero adjustment terminal that is easy to adjust and has a small temperature drift, and the output noise is reduced to a secondary factor.

In AC applications, zero drift and temperature drift do not need to be considered, and output noise or other indicators become the main factors, such as the use of high-speed bandwidth op amps.

Characteristics and limitations of bipolar input op amps and CMOS op amps
1. Bipolar input op amps are widely used, in which all devices including the input stage are composed of bipolar transistors (triodes). Its input bias and offset currents are hundreds of nA, the typical bias voltage is 10mV, and the open-loop input impedance is hundreds of K\Omega.

2.CMOS op amps have high input impedance and very low bias current. Their offset voltage is higher than that of bipolar op amps. CMOS amplifiers can operate within the rail-to-rail range. Because they consume little power, they are suitable for single-supply and low-voltage applications. Compared with bipolar types, CMOS amplifiers generally have higher noise.

3. BiFET op amp is the abbreviation of bipolar-field-effect transistor. It combines two technologies, using FETS at the front end or input stage and bipolar tubes in other parts. As a result, it can obtain wider bandwidth, lower input offset current, higher input impedance and stronger driving capability than bipolar op amps. However, the input offset voltage is generally higher than that of bipolar op amps.