How to determine the parameters and selection of op amps

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How to determine the parameters and selection of op amps [Copy link]

Bias Voltage and Input Bias Current

Bias voltage is a key factor in precision circuit design. Often overlooked parameters such as bias voltage drift and voltage noise over temperature must also be measured. Precision amplifiers require bias voltage drift less than 200μV and input voltage noise less than 6nV/√Hz. Bias voltage drift over temperature is required to be less than 1μV/℃.

The low bias voltage specification is important in high-gain circuit design because the bias voltage can cause a large voltage output after amplification and can occupy a large part of the output swing. Temperature sensing and strain measurement circuits are examples of applications that use precision amplifiers.

Low input bias current is sometimes required. Amplifiers in optical receiving systems must have low bias voltage and low input bias current. For example, the dark current of a photodiode is in the order of pA, so the amplifier must have a smaller input bias current. CMOS and JFET input amplifiers are currently available operational amplifiers with the lowest input bias current.

Because I am currently using a photocell as the data collection system, I am paying special attention to the bias voltage and current. If there are other needs, then other parameters should also be considered.

1. Input offset voltage VIO (Input Offset Voltage)

The input offset voltage is defined as the compensation voltage added between the two input terminals when the output voltage of the integrated operational amplifier is zero.

The input offset voltage actually reflects the circuit symmetry inside the op amp. The better the symmetry, the smaller the input offset voltage. The input offset voltage is a very important indicator of the op amp, especially for precision op amps or when used for DC amplification.

2. Input offset voltage temperature drift αVIO (Input Offset Voltage Drift)

The temperature drift (also called temperature coefficient) of the input offset voltage is defined as the ratio of the change in input offset voltage to the change in temperature over a given temperature range.

This parameter is actually a supplement to the input offset voltage, which is convenient for calculating the drift of the amplifier circuit due to temperature changes within a given working range. The input offset voltage temperature drift of a general op amp is between ±10~20μV/℃, and the input offset voltage temperature drift of a precision op amp is less than ±1μV/℃.

3. Input bias current IB (Input Bias Current)

When using an op amp, you may also encounter an input bias current IB, which refers to the base DC current of the input transistor of the first-stage amplifier. This current ensures that the amplifier operates in the linear range and provides a DC operating point for the amplifier.

The input bias current is defined as the average value of the bias current at its two input terminals when the output DC voltage of the op amp is zero.

The input bias current has a great impact on places that require input impedance, such as high-impedance signal amplification and integration circuits. 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; when field-effect transistors are used as input stages, the input bias current is generally less than 1nA.

For bipolar op amps, this value is very discrete but is almost unaffected by temperature; for MOS op amps, this value is the gate leakage current, which is very small but is greatly affected by temperature.

4. Input Offset Current

Input offset current refers to the error in the bias current of the two differential input terminals.

Input offset current is defined as the difference in bias current between its two input terminals when the output DC voltage of the op amp is zero.

The input offset current also reflects the circuit symmetry inside the op amp. The better the symmetry, the smaller the input offset current. Input offset current is a very important indicator of op amps, especially for precision op amps or when used for DC amplification. The input offset current is approximately one percent to one tenth of the input bias current. The input offset current has an important influence on small signal precision amplification or DC amplification, especially when a larger resistor (such as 10k or greater) is used outside the op amp. The impact of the input offset current on accuracy may exceed the impact of the input offset voltage on accuracy. The smaller the input offset current, the smaller the intermediate zero point offset during DC amplification, and the easier it is to handle. Therefore, it is an extremely important indicator for precision op amps.

5. Input impedance

(1) Differential mode input impedance

The differential input impedance is defined as the ratio of the voltage change at the two input terminals to the corresponding input current change when the op amp operates in the linear region. The differential input impedance includes input resistance and input capacitance, and only refers to input resistance at low frequencies.

(2) Common mode input impedance

Common-mode input impedance is defined as the ratio of the change in common-mode input voltage to the corresponding change in input current when the op amp is operating on an input signal (i.e. the same signal is input to both input terminals of the op amp). At low frequencies, it behaves as common-mode resistance.

6. Voltage gain

(1) Open-Loop Gain

In the case of no negative feedback (open loop), the amplification factor of the operational amplifier is called open loop gain, recorded as AVOL, and some datasheets write it as: Large Signal Voltage Gain. The ideal value of AVOL is infinite, generally about thousands to tens of thousands of times, and its expression can be expressed in dB and V/mV.

(2) Closed-Loop Gain

As the name suggests, it is the amplification factor of the operational amplifier when there is feedback.

7. Output Voltage Swing

When the op amp operates in the linear region, under a specified load, the op amp can output the maximum voltage amplitude when powered by the current power supply voltage.

8. Input voltage range

(1) Differential mode input voltage range

The maximum differential input voltage is defined as the maximum input voltage difference allowed between the two input terminals of the op amp.
When the input voltage difference allowed between the two input terminals of the op amp exceeds the maximum differential input voltage, the op amp input stage may be damaged.

(2) Common Mode Input Voltage Range

The maximum common-mode input voltage is defined as the common-mode input voltage at which the common-mode rejection ratio characteristic of the op amp deteriorates significantly when the op amp operates in the linear region.

Generally defined as the common mode input voltage corresponding to a 6dB drop in the common mode rejection ratio as the maximum common mode input voltage. The maximum common mode input voltage limits the maximum common mode input voltage range in the input signal. In the presence of interference, this issue needs to be taken into account in circuit design.

9. Common Mode Rejection Ratio

The common-mode rejection ratio is defined as the ratio of the op amp's differential-mode gain to its common-mode gain when the op amp operates in its linear region.

Common mode rejection ratio is an extremely important indicator, which can suppress common mode interference signals. Since the common mode rejection ratio is very large, the common mode rejection ratio of most op amps is generally tens of thousands of times or more. It is not convenient to compare directly with numerical values, so it is generally recorded and compared in decibels. The common mode rejection ratio of general op amps is between 80 and 120 dB.

10. Supply Voltage Rejection Ratio

The power supply voltage rejection ratio is defined as the ratio of the change in the op amp input offset voltage to the power supply voltage when the op amp operates in the linear region.

The power supply voltage rejection ratio reflects the impact of power supply changes on the output of the op amp. Therefore, when used for DC signal processing or small signal processing analog amplification, the power supply of the op amp needs to be carefully handled. Of course, an op amp with a high common mode rejection ratio can compensate for part of the power supply voltage rejection ratio. In addition, when using dual power supplies, the power supply voltage rejection ratios of the positive and negative power supplies may be different.

11. Static power consumption

The quiescent power of an op amp at a given supply voltage, usually under no load.

Here comes the concept of quiescent current IQ, which actually refers to the current consumed by the op amp when it is working at no load. This is the minimum current consumed by the op amp (excluding sleep state)

12. Slew Rate

The op amp conversion rate is defined as the output rise rate of the op amp measured from its output terminal when a large signal (including a step signal) is input into the op amp input terminal under the condition that the op amp is connected in a closed loop.

Since the input stage of the op amp is in a switching state during the conversion period, the feedback loop of the op amp does not work, that is, the conversion rate has nothing to do with the closed-loop gain. The conversion rate is a very important indicator for large signal processing. For general op amps, the conversion rate SR<=10V/μs, and the conversion rate SR of high-speed op amps>10V/μs. The current high-speed op amp has a maximum conversion rate SR of 6000V/μs. This is used in the selection of op amps in large signal processing.

13. Gain Bandwidth

(1) Gain Bandwidth Product

Gain Bandwidth Product, GBP, is the product of bandwidth and gain.

(2) Unity gain bandwidth

The bandwidth of an operational amplifier when its gain is 1.

The concepts of unity gain bandwidth and bandwidth gain product are somewhat similar, but different. It should be noted here that for voltage feedback op amps, the gain bandwidth product is a constant, but this is not the case for current mode op amps, because for current mode op amps, the relationship between bandwidth and gain is not linear.

14. Output impedance

Output impedance is defined as the ratio of the voltage change to the corresponding current change when a signal voltage is applied to the output of the op amp when the op amp is operating in the linear region. At low frequencies, it only refers to the output resistance of the op amp. This parameter is tested in an open-loop state.

15. Equivalent Input Noise Voltage

The equivalent input noise voltage is defined as any irregular AC interference voltage generated at the output of a well-shielded op amp with no signal input.

When this noise voltage is converted to the op amp input, it is called the op amp input noise voltage (sometimes also expressed as noise current). For broadband noise, the effective value of the input noise voltage of an ordinary op amp is about 10~20μV.