The operational amplifier (opamp) is the cornerstone of the entire analogue circuit design and the selection of an appropriate amplifier is essential to achieve system design specifications. By understanding the most important parameters of an amplifier, you will be able to find the most suitable operational amplifier. This article gives you an overview of the important parameters and important indicators of operational amplifiers.
What are the considerations for operational amplifier selection?
Bias voltage and input bias current
Bias voltage is a key factor in precision circuit design. Parameters that are often overlooked, such as bias voltage drift and voltage noise that vary with temperature, must also be measured. Precision amplifiers require bias voltage drift of less than 200 μV and input voltage noise of less than 6 nV/√Hz. bias voltage drift with temperature is required to be less than 1 μV/°C .
The low bias voltage specification is important in high gain circuit design as the bias voltage can be amplified to cause a large voltage output and can take up a large part of the output swing. Temperature sensing and tension measurement circuits are examples of applications that utilise precision amplifiers.
Low input bias currents are sometimes necessary. The amplifier in an optical receiver must have a low bias voltage and a low input bias current.
Of all amplifiers, chopper amplifiers provide the lowest bias voltage and the lowest temperature-dependent bias voltage drift. Many weight metering devices have high gain requirements and need to be equipped with high quality precision amplifiers, where chopper amplifiers are a good choice.
Be aware of the effects of power supply
Amplifiers in portable systems require very low supply voltage operation and low supply current to maximise battery life. These amplifiers must also generally have good output drive capability and high open loop gain.
Although many amplifiers are advertised as consuming very little current, care should be taken when selecting them. Be sure to read the data sheet carefully to watch out for performance problems that may arise from operating at low voltages. Some low power op amps have a wide range of supply currents when the output voltage is changed. At low supply voltages, the output current drive capability may also be significantly reduced. A table of parameters can be consulted to determine the output current drive capability that can be achieved at a particular supply voltage.
Another option is to use an amplifier with an "off" feature. Although these amplifiers have a high supply current, they can be switched off when not in operation and thus enter an ultra-low current state. The higher supply current allows the amplifier to be driven faster and with greater output capability.
The following three important selection indicators are given below.
1 、Input detuning voltage
Definition: The DC voltage applied between the two inputs that makes the amplifier DC output voltage 0 when the op-amp is in use.
The range of advantages and disadvantages: 1µV or less is excellent, 100µV or less is good. The maximum is a few tens of mV.
Countermeasures.
Select an amplifier with a VOS much smaller than the direct current being measured.
The zeroing measures of the over-operating amplifier eliminate this effect
If you are only concerned with the alternating component of the signal under test, you can add AC coupling circuitry to the input and output to eliminate it.
If IB1=IB2, then choosing R1=R2//RF can make the current-forming detuned voltage will disappear, but in practice IB1=IB2 is difficult to meet.
2、Drift of the detuning voltage
Definition: When the independent variables such as temperature change (µV/°C), time duration (µV/MO), supply voltage (µV/V) change, the input detuning voltage will change.
Consequence: very serious. Because it cannot be zeroed by the zeroing terminal, even if zeroing is completed, it introduces a new detuning.
Countermeasures.
The first is to choose an op-amp with high stability, i.e. a small drift factor as described above.
Second, some op-amps have self-zeroing technology, which can constantly measure the detuning and subtract the current detuning voltage in the process of processing the signal.
3、Input bias current
Definition: When the output is maintained at a specified level, the average of the current flowing into the two inputs. Ib=(Ib1+Ib2)/2
Superiority range: 60fA~100µA
Consequences.
Firstly, when an amplifier is connected as a transimpedance amplifier to measure a small external current, the excessive input bias current will split the measured current and make the measurement inaccurate.
Secondly, when the amplifier input is grounded through a resistor, this current will produce an undesired input voltage across the resistor.
Countermeasure: To avoid the effect of input bias current on the amplifier circuit, the main measure is to choose an amplifier with a small IB.
