1. What is an Operational Amplifier (Op-amp)?
An operational amplifier is an integrated circuit that can amplify weak electric signals.
An operational amplifier has two input pins and one output pin. Its basic role is to amplify and output the voltage difference between the two input pins.
2. What an Operational Amplifier Can Do
An operational amplifier is not used alone but is designed to be connected to other circuits to perform a great variety of operations. This article provides some typical examples of usage of circuits with operational amplifiers.
●Enables substantial amplification of an input signal
When an operational amplifier is combined with an amplification circuit, it can amplify weak signals to strong signals.It behaves like a megaphone where the input signal is a person’s voice and the megaphone is the operational amplifier circuit.For example, such a circuit can be used to amplify minute sensor signals.
Processing of sensor signals can be further improved by inputting the amplified signal to a microcontroller* unit (MCU).
* Microcontroller… A compact computer for controlling electronic devices. As the brain of electronic devices, MCUs operate according to input signals.
●Enables elimination of noise from an input signal
By operating as a filter of input signals, the operational amplifier circuit is able to extract the signal with the target frequency.For example, when an operational amplifier circuit is used for voice recognition or in a voice recorder, it can extract frequencies close to the targeted sound while shutting out all other frequencies as noise.An operational amplifier circuit can be tweaked to perform a broad range of functions such as arithmetical operations or signal synthesis.
3. Operational Amplifier Applications
As noted above, an operational amplifier is almost never used alone. By connecting resistors or capacitors, you can configure a circuit capable of the signal amplification, filtering or arithmetic circuit operations described in “2. What an Operational Amplifier Can Do”.
(1)Internal operational amplifier operations
The following describes the operations performed by the operational amplifier in the circuit. Let’s see how an operational amplifier behaves in an amplifier circuit, taking the example of a non-inverting amplifier circuit.In addition to this simple calculation, the operational amplifier’s performance must also be considered when configuring a circuit. This point is described later in this section under “4. Selecting an Operational Amplifier and Explanation of Terms.”
We describe here some typical operational amplifier applications.
[Non-inverting amplifier circuit]
As explained in (1), this also is a circuit for amplifying and outputting input signals.
[Inverting amplifier circuit]
An inverting amplifier circuit is indicated by a minus sign. If the VIN voltage increases, the VOUT voltage decreases.
[Voltage follower circuit]
This is a non-inverting amplifier circuit where R2 is short-circuited (R2=0Ω) and R1 is open (R1=infinity). Since VOUT=(1+R2/R1)×VIN= (1+0Ω/∞)×VIN=VIN, the output is the same voltage as the input signal. A voltage follower is used as a buffer circuit to convert the impedance or to separate circuits.
[Differential amplifier circuit]
This is a circuit for amplifying and outputting the difference between two input signals.
4. Selecting an Operational Amplifier and Explanation of Terms
Here, we will use the ABLIC operational amplifier S-89630A as an example of what items to check in selecting an operational amplifier and explaining operational amplifier attributes.
(1) Check the operating voltage.
[Operating power supply voltage range]
This is the range of the operable power supply voltage at the VDD pin.Check that the power supply voltage is within the range of the operational amplifier operating voltage range.
[Common-mode input voltage range]
The range of voltage of a signal that may be applied to the input pins. The operational amplifier will work as long as the input signal is within this range.An operational amplifier whose common-mode input voltage range covers VSS – VDD is called a “Rail-to-Rail input operational amplifier”; that is, an operational amplifier with an excellent input signal voltage range.
(2) Check the input signal frequency.
[Gain bandwidth product]
This specifies the maximum frequency that an operational amplifier can amplify a signal to. The maximum frequency varies with the factor (gain) you use to amplify a signal. At a gain of one (=0dB), the signal can be amplified to maximum frequency, the so called gain bandwidth product.
The graph on the right indicates that at a gain of one (=0dB), the maximum frequency that amplification allows using the S-89630A is 1.2MHz and that at a gain of 10 (=20dB), the maximum frequency is 120kHz.Make sure that the maximum frequency you want to amplify to is within the range of the factor by which you want to amplify.
(3) Check the current consumption.
This indicates the current value drawn from the VDD pin. The lower this value is, the more you can reduce the power of the system. Normally, an operational amplifier with low current consumption tends to also have low frequency of gain bandwidth.
(4) Check the signal amplification accuracy.
[Input offset voltage]
The input offset voltage is a conversion of the error voltage generated at the output to an input value when the input voltage is 0V. It is an essential attribute affecting the amplification accuracy of operational amplifiers. Generally, when the voltage amplitude of the input signal is on the order of mV, an input offset voltage on the order of μV is required. This makes it necessary to select a “zero drift operational amplifier” to handle such tiny offset voltages.
5. What is a Zero Drift Amplifier?
While an input offset voltage of 0V is ideal for an operational amplifier, the characteristic dispersion of the transistors that are used in building an operational amplifier generate an input offset voltage. Also, the input offset voltage changes with ambient temperature or elapsed time. It is these changes that are referred to as input offset voltage drift.
A zero drift amplifier is an operational amplifier that minimizes input offset voltage and input offset voltage drift (≒0).Selecting a zero-drift operational amplifier is a highly effective solution for applications that demand high-accuracy signal amplification.
Zero-drift amplifiers contain circuitry that automatically corrects offset voltage. This circuitry is categorized as one of the following three types:
- Auto zero amplifier
- Chopper amplifier
- Combined auto-zero and chopper amplifier
In addition to a regular operational amplifier (the main amp), an auto-zero amplifier comprises a correction amplifier, capacitors and switches for changing signal paths.The role of the correction amplifier is to measure the input offset voltage of the main amplifier. Measurement data is stored in a capacitor and is used to eliminate the input offset voltage of the main amplifier.This process virtually eliminates input offset voltage.
The operating principle of a chopper amplifier is a little more complex. The input signal is converted to an AC signal in an input modulation circuit that comprises a switch. The converted signal is amplified in the amplifier and converted back to a DC signal in an output modulation circuit. Since the input offset voltage does not go through the input modulation circuit, it is converted to an AC signal in the output demodulation circuit. As a result, only the input offset voltage data converted to an AC signal is removed in the low pass filter in the output unit. This process minimizes the input offset voltage.
ABLIC’s Zero-drift Amplifier
S-89630A operational amplifier combines auto-zero with a chopper system to provide zero drift operation, wide voltage range (4.0 – 36V) operation and low offset voltage attributes.
S-89713 Series provides auto-zero drift operation with low-voltage operation from 2.65V and low offset voltage attributes. We are also offering this product in an ultra small package (SNT-8A: 1.97 x 2.46mm size) lineup.
If you are looking for a zero drift operational amplifier, ABLIC’s operational amplifiers come highly recommended.