The input offset voltage (
) is a parameter defining the
differential DC
voltage
Voltage, also known as (electrical) potential difference, electric pressure, or electric tension, is the difference in electric potential between two points. In a Electrostatics, static electric field, it corresponds to the Work (electrical), ...
required between the inputs of an
amplifier
An amplifier, electronic amplifier or (informally) amp is an electronic device that can increase the magnitude of a signal (a time-varying voltage or current). It is a two-port electronic circuit that uses electric power from a power su ...
, especially an
operational amplifier
An operational amplifier (often op amp or opamp) is a direct coupling, DC-coupled Electronic component, electronic voltage amplifier with a differential input, a (usually) Single-ended signaling, single-ended output, and an extremely high gain ( ...
(op-amp), to make the output zero (for voltage amplifiers, 0
volts
The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI).
Definition
One volt is defined as the electric potential between two point ...
with respect to ground or between differential outputs, depending on the output type).
Details
An ideal op-amp amplifies the differential input; if this input difference is 0 volts (i.e. both inputs are at the same voltage), the output should be zero. However, due to manufacturing process, the differential input transistors of real op-amps may not be exactly matched. This causes the output to be zero at a non-zero value of differential input, called the input offset voltage.
Typical values for
are around 1 to 10 mV for cheap commercial-grade op-amp
integrated circuits
An integrated circuit (IC), also known as a microchip or simply chip, is a set of electronic circuits, consisting of various electronic components (such as transistors, resistors, and capacitors) and their interconnections. These components a ...
(IC). This can be reduced to several microvolts if nulled using the IC's offset null pins or using higher-quality or
laser-trimmed devices. However, the input offset voltage value may drift with temperature or age.
Chopper amplifiers actively measure and compensate for the input offset voltage, and may be used when very low offset voltages are required.
Input bias current and
input offset current also affect the net offset voltage seen for a given amplifier. The voltage offset due to these currents is separate from the input offset voltage parameter and is related to the impedance of the signal source and of the feedback and input impedance networks, such as the two resistors used in the basic inverting and non-inverting amplifier configurations.
FET
The field-effect transistor (FET) is a type of transistor that uses an electric field to control the current through a semiconductor. It comes in two types: junction FET (JFET) and metal-oxide-semiconductor FET (MOSFET). FETs have three termi ...
-input op-amps tend to have lower input bias currents than
bipolar-input op-amps, and hence incur less offset of this type.
Input offset voltage is symbolically represented by a voltage source that is in series with either the positive or negative input terminal (it is mathematically equivalent either way). Normally input offset voltage is measured in the terms of input voltage applied at the non-inverting terminal to make output zero.
[http://www.ti.com/lit/an/sloa059/sloa059.pdf page 3]
References
{{reflist
External links
Analog Devices tutorial on op-amp input offset voltage and mitigation techniques
Electrical parameters