In
electronics, the relationship between the
direct current (DC) through an
electronic device and the DC
voltage across its terminals is called a current–voltage characteristic of the device.
Electronic engineers use these charts to determine basic parameters of a device and to model its behavior in an
electrical circuit. These characteristics are also known as I–V curves, referring to the standard symbols for current and voltage. In
electronic components with more than two terminals, such as
vacuum tubes and
transistors, the current–voltage relationship at one pair of terminals may depend on the current or voltage on a third terminal. This is usually displayed on a more complex current–voltage graph with multiple curves, each one representing the current–voltage relationship at a different value of current or voltage on the third terminal. For example the diagram at right shows a family of I–V curves for a
MOSFET as a function of drain voltage with overvoltage (
VGS − Vth) as a parameter. The simplest I–V curve is that of a
resistor, which according to
Ohm's law exhibits a
linear relationship between the applied voltage and the resulting
electric current; the current is proportional to the voltage, so the I–V curve is a straight line through the
origin with positive
slope. The
reciprocal of the slope is equal to the
resistance. The I–V curve of an electrical component can be measured with an instrument called a
curve tracer. The
transconductance and
Early voltage of a
transistor are examples of parameters traditionally measured from the device's I–V curve.
Types of I–V curves The shape of an electrical component's characteristic curve reveals much about its operating properties. I–V curves of different devices can be grouped into categories: •
Active vs passive: Devices which have I–V curves which are limited to the first and third
quadrants of the I–V plane, passing through the
origin, are
passive components (loads), that consume
electric power from the circuit. Examples are
resistors,
light bulbs and
electric motors.
Conventional current always flows through these devices in the direction of the
electric field, from the positive voltage terminal to the negative, so the charges lose
potential energy in the device, which is converted to heat or some other form of energy. :In contrast, devices with I–V curves which pass through the second or fourth quadrants are
active components,
power sources, which can produce electric power. Examples are
batteries and
generators. When it is operating in the second or fourth quadrant, current is forced to flow through the device from the negative to the positive voltage terminal, against the opposing force of the electric field, so the
electric charges are gaining
potential energy. Thus the device is converting some other form of energy into electric energy. •
Linear vs nonlinear: A straight line through the origin represents a
linear circuit element, while a curved line represents a
nonlinear element. For example, resistors, capacitors, and inductors are linear, while
diodes and
transistors are nonlinear. An I–V curve which is a straight line through the origin with positive
slope represents a linear or ohmic resistor, the most common type of resistance encountered in circuits. It obeys
Ohm's law; the current is proportional to the applied voltage over a wide range. Its
resistance, equal to the
reciprocal of the slope of the line, is constant. A curved I–V line represents a nonlinear resistance, such as a diode. In this type the resistance varies with the applied voltage or current. •
Negative resistance vs positive resistance: If the I–V curve has a positive slope (increasing to the right) throughout, it represents a positive resistance. An I–V curve that is
nonmonotonic (having peaks and valleys) represents a device which has
negative resistance. Regions of the curve which have a negative slope (declining to the right) represent operating regions where the device has negative
differential resistance, while regions of positive slope represent positive differential resistance. Negative resistance devices can be used to make
amplifiers and
oscillators.
Tunnel diodes and
Gunn diodes are examples of components that have negative resistance. •
Hysteresis vs single-valued: Devices which have
hysteresis; that is, in which the current–voltage relation depends not only on the present applied input but also on the past history of inputs, have I–V curves consisting of families of closed loops. Each branch of the loop is marked with a direction represented by an arrow. Examples of devices with hysteresis include iron-core
inductors and
transformers,
thyristors such as
SCRs and
DIACs, and
gas-discharge tubes such as
neon lights. File:Voltage controlled negative resistance.svg|
I–V curve similar to a
tunnel diode characteristic curve. It has negative resistance in the shaded voltage region, between
v1 and
v2 File:Kennlinie DIAC.svg|
DIAC I–V curve.
VBO is the
breakover voltage. File:Pinched crossing hysteresis.png|
Memristor I–V curve, showing a pinched
hysteresis File:Gunn diode IV curve.svg|
Gunn diode I–V curve, showing negative differential resistance with hysteresis (notice arrows) ==In electrophysiology==