Bias tee
Encyclopedia
A bias tee is a three port network used for setting the DC bias point of some electronic components without disturbing other components. The bias tee is a diplexer
. The low frequency port is used to set the bias; the high frequency port passes the radio frequency signals but blocks the biasing levels; the combined port connects to the device, which sees both the bias and RF. It is called a tee because the 3 ports are often arranged in the shape of a T.
that allows AC
through but blocks the DC
bias and an ideal inductor
that blocks AC but allows DC. Although some bias tees can be made with a simple inductor and capacitor, wideband bias tees are considerably more complicated because practical components have parasitic elements.
Bias tees are designed for transmission line environments. Typically, the characteristic impedance Z0 will be 50 ohms or 75 ohms. The impedance
of the capacitor (XC) is chosen to be much less than Z0, and the impedance of the inductor (XL) is chosen to be much greater than Z0.
Where ω is the frequency in radians per second and f is the frequency in hertz.
Bias tees are designed to operate over a range of signal frequencies. The reactances are chosen to have minimal impact at the lowest frequency.
For wide range bias tees, the inductor must be large at the lowest frequency. A large inductor will have a stray capacitance (which creates its self-resonant frequency). At a high enough frequency, the stray capacitance presents a low impedance shunt path for the signal, and the bias tee becomes ineffective. Practical wide band bias tees must use circuit topologies that avoid the shunt path. For example, a Picosecond Pulse Labs 5580 works from 10 kHz to 15 GHz. Consequently, the simple design would need an inductance of at least 800 μH (XL about j500 ohms at 10 kHz), and that inductor must still look like an inductor at 15 GHz. However, a commercial 820 μH inductor has a self-resonant frequency of only 1.8 MHz -- four orders of magnitude too low.
power into an AC
signal
to power remote antenna
amplifier
s or other devices. It is usually positioned at the receiving end of the coaxial cable to pass DC power from an external source to the coaxial cable running to powered device. A bias “T” consists of a feed inductor
to deliver DC to a connector
on the device side and a blocking capacitor
to keep DC from passing through to the receiver
. The RF
signal is connected directly from one connector to the other with only the blocking capacitor in series. The internal blocking diode
prevents damage to the bias “T” if reverse supply voltage
is applied.
Biasing for photodiodes (vacuum and solid state), Microchannel plate detector
s, transistor
s, and triode
s. High frequencies are not leaking into a common power supply rail and noise from the power supply does not appear on the signal line.
Bias "T" has been used in a variety of applications, but is generally used to provide an RF signal and power to a remote device where running two separate cables would not be advantageous. Examples of this are: Power over Ethernet
, active antennas, low-noise amplifiers, and down converters
with air as dielectric. The radius is chosen to be as large as possible without allowing higher modes.
The design of a bias "T" is based upon power going out to the remote device, but not being seen by the base station or receiver. It does this by using a capacitor on the RF output terminal, effectively creating an open circuit for the DC current. The incoming RF signal, or the one from the antenna, is the output for the DC power. This front-end of a bias "T" typically consists of a bandpass filter, a low noise amplifier
, and a mixer coupled to a local oscillator.
are damped by the dielectric and the core. Also the small coil should have about 10 ohm resistance to further damp oscillations and avoid ripple on the transmitted spectrum.
Diplexer
A diplexer is a passive device that implements frequency domain multiplexing. Two ports are multiplexed onto a third port . The signals on ports L and H occupy disjoint frequency bands...
. The low frequency port is used to set the bias; the high frequency port passes the radio frequency signals but blocks the biasing levels; the combined port connects to the device, which sees both the bias and RF. It is called a tee because the 3 ports are often arranged in the shape of a T.
Design
Conceptually, the bias tee can be viewed as an ideal capacitorCapacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
that allows AC
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
through but blocks the DC
Direct current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
bias and an ideal inductor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
that blocks AC but allows DC. Although some bias tees can be made with a simple inductor and capacitor, wideband bias tees are considerably more complicated because practical components have parasitic elements.
Bias tees are designed for transmission line environments. Typically, the characteristic impedance Z0 will be 50 ohms or 75 ohms. The impedance
Electrical impedance
Electrical impedance, or simply impedance, is the measure of the opposition that an electrical circuit presents to the passage of a current when a voltage is applied. In quantitative terms, it is the complex ratio of the voltage to the current in an alternating current circuit...
of the capacitor (XC) is chosen to be much less than Z0, and the impedance of the inductor (XL) is chosen to be much greater than Z0.
Where ω is the frequency in radians per second and f is the frequency in hertz.
Bias tees are designed to operate over a range of signal frequencies. The reactances are chosen to have minimal impact at the lowest frequency.
For wide range bias tees, the inductor must be large at the lowest frequency. A large inductor will have a stray capacitance (which creates its self-resonant frequency). At a high enough frequency, the stray capacitance presents a low impedance shunt path for the signal, and the bias tee becomes ineffective. Practical wide band bias tees must use circuit topologies that avoid the shunt path. For example, a Picosecond Pulse Labs 5580 works from 10 kHz to 15 GHz. Consequently, the simple design would need an inductance of at least 800 μH (XL about j500 ohms at 10 kHz), and that inductor must still look like an inductor at 15 GHz. However, a commercial 820 μH inductor has a self-resonant frequency of only 1.8 MHz -- four orders of magnitude too low.
Application
A bias tee is used to insert DCDirect current
Direct current is the unidirectional flow of electric charge. Direct current is produced by such sources as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through...
power into an AC
Alternating current
In alternating current the movement of electric charge periodically reverses direction. In direct current , the flow of electric charge is only in one direction....
signal
Signal (electrical engineering)
In the fields of communications, signal processing, and in electrical engineering more generally, a signal is any time-varying or spatial-varying quantity....
to power remote antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
s or other devices. It is usually positioned at the receiving end of the coaxial cable to pass DC power from an external source to the coaxial cable running to powered device. A bias “T” consists of a feed inductor
Inductor
An inductor is a passive two-terminal electrical component used to store energy in a magnetic field. An inductor's ability to store magnetic energy is measured by its inductance, in units of henries...
to deliver DC to a connector
Electrical connector
An electrical connector is an electro-mechanical device for joining electrical circuits as an interface using a mechanical assembly. The connection may be temporary, as for portable equipment, require a tool for assembly and removal, or serve as a permanent electrical joint between two wires or...
on the device side and a blocking capacitor
Capacitor
A capacitor is a passive two-terminal electrical component used to store energy in an electric field. The forms of practical capacitors vary widely, but all contain at least two electrical conductors separated by a dielectric ; for example, one common construction consists of metal foils separated...
to keep DC from passing through to the receiver
Receiver (radio)
A radio receiver converts signals from a radio antenna to a usable form. It uses electronic filters to separate a wanted radio frequency signal from all other signals, the electronic amplifier increases the level suitable for further processing, and finally recovers the desired information through...
. The RF
Radio frequency
Radio frequency is a rate of oscillation in the range of about 3 kHz to 300 GHz, which corresponds to the frequency of radio waves, and the alternating currents which carry radio signals...
signal is connected directly from one connector to the other with only the blocking capacitor in series. The internal blocking diode
Diode
In electronics, a diode is a type of two-terminal electronic component with a nonlinear current–voltage characteristic. A semiconductor diode, the most common type today, is a crystalline piece of semiconductor material connected to two electrical terminals...
prevents damage to the bias “T” if reverse supply voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
is applied.
Biasing for photodiodes (vacuum and solid state), Microchannel plate detector
Microchannel plate detector
A micro-channel plate is a planar component used for detection of particles and impinging radiation . It is closely related to an electron multiplier, as both intensify single particles or photons by the multiplication of electrons via secondary emission...
s, transistor
Transistor
A transistor is a semiconductor device used to amplify and switch electronic signals and power. It is composed of a semiconductor material with at least three terminals for connection to an external circuit. A voltage or current applied to one pair of the transistor's terminals changes the current...
s, and triode
Triode
A triode is an electronic amplification device having three active electrodes. The term most commonly applies to a vacuum tube with three elements: the filament or cathode, the grid, and the plate or anode. The triode vacuum tube was the first electronic amplification device...
s. High frequencies are not leaking into a common power supply rail and noise from the power supply does not appear on the signal line.
Bias "T" has been used in a variety of applications, but is generally used to provide an RF signal and power to a remote device where running two separate cables would not be advantageous. Examples of this are: Power over Ethernet
Power over Ethernet
Power over Ethernet or PoE technology describes a system to pass electrical power safely, along with data, on Ethernet cabling. The IEEE standard for PoE requires category 5 cable or higher for high power levels, but can operate with category 3 cable for low power levels...
, active antennas, low-noise amplifiers, and down converters
A particular construction
The construction of the horizontal bar of the T is based on the rigid coaxial cableCoaxial cable
Coaxial cable, or coax, has an inner conductor surrounded by a flexible, tubular insulating layer, surrounded by a tubular conducting shield. The term coaxial comes from the inner conductor and the outer shield sharing the same geometric axis...
with air as dielectric. The radius is chosen to be as large as possible without allowing higher modes.
The design of a bias "T" is based upon power going out to the remote device, but not being seen by the base station or receiver. It does this by using a capacitor on the RF output terminal, effectively creating an open circuit for the DC current. The incoming RF signal, or the one from the antenna, is the output for the DC power. This front-end of a bias "T" typically consists of a bandpass filter, a low noise amplifier
Amplifier
Generally, an amplifier or simply amp, is a device for increasing the power of a signal.In popular use, the term usually describes an electronic amplifier, in which the input "signal" is usually a voltage or a current. In audio applications, amplifiers drive the loudspeakers used in PA systems to...
, and a mixer coupled to a local oscillator.
Capacitor
At one point a small slice is cut out of the center conductor, therefore a capacitor is formed and low frequencies are blocked. This kind of capacitor has the advantage that it is nearly invisible to higher frequencies. To pass frequencies down to 1 MHz the capacitance has to be increased. A dielectric like NPO multiplies the capacitance by a factor of 65. The thickness of the capacitor has to be minimal without leading to electric breakdown in the dielectric, this means to avoid any peaks in the electric field and this means smooth electrodes with rounded edges and a dielectric protruding between the electrodes (doorknob design). A stack of capacitors can be used, but every capacitor needs access to the surface of the inner conductor, because if it's hidden behind another capacitor the high frequencies won't see it, because the electric field needs a lot of time to travel through a dielectric with a high dielectric constantCoil
A small coil made of fine wire with an air core or MnFeZn-core connects the inner conductor of one of the sides of the capacitor with the a port in the outer conductor leading down the T. Frequencies above 1 GHz hit the coil from the side and apply an equal electric field to the whole coil. Therefore no higher modes are excited within the coil. Because of the inductiveness of the coil almost no current leaks from the center conductor to the port. Frequencies between 1 MHz and 1 GHz do leak into this port, so there is a second coil with a cone shaped core outside of the outer conductor, but inside of a housing to avoid interference with other components. This cone acts like a tapered transmission line transformer. It starts with a high impedance, so a lot of power will be reflected, but the rest will travel down the coil and there is some leakage into the low frequency port.Oscillations
Any oscillations in the capacitor or the coil or the composed LC circuitLC circuit
An LC circuit, also called a resonant circuit or tuned circuit, consists of an inductor, represented by the letter L, and a capacitor, represented by the letter C...
are damped by the dielectric and the core. Also the small coil should have about 10 ohm resistance to further damp oscillations and avoid ripple on the transmitted spectrum.