Grid bias
Encyclopedia
Grid bias is a DC
voltage applied to electron tubes
(or valves in British English) with three electrodes or more, such as triodes. The control grid (usually the first grid) of these devices is used to control the electron flow from the heated cathode
to the positively charged anode
. Bias point in small-signal applications is set to minimize distortion
and achieve sufficiently low power draw. In high-power applications, biasing is typically set for maximum available output power or voltage, with a secondary target of either low distortion or high efficiency.
Bias voltage is obtained through:
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...
voltage applied to electron tubes
Vacuum tube
In electronics, a vacuum tube, electron tube , or thermionic valve , reduced to simply "tube" or "valve" in everyday parlance, is a device that relies on the flow of electric current through a vacuum...
(or valves in British English) with three electrodes or more, such as triodes. The control grid (usually the first grid) of these devices is used to control the electron flow from the heated cathode
Cathode
A cathode is an electrode through which electric current flows out of a polarized electrical device. Mnemonic: CCD .Cathode polarity is not always negative...
to the positively charged anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
. Bias point in small-signal applications is set to minimize distortion
Distortion
A distortion is the alteration of the original shape of an object, image, sound, waveform or other form of information or representation. Distortion is usually unwanted, and often many methods are employed to minimize it in practice...
and achieve sufficiently low power draw. In high-power applications, biasing is typically set for maximum available output power or voltage, with a secondary target of either low distortion or high efficiency.
- In a typical voltage amplifier, including power stages of most audio power amplifiers, DC bias voltage is negative relative to cathode potential. Instant grid voltage (sum of DC bias and AC input signal) should never rise above cathode potential to prevent grid-to-cathode currents that overload preceding amplifier stages and may cause severe even-order distortion. High transconductance tubes develop significant grid currents even with small negative bias; in these cases, maximum instant voltage ceiling is lowered to -1.0..-0.5 VoltVoltThe volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
. - High efficiency Class B+ push-pull amplifiers operate at higher bias points (near zero or even positive values). These designs take care of grid currents through the use of cathode followers or interstage transformers easing current load on the driver stages, and deep negative feedbackNegative feedbackNegative feedback occurs when the output of a system acts to oppose changes to the input of the system, with the result that the changes are attenuated. If the overall feedback of the system is negative, then the system will tend to be stable.- Overview :...
to minimize distortion. - High power transmitter tubes (oscillators and modulators) are frequently positively biased to maximize radio frequency output. Distortion is minimized by using band-pass filterBand-pass filterA band-pass filter is a device that passes frequencies within a certain range and rejects frequencies outside that range.Optical band-pass filters are of common usage....
loads tuned to the desired radio frequency.
Bias voltage is obtained through:
- An external voltage source (fixed bias) - a battery or a dedicated DC power supply. When the cathode potential is raised above ground (as in cascodeCascodeThe cascode is a two-stage amplifier composed of a transconductance amplifier followed by a current buffer. Compared to a single amplifier stage, this combination may have one or more of the following characteristics: higher input-output isolation, higher input impedance, high output impedance,...
circuits), bias voltage is obtained by tapping into main (positive) plate power supply. - Automatic bias or self bias - using a cathode resistorResistorA linear resistor is a linear, passive two-terminal electrical component that implements electrical resistance as a circuit element.The current through a resistor is in direct proportion to the voltage across the resistor's terminals. Thus, the ratio of the voltage applied across a resistor's...
to raise cathode potential above grid (tied to ground) and stabilize plate current; - Grid leak bias - diverting DC grid current through a high value grid resistor.