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What is Bias Voltage?

Paul Scott
Paul Scott

Bias voltage typically refers to an electrical charge applied to the gate or base of a transistor or electronic vacuum tube. This applied voltage causes the component to become conductive, thereby passing electrical current through its collector and emitter stages. The amount of current passed through the component is also directly related to the amount of bias voltage applied. Depending on the type of transistor, this voltage is either positive or negative, and is referred to as a forward or reverse bias. These voltages are, therefore, used to switch electronic components on or off, keep them active, or control the amount of current passing through them.

Transistors typically have three points: a collector, an emitter, and a base or gate. The path between the collector and emitter may be seen as the road used by electrical current to pass through the transistor. The base would then be a turnstile or gate across that road used to control the traffic, and the bias voltage would be the guard that opens the gate to allow traffic to pass and controls the volume of traffic passing through. The direction of current flow is also a variable factor in transistor theory; common base, collector, or emitter configurations allow for various end results to be attained. In all configurations though, it is this voltage that marshals the flow of traffic.

Bias voltage is an electrical charge applied to the base of transistors.
Bias voltage is an electrical charge applied to the base of transistors.

By controlling this voltage, circuit designers can not only allow for switching functions with transistors and vacuum tubes, but they can also direct the amount of current passed to other parts of the circuit. This characteristic allows the gain factor or amplification of a transistor to be very finely controlled. Whether the transistor is switched on by the applied voltage or kept off until such time as it needs to conduct can be controlled by applying forward or reverse bias voltages. This effect is achieved by adjusting the polarity of the applied voltage, which allows for a large margin of flexibility in how transistors are applied in electronic circuits.

Transistors and vacuum tubes are some of the most basic and fundamentally important building blocks of the science of electronics. Their unique, flexible switching or current controlling characteristics are critical to most electronic circuits and have formed the basis of much of the advancement that has been made in the field of electronic engineering. This extremely useful controllable switching ability is made possible by the controlled application of bias current and makes it an essential part of transistor theory.

Discussion Comments


Yeah, it's a bit hard for beginners to understand bias voltage, but i bias voltage is that voltage which makes the current flow in the emitter and collector and makes the circuit conductive.


@GlassAxe- The bias voltage definition can be difficult to understand, especially for the uninitiated. In the application you are exploring in your class, bias voltage is a very important factor in the implementation of HVDC technology. HVDC has been around since at least the 1930s but has never really been a viable option because the American grid infrastructure has always been dependent on AC current. It was not until the switching techniques developed in the 1970s that it was easy and cost effective to switch between AC and DC current.

Anyway, interconnecting an HVDC system to the existing AC electricity grid requires conversion when switching AC to DC, and Inversion when switching DC to AC. Conversion requires the use of special circuits called rectifiers and inversion requires circuits called inverters. Think of it in the terms of an inverter used to convert your car's DC current to an AC outlet so you can use your laptop on the road. Together, the generic term converters refer to inverters and rectifiers.

Converters work by using switching devices called valves. Valves can be controlled, or non-controlled. Non-controlled valves act as diodes and are "on" when the switch is opened and forward biased (voltage is positive). When the converter is “off”, the switch is closed, and the voltage is negative.

Controlled valves do the same, but they require a gate pulse to turn the converter on. Besides the gate being pulsed, everything about the operation of the switch remains the same.


What is DC bias voltage, and what does it have to do with converting AC to DC voltage and vice versa? We were discussing the process of upgrading our electricity grid and a somewhat technical discussion over the transmission system came up.

We were discussing the pros and cons of high voltage alternating current lines (HVAC) versus high voltage direct current lines (HVDC). HVDC lines are much more efficient, but they are not easily converted to AC power, so they have trouble servicing communities located along these long distance HVDC lines.

Someone (one of the engineering grad students) was talking about the need for DC bias voltage to split the voltage signal either into a positive or negative signal so it can be easily converted between DC and the AC currents that are used at the consumer level. I just don't really understand what a bias voltage is and how it works.


I find it amazing that things like voltage amplifiers, transistors, and electronic vacuum tubes were ever conceived. The inventions of these tiny electrical components have been partially responsible for the greatest technological advancements in our society. None of the computers, phones, televisions, and other devices that are so central to modern life would be possible without the brilliant minds who designed these things.

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    • Bias voltage is an electrical charge applied to the base of transistors.
      By: yurazaga
      Bias voltage is an electrical charge applied to the base of transistors.