# What is the purpose of a bias resistor?

Could someone explain what "bias" is.

And why do some devices need an external bias resistor?

If your device is powered from a single voltage and ground, it can't output anything below ground. In order to reproduce a signal that varies between +V and -V, you need to shift it upwards so that it varies from 0 to +2V instead. The DC offset is the bias.

http://en.wikipedia.org/wiki/Biasing

• I've just been working with an IC that does this, now I understand why! But how do you "undo" the bias at the other end to move the signal back to +V/-V? – Malvineous Mar 27 '10 at 8:09
• Use a coupling capacitor with a bias resistor to ground. :) – endolith Mar 27 '10 at 12:53
• To expand on endolith's comment, capacitors block DC and pass AC. So if you put a capacitor in series with the output, it will block the DC bias. In combination with a resistor to ground (on the non-biased side of the capacitor!), it will set the corner frequency of the filter, adjusting how much AC will pass. A higher resistance will lower the corner frequency, allowing more AC to pass. The formula for corner frequency is 1/2piRC. – ajs410 Apr 21 '10 at 19:12

The bias is the operating point.

For a bipolar transistor (BJT) the bias resistor will maintain enough current into the base so that the transistor is neither saturated (fully on) or cut-off (fully off).

Some BJTs come with an internal bias resistor to reduce the parts count in a design. If you are switching BJTs on or off you don't need a "bias" resistor but you may need a resistor to limit the current into the base.

In analogue working, transistors (and before that, valves (or vacuum tube devices)) do not have a fully linear response, ie the output is not exactly proportional to the input over the full operating range. If you are wanting a linear response, you move the input signal into the middle of the linear part of the operating range by using bias resistors (and you restrict the input signal such that it does not go outside the linear range).

For the AC input signal to be amplified correctly by the transistor,so that there is proper flow of zero signal collector current and the maintaince of proper collector-emmitter voltage during the passage of signal.

Biasing resistors are also used in RS485 interface.

There are two signals in the RS485 - A and B (some people call these TRX+ and TRX-). And the RS485 transceiver outputs signal to the UART controller according to the difference between A and B as followed:

• A-B > 150mV: outputs High
• A-B < -150mV: outputs Low

If the A-B is between -150mV and +150mV, the output state is unpredictable. So the biasing resistors are required in RS485 circuit.

The biasing resistors in the RS485 circuit keep the A and B signal line a High or Low level in RS485 idle state. (See the below.)

• A: should stays high state
• B: should stays low in idle state

The answer is no, no one who understands electronics well enough to design, build, or manipulate circuits can answer an easy, common question in layman's terms. Example of biasing: A Fender guitar amp uses biasing on their dual power output tubes. The reason is that, many times you can build 2 items that seem or look identical but when you put them into operation they vary from one another. The "biasing" is like an adjustment to pair them into similarity so they both operate at the same degree or position as they move throughout their linear ranges together. Be advised that an O2 sensor ahead or behind an automotive catalytic converter that becomes "biased" rich or lean means that it is "stuck". A different deal. In this case a lower voltage reading, .9V would be rich meaning the brain is telling the injectors to stay open a few milliseconds too long, while a higher voltage, 9V indicates biased lean, where the injectors don't open long enough. Correct would be a varying voltage between .9V and 9V, flip-flop every second or two.