What is the purpose of a bias resistor?

Question

Could someone explain what "bias" is.

And why do some devices need an external bias resistor?

Answer 1

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

Answer 2

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.

Answer 3

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).

Answer 4

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.

Answer 5

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

Answer 6

Summary

If the driving stage output is of 'relatively low' impedance (and is thus more of a voltage source in actuality) the series resistor must be present to convert it to a current source, just like in any other case of driving a diode or generalized low-impedance target. Quite often for instance, current is set through an LED to meet its specifications in exactly this way.

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Whatever source or sink of current (output) is driving the transistor (an open collector / level shifter, a CMOS output, what have you) is ultimately to be driving such current to a silicon diode (or two in the Darlington case), for all it knows of the base-emitter junction(s) of the transistor(s) in question. Speaking of which [plural case], when power transistors are to be paralleled, it may be more important to degenerate their emitters to prevent fallen hero syndrome, a.k.a. thermal runaway.

Barring excessive resistance values and low current gains, it has little do with current gain, as any such design case would fundamentally tend to include transistor saturation on the real-time operational menu (i.e., using as a switch or other high-gain or logic-drive scenario rather than a linear amplifier) and the principal pitfall would be excessive resistance which could choke the transistor at an unreliable threshold, short of saturation (generally non-applicable in Darlington case), and this would be load-dependent (as a consequence of hFE), probably thermally adverse, and not CMOS-input-compliant. Analog or mixed (ADC) detection could probably make sense of it though, but now I'm headed out on a tangent based on random and frivolous guesses about your application.

The series resistance can also provide a certain meaning to Miller capacitance (collector-base) in the same way (or reveal the parasitic Miller cap) via the impedance increase, which could be good or bad but sort of brings linear amplification into the realm of inclusion of a series base resistor, particularly where interstage/open-loop gains are high and a slew-rate limitation must be imposed to avoid spontaneous oscillation. An editorial here might open by stating a firm belief that one has not applied a particularly linear discipline of spectral amplification if this is proven necessary, but that's for another time, and there are those who feel differently. It should suffice to say that in linear circuits this series resistance could reduce the gain-bandwidth product by giving more potency to Miller capacitance, as in any case where [parasitic] capacitances lurk in an increased-impedance environment.

Answer 7

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.