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What is Bias?

Latest Updated:03/01/2007

Question:

What is Bias?

Answer:

This originally means to lift up the basic level in order to make processing possible.
For example, when dealing with numbers ranging from -128 to +127 in a program processing, it is also possible to express negative numbers using a range of 0 to 255 (8 bits from 0 to FFH) by defining an offset of 128 (80H) as the bias. In this case, 00H to 80H correspond to -128 to 0, and 81H to FFH correspond to 1 to 127. If the operation result is smaller than 80H, it is a negative number.

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To express negative numbers, one can use 2's complements with MSB regarded as a sign. In this case as well, numbers in the range of -128 to +127 are handled using just 8 bits. When bit 7 (MSB) is 1, this indicates a negative number; 00H to 7FH correspond to 0 to 127, and 80H to FFH correspond to -128 to -1.


The bias in an electric circuit consists of the application of a DC voltage to obtain a practicable state. In amplifiers, to enable operation of the negative voltage part of an AC signal, an offset is applied with a bias voltage to raise it to the positive voltage side. For example, in the case of an A class amplifier consisting of bipolar transistors, an active state is kept by applying a bias voltage to the base to prevent the voltage from fluctuating below 0 V.



The A class amplifier consumes additional power for the bias. In comparison, a B class amplifier, consisting of a push-pull circuit combining an NPN transistor and a PNP transistor, can reduce the power consumption by operating alternately with a positive and negative voltage and synthesizing the output. A push-pull circuit uses two transistors with complementarity (whose characteristics are equivalent while of opposite polarity).



Since the circuit does not operate in a base signal that is closer to 0 V than the on-voltage (on the order of 0.6 V) between the base and emitter of each transistor, in the vicinity of 0 V, the synthesized output waveform of a B class amplifier warps. In the case of an AB class amplifier, the synthesized waveform is kept free of distortion through the application of a bias equivalent to the on-voltage.

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In the case of transistors and vacuum tube amplifiers, there are, in addition to the above A, B, and AB class amplifiers, C class and D class amplifiers.
C class amplifiers, which do not apply a bias to elements, but instead amplify only the high input voltage parts and extract the high-harmonic component with a filter, are used for frequency multiplication circuits, etc.
D class amplifiers perform digital switching by using the saturation region of elements, and are not pure amplifiers.
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