Many solid-state devices, like the earlier the earlier vacuum tubes that they have largely replaced, require a unvarying dc voltage applied at one or more specific terminals. This is known as a bias. In an amplifier, for example, the voltage coexists with the varying ac signal that is applied at the same terminal. With the ac signal at ground potential (that is, with no ac signal applied), the amount of bias for a given device in a particular circuit configuration is termed the bias, quiescent or Q-point. The power supply provides these dc voltages as needed throughout the electronic equipment.
Bias voltages and currents aren’t necessarily dc. An ac bias is applied to the recording head in a magnetic tape recorder. This is 40-150 kHz, and its purpose is to both operate the tape substantially within the region in which its response is linear and to randomize any magnetic patterns that remain on the tape from previous recordings.
DC bias at a device terminal in an electronic circuit facilitates correct operation of a semiconductor by forward- or reverse-biasing the junction to which it is applied as needed. In vacuum tubes, a much higher grid bias was required. A greater bias would suppress crossover distortion but would shorten component life.
Here R1 and R2 form a voltage divider applying a dc voltage to the transistor base. A first-pass analysis might ignore the bias contribution from the transistor base current Ib because current running through R2 is generally designed to be on the order of 10X that of Ib, as a way of swamping out operating point variations caused by temperature and transistor beta.
In a semiconductor acting as an amplifier, if the dc bias added to the ac signal applied at the input is too high, the device will be driven into saturation, causing clipping, which can be seen in the display of an oscilloscope connected at the output. In an audio amplifier output, this distortion will take various forms depending on the fundamental frequency and the harmonic layout.
An example of clipping, possibly caused by improper input biasing.
Device performance is highly temperature-dependent. And a bias level that appears correct at first may in the overall equipment environment exhibit unanticipated saturation or cutoff. If thermal runaway occurs, the device will fail.
The bipolar transistor has been replaced in the majority of applications by the metal-oxide semiconductor field effect transistor (MOSFET). This device may be either discrete or part of an integrated circuit (IC). DC current sources are generally used for biasing a MOSFET that is used as an amplifier in an IC. Here again operating temperature, as well as utility or branch-circuit voltage fluctuations, may have to be factored in. In selecting bias voltage, care must be taken to balance high performance and long operating life.
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