![]() Types of bias circuit for class-A amplifiers The circuits below primarily demonstrate the use of negative feedback to prevent thermal runaway. Runaway is then impossible because increasing collector current leads to a decrease in dissipated power this notion is known as the half-voltage principle. The transistor can be biased so that its collector is normally less than half of the power supply voltage, which implies that collector–emitter power dissipation is at its maximum value.Heat sinks can be used that carry away extra heat and prevent the base–emitter temperature from rising.Hence, the increasing collector current throttles its source. Negative feedback can be built into the biasing circuit so that increased collector current leads to decreased base current.There are several approaches to mitigate bipolar transistor thermal runaway. This deleterious positive feedback results in thermal runaway. ![]() Depending on the bias point, the power dissipated in the transistor may also increase, which will further increase its temperature and exacerbate the problem. By the Ebers–Moll model, if the base–emitter voltage V BE is held constant and the temperature rises, the current through the base–emitter junction I B will increase, and thus the collector current I C will also increase. Often, the Q-point is established near the center of the active region of a transistor characteristic to allow similar signal swings in positive and negative directions.įor digital operation, the Q-point is instead chosen so the transistor switches from the "on" (saturation) to the "off" (cutoff) state.Īt constant current, the voltage across the emitter–base junction V BE of a bipolar transistor decreases by 2 mV (silicon) and 1.8 mV (germanium) for each 1 ☌ rise in temperature (reference being 25 ☌). The leakage current also increases with temperature.Ī bias circuit may be composed of only resistors, or may include elements such as temperature-dependent resistors, diodes, or additional voltage sources, depending on the range of operating conditions expected.įor analog operation of a class-A amplifier, the Q-point is placed so the transistor stays in active mode (does not shift to operation in the saturation region or cut-off region) across the input signal's range.Both gain and base–emitter voltage depend on the temperature.Due to the Early effect, the current gain is affected by the collector–emitter voltage.The gain of a transistor can vary significantly between different batches, which results in widely different operating points for sequential units in serial production or after replacement of a transistor.The operating point of a device, also known as bias point, quiescent point, or Q-point, is the point on the output characteristics that shows the DC collector–emitter voltage ( V ce) and the collector current ( I c) with no input signal applied.Ī bias network is selected to stabilize the operating point of the transistor, by reducing the following effects of device variability, temperature, and voltage changes: By selecting the proper resistor values, stable current levels can be achieved that vary only little over temperature and with transistor properties such as β. The voltage divider configuration achieves the correct voltages by the use of resistors in certain patterns. Much more elaborate biasing arrangements are used in integrated circuits, for example, bandgap voltage references and current mirrors. In circuits made with individual devices (discrete circuits), biasing networks consisting of resistors are commonly employed. Process necessary for BJT amplifiers to work correctly A load line diagram, illustrating an operating point in the transistor's active regionīipolar transistors must be properly biased to operate correctly.
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