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What is DC biasing? If in terms of how a device is supplied with electricity, it is a method of providing a fixed value of either voltage or current.For instance, a power supply gives a constant value of voltage or current. A parametric analyzer sweep a value, and it is used to obtain the DC transfer function. Now, nothing prevent people using an SMU to bias a circuit (i.e.use it as power supply). If instead of sweeping a voltage (or current) have the SMU set to deliver a fixed value. A SMU provides a voltage and current,it is controlled in a way that you can sweep either. So SMU can be used as a DC supply or to characterize device in DC. From diodes prospective, biasing accounts for: Externally applying a diode a DC voltage source, which is presented by a battery, there will be a pn junction potential formed internally. Thus the current is conducted only in one direction, but blocked in the opposite direction. This is how forward-biased and reverse-biased de_ned.[11] From BJTs prospective, bias-ing means externally providing each terminal with a constant voltage value by externally connecting to a DC voltage source which can be batteries or DC power supplies. Because BJT has three terminals, its DC supply voltages are normally indicated as: VEE, VBB and VCC. ..Figure 8.1: A transistor is characterized by the relationships between currents and voltages at both input and output. The Input Characteristics is the IV characteristics curves that are measured at the emitter base junction.Iin = f(Vin). The Output Characteristics is the IV characteristic curves that are measured at the base collector junction.A SMU provides a voltage and current, it is controlled in a way that you can sweep between specified values. So it can be used as a DC supply or to characterize device in DC. indicated as: VE, VB and VC, respectively. Two junctions voltages VEB and VBC are formed by its three terminals VE VB VC, respectively. For the former junction VEB, forward-biased (VEB > 0) is always needed in order to switch on the device. But for the later junction VBC, both forward-biased (VBC > 0) and reverse-biased (VBC < 0) are possible. The two biasing conditions will result in two complete di_erent operation modes. One is the saturation mode,which is used in digital electronics. The other is the active mode, used in analogue electronics. When VBC is forward-biased(VBC > 0), BJT is used as a switch in digital electronics. When VBC is reverse-biased (VBC < 0), BJT is functioning as an ampli_er in analogue electronics. [11]The established constant values at BJTs three terminals are named as: the bias point or the quiescent point (Q-point) or the DC operating point. The Q-point can be further stabilized against and temperature by replacing the Bias Schemes, such as : voltage divider bias , emitter feedback bias, current source bias...etc. The position of the DC voltage level decides if the waveform of the AC signal that fed in the input port can expand its fully swing( amplification) without being clipped the output port. To put it simply, when a BJT is biased, regardless its operation modes, all its threes terminals are provided with constant values of voltages and currents.[11] From amplifiers prospective, it does not matter what is inside the black box. What does matter is the connection outside the black box.[37] "Biasing" refers to all the external power supply networks or circuits arrangement that connected to its input and output ports to ensure the constant (V, I) levels. Figure 8.2: The _gure on the top left is the input characteristics, which plots IB versus VBE with a constant VCE. Where IB is an exponential function of VBE.The input bias point Qin(VBE, IB) is determined from the intersection between the input load line and the input characteristics. The figure on the top right is the output characteristics, which plots IC versus VCE with IB as the varing parameter. The output bias point Qout(VCE, IC) is determined. The output bias point Qout (VCE, IC) is the intersection between the output load line and the output characteristics. The two schematics below are for DC biasing BJT in CE con_guration. In the measurement of output characteristics, instead of forcing a voltage source VBB to the base, a current source IB is employed. The replacement in the bias current source at the base is to prevent the self-heating effects. 65

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