CHAPTER-1 (Lecture-1 & 2) 1.1The The Diode as a Circuit Element Diodes are referred to as non-linear linear circuit elements because of the diode characteristic curve i.e. = ( -1) 1) ...(1.1) Where =reverse saturation current in the range of pA for low-power low power diode. = is thermal voltage(about 26 mV at room ttemperature, emperature, T = 300 K). =ideality factor(1< <2). Figure 1.1: a) Circuit symbol for a diode and b) current current versus voltage for a semiconductor diode. For most applications the non-linear non linear region can be avoided and the device can be modeled by piece-wise wise linear circuit elements. Qualitatively we can just think of an ideal diode has having two regions: a conduction region of zero resistance and an infinite resistance non-conduction non region. For many circuit applications, this ideal diode diode model is an adequate representation of an actual diode and simply requires that the circuit analysis be separated into two parts: forward
current and reverse current. Figure 1.1 shows a schematic symbol for a diode and the currentvoltage curve for an ideal diode. Figure 1.1: a) Schematic symbol for a diode and b) current versus voltage for an ideal diode. A diode can more accurately be described using the equivalent circuit model shown in figure 1.2. ) in series with a If a diode is forward biased with a high voltage it acts like a resistor ( voltage source ( ). For reverse biasing, it acts simply as a resistor ( ). These approximations are referred to as the linear element model of a diode. Figure 1.2: Equivalent circuit model of a junction diode. 1.2 Load line concept of Diode: The applied load will normally have an important impact on the behavior of a device. If the analysis is performed in a graphical manner, a line can be drawn on the characteristics of the device that represents the applied load. The intersection of the load line with the characteristics will determine the point of operation of the system. Such an analysis is called load-line analysis. Consider the network of Fig. 1.2 using a diode having diode voltage , resistor R and a voltage source . Due to the voltage source a current is established through a series circuit in clock wise manner. The current direction and the defined direction of conduction of the diode is matched so the diode is in the “on” state and conduction has been established.
(a) (b) Figure 2.1: Series diode configuration (a) circuit (b)) characteristics. characteristics Applying Kirchhoff’s voltage law to the series circuit of Fig. 2.1 will result in -R - =0 =R ...(2.1) The intersections of the load line on the characteristics can easily be determined if one simply employs the fact that anywhere on the horizontal axis =00 A and anywhere on the vertical axis = 0 V. when =0 , then the above equation will be =0+R = ...(2.2) Similarly when = = =0, then the equation 2.1 will be + (0A) R | ...(2.3)
(a) (b) Figure 2.2 :(a)load load line graph of the above network (b) intersection of the load line with the characteristics plot Example on load line analysis: analysis 1.3 Clipping (limiting) circuit: