A negative clamper shifts its input waveform in a negative direction, so that it lies below a dc reference voltage. The direction of the diode determines whether the circuit is a positive or negative clamper. Clamper operation is based on the concept of switching time constants. Circuit Diagram Input Output 7. Explain Varactor diode with its characteristic curves. (3 marks) (June 2014) Varactor [also called varicap, VVC (voltage-variable capacitance), or tuning] diodes are semiconductor, voltage-dependent, variable capacitors. Their mode of operation depends on the capacitance that exists at the p-n junction when the element is reverse biased. Under reverse-bias conditions, it was established that there is a region of uncovered charge on either side of the junction that together the regions make up the depletion region and define the depletion width Wd. As the reverse-bias potential increases, the width of the depletion region increases, which in turn reduces the transition capacitance. The characteristics of an typical commercially available varicap diode in figure above. Note the initial sharp decline in CT with increase in reverse bias. The range of VR for VVC diodes is limited to about 20 V. When reverse voltage across a varactor diode is increased, the width Wd of the depletion layer increases. Therefore, the total junction capacitance CT of the junction decreases. On the other hand, if the reverse voltage across the diode is lowered, the width Wd of the depletion layer decreases. Consequently, the total junction capacitance CT increases. 8. Explain how transistor can be used as switch (5 marks) (Dec 2013/ Dec 2014 2015) Saturation region, - switch closed, Cut-off region – switch open, with Explanation of Propagation delay, Timing state diagram. state switches are one of the main applications for the use of Transistors. In the application of transistor as a switching element, the device is operated alternately between the cut-off condition and the saturation condition the transistor is allowed to make a transition from cut-off to saturation state and vice-versa by using a pulse wave form. Consider Cut-off characteristics The input and base are grounded (0V) , Base-emitter voltage VBB< 0.7V , Base-emitter junction is reverse biased.
Figure: SCR Characteristics It is the curve between anode-cathode voltage (V) and anode current (I) of an SCR at constant gate current. Fig. 20.7 shows the V-I characteristics of a typical SCR. Forward characteristics. When anode is positive w.r.t. cathode, the curve between V and I is called the forward characteristic. In Fig. 20.7, OABC is the forward characteristic of SCR at IG = 0. If the supply voltage is increased from zero, a point is reached (point A) when the SCR starts conducting. Under this condition, the voltage across SCR suddenly drops as shown by dotted curve AB and most of supply voltage appears across the load resistance RL. If proper gate current is made to flow, SCR can close at much smaller supply voltage. Reverse characteristics. When anode is negative w.r.t. cathode, the curve between V and I is known as reverse characteristic. The reverse voltage does come across SCR when it is operated with a.c. supply. If the reverse voltage is gradually increased, at first the anode current remains small (i.e. leakage current) and at some reverse voltage, avalanche breakdown occurs and the SCR starts conducting heavily in the reverse direction as shown by the curve DE. This maximum reverse voltage at which SCR starts conducting heavily is known as reverse breakdown voltage.