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MAHATMA GANDHI INSTITUTE FOR TECHNICAL RESEARCH AND EDUCATION Guided by:Anita Mam Sapna Mam TWO LOAD LINE OF VDB AMPLIFIER Fig (a) shows Voltage Divider Biased (VDB) amplifier. In VDB two resistors R1&R2 is used instead of VBB. In dc circuit there is no capacitor. For finding ,VBB = R2 *Vcc R1+R2 Fig (b) shows ac equivalent circuit. As emitter in ac gnd so no effect. a b DC LOAD LINES & AC LOAD LINES:The Saturation &Cut off points are different in ac &dc load line. The collector &Emitter resistance are less than dc resistance. The ac &dc load lines intersect at Q point. DC LOAD LINE: Q point vary by R2. R2>>Transistor in Saturation ¤t given as Ic(sat) = Vcc RC+RE R2<< Transistor in Cutoff & vtg given as VCE(cut off) = Vcc. Fig (c) shows DC Load Line. c AC LOAD LINE: The Collector current is IC= ICQ + VCEQ – VCE rc Transistor in saturation, VCE=0rc ic(sat)=ICQ + VCEQ rc Transistor in cut off , Ic=0 vce(cut off) = VCEQ + ICQrc d The ac load line has higher slope than dc load line so max. peak to peak o/p always less then Vcc. Fig(d) shows AC Load Line. CLASS A OPERATIONAL AMPLIFIER Class A operation of an amplifier means that the transistor operates in the active region at all times. Fig(a) shows Class A Amplifier. Output is not clipped. Collector current flows for 360° throughout the cycle. Some parameters useful in Class A Amplifiers are Power Gain Output Power, etc. With a class A amplifier, the designer usually tries to locate the Q point somewhere near the middle of the load line so signal do not distorted. In output we get the signal amplified . The efficiency is 25%. DEFINATIONS:1. OUTPUT POWER Fig (a)The output power is given by, Pout = Vrms2 RL Usually, we measure the output voltage in peak-to-peak volts with an oscilloscope. In this case, a more convenient equation to use for output power is: Pout=Vout2 8RL Fig(b) The maximum output power is given by, P(out)=MPP2 8RL 2. TRANSISTOR POWER DISSIPATION: When no signal drive amplifier then , PDQ = VCEQICQ. When signal present the power dissipation of transistor decrease bze some power converts in signal power. 3. CURRENT DRAIN: The dc voltage source supply a dc current Idc to the amplifier. This dc current has two components: the biasing current through the voltage divider and the collector current through the transistor. The dc current is called the current drain of the stage. 4. EFFICIENCY:The dc power supplied to an amplifier by the dc source is: Pdc = VCCIdc. The efficiency is defined as n=Pout *100% Pdc The efficiency of any amplifier is between 0 and 100 percent. The higher the efficiency, the better the amplifier is at converting dc power to ac power. CLASS B OPERATIONAL PUSH PULL AMPLIFIER Class B operation is different. It means that collector current flows for only half the cycle (180°), as shown in Fig. designer locates the Q point at cutoff. Then, only the positive half of ac base voltage can produce collector current. This reduces the wasted heat in power transistors. Push-pull means that one transistor conducts for half a cycle while the other is off, and vice versa. On the positive half cycle of input voltage, the secondary winding of T1 has voltage v1 and v2, as shown. Therefore, the upper transistor (Q1)conducts and the lower one(Q2) cuts off. The collector current through Q1 flows through the upper half of the output primary winding. This produces an amplified and inverted voltage, which is transformer-coupled to the loudspeaker. On the next half cycle of input voltage, the polarities reverse. Now, the lower transistor (Q2) turns on and the upper transistor(Q1) turns off. The lower transistor amplifies the signal, and the alternate half cycle appears across the loudspeaker. Since each transistor amplifies one-half of the input cycle, the loudspeaker receives a complete cycle of the amplified signal. Advantages and Disadvantages:Advantages are as below Each transistor is at cutoff when there is no input signal Improved efficiency where there is an input signal. The maximum efficiency of a class B push-pull amplifier is 78.5 %. Disadvantage are as below use of transformers. Audio transformers are bulky and expensive. So nowadays not used more.