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UNIT-I Discrete Electronic Circuits – SEIA1301 SCHOOL OF ELECTRICAL & ELECTRONICS ENGINEERING DEPARTMENT OF ELECTRONICS & INSTRUMENTATION UNIT- I Discrete Electronic Circuits – SEIA1301 1.Introduction to BJT Amplifiers BJT amplifiers : CE, CB and CC amplifiers - multistage amplifiers - differential amplifier - designing BJT amplifier networks(analysis using hybrid -π model) FET amplifiers : CS, CG and CD amplifiers -designing FET amplifier networks Frequency response : low frequency response of BJT and FET amplifiers - Miller effect capacitance - high frequency response of BJT and FET amplifiers. 1.1 Common Base Amplifier: The common base amplifier circuit is shown The VEE source forward biases the emitter diode and VCC source reverse biased collector diode. The ac source vin is connected to emitter through a coupling capacitor so that it blocks dc. This ac voltage produces small fluctuation in currents and voltages. The load resistance RL is also connected to collector through coupling capacitor so the fluctuation in collector base voltage will be observed across RL. The dc equivalent circuit is obtained by reducing all ac sources to zero and opening all capacitors. The dc collector current is same as IE and VCB is given by VCB = VCC - IC RC. These current and voltage fix the Q point. The ac equivalent circuit is obtained by reducing all dc sources to zero and shorting all coupling capacitors. r'e represents the ac resistance of the diode as shown in Fig. Figure 1.1 Common base diagram 2 Figure 1.2 Equivalent Circuit diagram of CB Fig. shows the diode curve relating IE and VBE. In the absence of ac signal, the transistor operates at Q point (point of intersection of load line and input characteristic). When the ac signal is applied, the emitter current and voltage also change. If the signal is small, the operating point swings sinusoidally about Q point (A to B). Figure 1.3 Characteristics of CB If the ac signal is small, the points A and B are close to Q, and arc A B can be approximated by a straight line and diode appears to be a resistance given by If the input signal is small, input voltage and current will be sinusoidal but if the input voltage is large then current will no longer be sinusoidal because of the non linearity of diode curve. The emitter current is elongated on the positive half cycle and compressed on negative half cycle. Therefore the output will also be distorted. 3 r'e is the ratio of ΔVBE and Δ IE and its value depends upon the location of Q. Higher up the Q point small will be the value of r' e because the same change in VBE produces large change in IE. The slope of the curve at Q determines the value of r'e. From calculation it can be proved that. r'e = 25mV / IE 1.2 Common Collector Amplifier: If a high impedance source is connected to low impedance amplifier then most of the signal is dropped across the internal impedance of the source. To avoid this problem common collector amplifier is used in between source and CE amplifier. It increases the input impedence of the CE amplifier without significant change in input voltage. Fig. shows a common collector (CC) amplifier. Since there is no resistance in collector circuit, therefore collector is ac grounded. It is also called grounded collector amplifier. When input source drives the base, output appears across emitter resistor. A CC amplifier is like a heavily swamped CE amplifier with a collector resistor shorted and output taken across emitter resistor. vout = vin - vBE Figure 1.4 Dc load line Therefore, this circuit is also called emitter follower, because VBE is very small. As vin increases, vout increases.If vin is 2V, vout = 1.3V. If vin is 3V, vout = 2.3V. Since vout follows exactly the vin therefore, there is no phase inversion between input and output. The output circuit voltage equation is given by VCE = VCC – IE RE Since IE >>IC IC = (VCC – VCE ) / RE 4 AC Load line:Consider the dc equivalent circuit Figure 1.5 Equivalent circuit Assuming IC = IC(approx), the output circuit voltage equation can be written as The slop of the d.c load line is . When considering the ac equivalent circuit, the output impedance becomes RC || RL which is less than (RC +RE). In the absence of ac signal, this load line passes through Q point. Therefore ac load line is a line of slope (-1 / ( RC || RL) ) passing through Q point. Therefore, the output voltage fluctuations will now be corresponding to ac load line as shown in fig. Under this condition, Q-point is not in the middle of load line, therefore Q-point is selected slightly upward, means slightly shifted to saturation side. Figure 1.6 AC and DC Load Line 5 2.Types of Coupling: In a multistage amplifier the output of one stage makes the input of the next stage. Normally a network is used between two stages so that a minimum loss of voltage occurs when the signal passes through this network to the next stage. Also the dc voltage at the output of one stage should not be permitted to go to the input of the next. Otherwise, the biasing of the next stage are disturbed. The three couplings generally used are. 1. RC coupling 2. Impedance coupling 3. Transformer coupling. 2.1 RC coupling: Fig. shows RC coupling the most commonly used method of coupling from one stage to the next. An ac source with a source resistance R S drives the input of an amplifier. The grounded emitter stage amplifies the signal, which is then coupled to next CE stage the signal is further amplified to get larger output. In this case the signal developed across the collector resistor of each stage is coupled into the base of the next stage. The cascaded stages amplify the signal and the overall gain equals the product of the individual gains. Figure 1.7 Circuit Diagram for RC Coupling The coupling capacitors pass ac but block dc Because of this the stages are isolated as for as dc is concerned. This is necessary to avoid shifting of Q-points. The drawback of this approach is the lower frequency limit imposed by the coupling capacitor. The bypass capacitors are needed because they bypass the emitters to ground. Without them, the voltage gain of each stage would be lost. These bypass capacitors also place a lower limit on the frequency response. As the frequency keeps decreasing, a point is reached at which capacitors no longer look like a.c. shorts. At this frequency the voltage gain starts to decrease because of the local feedback and the overall gain of the amplifier drops significantly. These amplifiers are suitable for frequencies above 10 Hz. 6 2. 2 Impedance Coupling: At higher frequency impedance coupling is used. The collector resistance is replaced by an inductor as shown in fig. As the frequency increases, XL approaches infinity and each inductor appears open. In other words, inductors pass dc but block ac. When used in this way, the inductors are called RFchokes. Figure 1.8 Circuit Diagram for Impedance Coupling The advantage is that no signal power is wasted in collector resistors. These RF chokes are relatively expensive and their impedance drops off at lower frequencies. It is suitable at radio frequency above 20 KHz. 2.3 Transformer Coupling: In this case a transformer is used to transfer the ac output voltage of the first stage to the input of the second stage. Fig,, the resistors RC is replaced by the primary winding of the transformer. The secondary winding is used to give input to next stage. There is no coupling capacitor. The dc isolation between the two stages provided by the transformer itself. There is no power loss in primary winding because of low resistance. Figure 1.9 Circuit Diagram for Transformer Coupling 7 At low frequency the size and cost of the transformer increases. Transformer coupling is still used in RF amplifiers. In AM radio receivers, RF signal have frequencies 550 to 1600 KHz. InTV receivers, the frequencies are 54 to 216 MHz. At these frequency the size and cost of the transformer reduces. CS capacitor is used to make other point of transformer grounded, so that ac signal is applied between base and ground. 2.4 Tuned Transformer Coupling: In this case a capacitor is shunted across primary winding to get resonance as shown in fig. At this frequency the gain is maximum and at other frequencies the gain reduces very much. This allows us to filter out all frequencies except the resonant frequency and those near it. This is the principle behind tuning in a radio station or TV channel. Figure 1.10 Circuit Diagram for Transformer Coupling for Tuned circuits 3.Differential Amplifiers: Differential amplifier is a basic building block of an op-amp. The function of a differential amplifier is to amplify the difference between two input signals. How the differential amplifier is developed? Let us consider two emitter-biased circuits as shown in fig. Figure 1.11 Differential Amplifier 8 The two transistors Q1 and Q2 have identical characteristics. The resistances of the circuits are equal, i.e. RE1 = R E2, RC1 = R C2and the magnitude of +VCC is equal to the magnitude of –VEE. These voltages are measured with respect to ground. To make a differential amplifier, the two circuits are connected as shown in fig. The two +VCC and –VEE supply terminals are made common because they are same. The two emitters are also connected and the parallel combination of RE1 and RE2 is replaced by a resistance RE.
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