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Compound Configurations Cascade Connections CASCADED SYSTEMS Important Characteristics: The output of one amplifier is the input to the next amplifier The overall voltage gain is determined by the product of gains of the individual stages The DC bias circuits are isolated from each other by the coupling capacitors The DC calculations are independent of the cascading The AC calculations for gain and impedance are interdependent two-port systems approach AvNL = voltage gain (no-load value) Thevenin’s Theorem. Thevenin’s Theorem states that any complicated network across its load terminals can be substituted by a voltage source with one resistance in series. Thevenin’s Theorem. Thevenin’s Theorem states that any complicated network across its load terminals can be substituted by a voltage source with one resistance in series. CASCADED SYSTEMS The two-port systems approach is particularly useful for cascaded systems such as that appearing in Fig. 5.67 , where Av1 , Av2 , Av3 , and so on, are the voltage gains of each stage under loaded conditions. CASCADED SYSTEMS That is, Av1 is determined with the input impedance to Av2 acting as the load on Av1 . For Av2 , Av1 will determine the signal strength and source impedance at the input to Av2 . The total gain of the system is then determined by the product of the individual gains as follows: and the total current gain is given by: EXAMPLE 5.14 The two-stage system of Fig. 5.68 employs a transistor emitter-follower configuration prior to a common-base configuration to ensure that the maximum percentage of the applied signal appears at the input terminals of the common-base amplifier. In Fig. 5.68 , the no-load values are provided for each system, with the exception of Z i and Z o for the emitter-follower, which are the loaded values. For the configuration of Fig. 5.68 , determine: EXAMPLE 5.14 a. The loaded gain for each stage. b. The total gain for the system, A v and Avs . c. The total current gain for the system. d. The total gain for the system if the emitter-follower configuration were removed. Notes: loaded gain and input voltage EXAMPLE 5.14 Solution: EXAMPLE 5.14 Solution: EXAMPLE 5.14 Solution: EXAMPLE 5.14 Solution: In total, therefore, the gain is about 25 times greater with the emitter-follower configuration to draw the signal to the amplifier stages. CASCADED SYSTEMS One popular connection and the name is derived from the capacitive coupling capacitor C c and the fact that the load on the first stage is an RC combination. The coupling capacitor isolates the two stages from a dc viewpoint but acts as a short-circuit equivalent for the ac response. CASCADED SYSTEMS What is a RC Coupled Amplifier? A Resistance Capacitance (RC) Coupled Amplifier is basically a multistage amplifier circuit extensively used in electronic circuits. Here the individual stages of the amplifier are connected together using a resistor–capacitor combination due to which it bears its name as RC Coupled. CASCADED SYSTEMS Figure 1 shows such a two-stage amplifier whose individual stages are nothing but the common emitter amplifiers. Hence the design of individual stages of the RC coupled amplifiers is similar to that in the case of common emitter amplifiers in which the resistors R1 and R2 form the biasing network while the emitter resistor RE form the stabilization network. Here the CE is also called bypass capacitor which passes only AC while restricting DC, which causes only DC voltage to drop across RE while the entire AC voltage will be coupled to the next stage. biasing CASCADED SYSTEMS Further, the coupling capacitor CC also increases the stability of the network as it blocks the DC while offers a low resistance path to the AC signals, thereby preventing the DC bias conditions of one stage affecting the other. In addition, in this circuit, the voltage drop across the collector-emitter terminal is chosen to be 50% of the supply voltage VCC in order to ensure appropriate biasing point. Next meeting… Laboratory Activity CASCADED SYSTEMS Laboratory Activity: Laboratory Report I. II. Introduction Objective A. Understand the operation and circuit connection of an RC-Coupled BJT Amplifier. B. Recognize the signal generated per stages of the circuit. C. Apply DC biasing and compute the voltage gain, output voltage and input impedance on each stage of the circuit. III. Data Results A. Input Signal B. Output signal at first stage C. . Output signal at 2nd stage IV. Computation/ Analysis Solve Example 5.15 (see next slide) V. Discussion (Summary) A. B. C. D. VI. VII. What is an RC Coupled Amplifier Operation / Diagram/ Concept Advantages/ Disadvantages Applications Conclusion Reference/s EXAMPLE 5.15 a. Calculate the no-load voltage gain and output voltage of the RC -coupled transistor amplifiers of Fig. 5.69 . b. Calculate the overall gain and output voltage if a 4.7 k load is applied to the second stage, and compare to the results of part (a). c. Calculate the input impedance of the first stage and the output impedance of the second stage. CASCADED SYSTEMS Good luck!