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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!