"Chapter 1 - The Op Amp's Place in the World" - HTL Wien 10

Block Diagram Math and Manipulations
5-2
The input impedance of each block is assumed to be infinite to preclude loading. Also,
the output impedance of each block is assumed to be zero to enable high fan-out. The
systems designer sets the actual impedance levels, but the fan-out assumption is valid
because the block designers adhere to the system designer’s specifications. All blocks
multiply the input times the block quantity (see Figure 5–1) unless otherwise specified
within the block. The quantity within the block can be a constant as shown in Figure
5–1(c), or it can be a complex math function involving Laplace transforms. The blocks can
perform time-based operations such as differentiation and integration.
Figure 5–1. Definition of Blocks
INPUT
A
VI
VO
(a) Input/Output Impedance
Block
Description
(b) Signal Flow Arrows
OUTPUT
A K B B = AK
(c) Block Multiplication
d
dt
(d) Blocks Perform Functions as Indicated
B
VO = dVI
dt
Adding and subtracting are done in special blocks called summing points. Figure 5–2
gives several examples of summing points. Summing points can have unlimited inputs,
can add or subtract, and can have mixed signs yielding addition and subtraction within
a single summing point. Figure 5–3 defines the terms in a typical control system, and Figure
5–4 defines the terms in a typical electronic feedback system. Multiloop feedback systems
(Figure 5–5) are intimidating, but they can be reduced to a single loop feedback system,
as shown in the figure, by writing equations and solving for V OUT/V IN. An easier method
for reducing multiloop feedback systems to single loop feedback systems is to follow
the rules and use the transforms given in Figure 5–6.