Page 181 - Build Your Own Transistor Radios a Hobbyists Guide to High-Performance and Low-Powered Radio Circuits

the phasing circuit in the figure is taken. I cannot say enough about how great this

book is in terms of showing how to design all types of filters with coils or active
components. The gyrator circuit in Chapter 11 came from this book. Any hobbyist
or engineer will find that this book contains practical designs without long
mathematical derivations on filter design. In short, this is a great filter book for
designers.
Figure 12-5 shows a phase-shifting circuit with six amplifiers: U1A, U1B, U2A, U2B,

U3A, and U3B.

-~
-
Note

A low-output-impedance amplifier (e.g., op amp UOA) should drive the resistors and
capacitors associated with U1A and U1B.

Each amplifier section constitutes an all-pass network. So the output of any of the
amplifiers will provide constant amplitude over frequency while phase shifting the
input signal. The actual phase shift varies with the frequency of the input signal. So
the output of the A section of amplifiers at U3A is the summation of phase shifts
from U1A, U2A, and U3A. Each amplifier has a different RC time constant, such as
C1A R1A, C2A R2A, and C3A R3A. These time constants cause the phase (as

compared with the input signal) at the output of U3A to vary quite "wildly" as the
frequency is varied. At first glance, this may look all wrong. But there is the B
section that also generates phase shifts from U1B, U2B, and U3B in a similarly
"wild" manner but with a constant difference of gO degrees from the A section.
The amplifier gain resistors R are identical 1 percent resistors and can be in the

range of 1,000
n

to 10,000
n

as long as the resistors are the same value for each amplifier. So, for example, all
the resistors R can be 1,000
n
1 percent (or better, such as 0.25 percent tolerance).

So we have one set of phasing circuits in the A section and another set in the B
section, both generating lots of phase shift when compared with the input. But by
carefully designing the RC time constants for each amplifier, a gO-degree phase
difference is achieved with constant amplitude for an input signal having a lower
and higher frequency limit.

The phasing circuit in Figure 12-5 maintains a constant gO-degree phase difference
over a frequency range. This range can be expressed as the ratio of the higher
frequency limit to the lower frequency limit. For example, from the Arthur B.
Williams book, the following values are calculated for a ratio of the higher