COOKBOOK — PART 8
Ray Marston describes a miscellaneous collection of useful
transistor circuits and gadgets in this month's final episode
of an eight-part series.
by Ray Marston
The opening piece of this eight-part series
described basic transistor principles and configurations; subsequent articles went on to describe a
wide variety of practical transistor circuits ranging from
common-collector amplifiers (Part 2), common-emitter
and common-base amplifiers (Part 3), and small-signal
audio amplifiers (Part 4), to various practical oscillator
(Part 5), multivibrator waveform generator (Part 6), and
audio power amplifier (Part 7) circuits. This month's final
episode rounds off the "Transistor Cookbook" subject by
presenting a miscellaneous collection of practical and
useful transistor circuits and gadgets.
it unintelligible. This problem can often be overcome by
using the noise limiter circuit in Figure 1. Here, the signal-plus-noise waveform is fed to amplifier Q1 via RV1. Q1
amplifies both waveforms equally, but D1 and D2 automatically limit the peak-to-peak output swing of Q1 to
about 1.2 V. Thus, if RV1 is adjusted so that the signal output is amplified to this peak level, the noise peaks will not
be able to greatly exceed the signal output, and intelligibility is greatly improved.
A NOISE LIMITER CIRCUIT
Unwanted electronic "noise" can be a great nuisance;
when listening to very weak broadcast signals, for example, peaks of background noise often completely swamp the broadcast signal, making
Figure 1. Noise limiter.
The astable multivibrator circuit has many practical
uses. It can be used to generate a non-symmetrical 800
Hz waveform that produces a monotone audio signal in
the loudspeaker when S1 is closed (Figure 2). The circuit
can be used as a Morse code
practice oscillator by using a
Morse key as S1; the tone frequency can be changed by altering the C1 and/or C2 values.
Figure 3 shows an astable
multivibrator used as the basis
of a "signal injector-tracer" item
of test gear. When SW1 is in
INJECT position 1, Q1 and Q2
are configured as a 1 kHz
astable, and feed a good square
wave into the probe terminal via
R1-C1. This waveform is rich in
harmonics, so if it is
injected into any AF
or RF stage of an
AM radio, it produces an audible
output via the
unless one of the
radio's stages is
faulty. By choosing
a suitable injection
point, the injector
can be used to trou-
Figure 2. Morse code practice oscillator.
NUTS & VOLTS
Figure 3. Signal injector-tracer.
Figure 4. Simple lie detector.