Amplifier Design 70320.1 The Necessity for AmplifiersThe necessity for amplification becomes apparent from
an analysis of the unlikely arrangement depicted in Fig.
20-1 wherein a dynamic microphone is connected
directly to a loudspeaker.The microphone typically, with moderate excitation,
would generate an open circuit voltage of 10 mV and
possess an internal impedance of 200:. The loud-
speaker typically would have an impedance of 8: and
an efficiency of 10%. The electrical power delivered to
the loudspeaker assuming that the microphone and
loudspeaker impedances are predominantly resistive
would be 1.8 × 10^8 W while the acoustical output of
the loudspeaker would only be 1.8 × 10^9 W. E v e n i f a
matching transformer is interposed between the micro-
phone and the loudspeaker, the improvement is hardly
significant. The acoustical output in this event becomes
only 1.25 × 10^8 W which is several orders of magni-
tude below the acoustical power requirements of most
applications.
20.2 Types and Descriptions of AmplifiersThe initial description for an amplifier is based on the
nature of the active elements involved such as vacuum
tube, bipolar transistor, field effect transistor, integrated
circuit, magnetic field, or a mixture of two or more of
these technologies, in which case it is called a hybrid.
The second descriptor is associated with the principal
quantity being amplified and indirectly with the input
output relationships exhibited by the amplifier.
For example, a voltage amplifier is excited at its
input by a signal in the form of a voltage and responds
by producing a related voltage at its output. In this
instance, it is desirable that the input impedance of a
voltage amplifier be large compared with the impedance
of the signal source and that the output impedance of
the amplifier be small compared with the load imped-
ance connected at its output. As a result, the signal
source impresses a maximum voltage across the ampli-
fier’s input and the amplifier subsequently produces a
maximum voltage across its associated load.
A current amplifier is excited at its input by a signal
in the form of a current and responds by producing a
related current in its associated load. Current amplifiers
have low input impedances and high output impedances.
A transconductance amplifier is excited at its input
by a voltage and responds by producing a related current
in its associated load. Transconductance amplifiers have
high input impedances and high output impedances.
Transresistance amplifiers are excited at the input by
a signal current and respond by producing a related
voltage at the output. A transresistance amplifier has a
low input impedance as well as a low output impedance.
Another useful amplifier descriptor describes the
functional relationship, in a mathematical sense, which
exists between the input and output signals. For
example, in linear amplifiers the output signal is a
linear function of the input signal whereas in loga-
rithmic amplifiers, the output signal is proportional to
the logarithm of the input signal. The majority of the
amplifiers employed in audio are linear but a significant
number of logarithmic or other special function ampli-
fiers find use in signal processing applications.
Additional descriptions are associated with the phys-
ical location of an amplifier in the overall amplification
chain. For example, a preamplifier is usually placed
immediately after a transducer where the signal levels
are quite low and noise characteristics are of consider-
able importance. Certain preamplifiers will incorporate
special equalization circuitry; for instance, a phono
preamplifier provides the required RIAA playback char-
acteristic.
Preamplifiers are followed by mixing amplifiers that
can combine and individually control the signals from
several different sources. Although there may exist
several other intermediate steps, the power amplifier is
the last step.
Power amplifiers in audio work have the input-
output impedance characteristics of a voltage amplifier
along with the ability to deliver large amounts of elec-
trical power. Fig. 20-2 illustrates a typical arrangement
in a reinforcement chain.
The final descriptor to be discussed concerns ampli-
fier terminal connections. Amplifiers are essentially two
port devices, i.e., they are constituted with a pair of
input terminals and a pair of output terminals as indi-
cated in Fig. 20-3.
If neither of the input terminals is connected directly
to ground and if both of the input terminals are electri-
cally symmetric with respect to ground, the input is said
to be balanced. If one of the input terminals is
connected to ground, the input is unbalanced and is
described as being single ended. Similar statements are
applicable to the output pair of terminals. All possible
combinations are encountered in practice. The inputFigure 20-1. The impossible sound reinforcement system.Microphone Loudspeaker