Input and output coupling
To overcome the challenge of creating
necessary DC bias voltage for an amplifier's input signal without
resorting to the insertion of a battery in series with the AC signal
source, we used a voltage divider connected across the DC power source.
To make this work in conjunction with an AC input signal, we "coupled"
the signal source to the divider through a capacitor, which acted as a
high-pass filter. With that filtering in place, the low impedance of the
AC signal source couldn't "short out" the DC voltage dropped across the
bottom resistor of the voltage divider. A simple solution, but not
without any disadvantages.
Most obvious is the fact that using a
high-pass filter capacitor to couple the signal source to the amplifier
means that the amplifier can only amplify AC signals. A steady, DC
voltage applied to the input would be blocked by the coupling capacitor
just as much as the voltage divider bias voltage is blocked from the
input source. Furthermore, since capacitive reactance is
frequency-dependent, lower-frequency AC signals will not be amplified as
much as higher-frequency signals. Non-sinusoidal signals will tend to be
distorted, as the capacitor responds differently to each of the signal's
constituent harmonics. An extreme example of this would be a
low-frequency square-wave signal:
Incidentally, this same problem occurs
when oscilloscope inputs are set to the "AC coupling" mode. In this
mode, a coupling capacitor is inserted in series with the measured
voltage signal to eliminate any vertical offset of the displayed
waveform due to DC voltage combined with the signal. This works fine
when the AC component of the measured signal is of a fairly high
frequency, and the capacitor offers little impedance to the signal.
However, if the signal is of a low frequency, and/or contains
considerable levels of harmonics over a wide frequency range, the
oscilloscope's display of the waveform will not be accurate.
In applications where the limitations of
capacitive coupling would be intolerable, another solution may be used:
direct coupling. Direct coupling avoids the use of capacitors or
any other frequency-dependent coupling component in favor of resistors.
A direct-coupled amplifier circuit might look something like this:
With no capacitor to filter the input
signal, this form of coupling exhibits no frequency dependence. DC and
AC signals alike will be amplified by the transistor with the same gain
(the transistor itself may tend to amplify some frequencies better than
others, but that is another subject entirely!).
If direct coupling works for DC as well
as for AC signals, then why use capacitive coupling for any
application? One reason might be to avoid any unwanted DC bias
voltage naturally present in the signal to be amplified. Some AC signals
may be superimposed on an uncontrolled DC voltage right from the source,
and an uncontrolled DC voltage would make reliable transistor biasing
impossible. The high-pass filtering offered by a coupling capacitor
would work well here to avoid biasing problems.
Another reason to use capacitive coupling
rather than direct is its relative lack of signal attenuation. Direct
coupling through a resistor has the disadvantage of diminishing, or
attenuating, the input signal so that only a fraction of it reaches the
base of the transistor. In many applications, some attenuation is
necessary anyway to prevent normal signal levels from "overdriving" the
transistor into cutoff and saturation, so any attenuation inherent to
the coupling network is useful anyway. However, some applications
require that there be no signal loss from the input connection to
the transistor's base for maximum voltage gain, and a direct coupling
scheme with a voltage divider for bias simply won't suffice.
So far, we've discussed a couple of
methods for coupling an input signal to an amplifier, but haven't
addressed the issue of coupling an amplifier's output to a load.
The example circuit used to illustrate input coupling will serve well to
illustrate the issues involved with output coupling.
In our example circuit, the load is a
speaker. Most speakers are electromagnetic in design: that is, they use
the force generated by an lightweight electromagnet coil suspended
within a strong permanent-magnet field to move a thin paper or plastic
cone, producing vibrations in the air which our ears interpret as sound.
An applied voltage of one polarity moves the cone outward, while a
voltage of the opposite polarity will move the cone inward. To exploit
cone's full freedom of motion, the speaker must receive true (unbiased)
AC voltage. DC bias applied to the speaker coil tends to offset the cone
from its natural center position, and this tends to limit the amount of
back-and-forth motion it can sustain from the applied AC voltage without
overtraveling. However, our example circuit applies a varying voltage of
only one polarity across the speaker, because the speaker is
connected in series with the transistor which can only conduct current
one way. This situation would be unacceptable in the case of any
high-power audio amplifier.
Somehow we need to isolate the speaker
from the DC bias of the collector current so that it only receives AC
voltage. One way to achieve this goal is to couple the transistor
collector circuit to the speaker through a transformer:
Voltage induced in the secondary
(speaker-side) of the transformer will be strictly due to variations
in collector current, because the mutual inductance of a transformer
only works on changes in winding current. In other words, only
the AC portion of the collector current signal will be coupled to the
secondary side for powering the speaker. The speaker will "see" true
alternating current at its terminals, without any DC bias.
Transformer output coupling works, and
has the added benefit of being able to provide impedance matching
between the transistor circuit and the speaker coil with custom winding
ratios. However, transformers tend to be large and heavy, especially for
high-power applications. Also, it is difficult to engineer a transformer
to handle signals over a wide range of frequencies, which is almost
always required for audio applications. To make matters worse, DC
current through the primary winding adds to the magnetization of the
core in one polarity only, which tends to make the transformer core
saturate more easily in one AC polarity cycle than the other. This
problem is reminiscent of having the speaker directly connected in
series with the transistor: a DC bias current tends to limit how much
output signal amplitude the system can handle without distortion.
Generally, though, a transformer can be designed to handle a lot more DC
bias current than a speaker without running into trouble, so transformer
coupling is still a viable solution in most cases.
Another method to isolate the speaker
from DC bias in the output signal is to alter the circuit a bit and use
a coupling capacitor in a manner similar to coupling the input signal to
the amplifier:
This circuit resembles the more
conventional form of common-emitter amplifier, with the transistor
collector connected to the battery through a resistor. The capacitor
acts as a high-pass filter, passing most of the AC voltage to the
speaker while blocking all DC voltage. Again, the value of this coupling
capacitor is chosen so that its impedance at the expected signal
frequency will be arbitrarily low.
The blocking of DC voltage from an
amplifier's output, be it via a transformer or a capacitor, is useful
not only in coupling an amplifier to a load, but also in coupling one
amplifier to another amplifier. "Staged" amplifiers are often used to
achieve higher power gains than what would be possible using a single
transistor:
While it is possible to directly couple
each stage to the next (via a resistor rather than a capacitor), this
makes the whole amplifier very sensitive to variations in the DC
bias voltage of the first stage, since that DC voltage will be amplified
along with the AC signal until the last stage. In other words, the
biasing of the first stage will affect the biasing of the second stage,
and so on. However, if the stages are capacitively coupled as shown in
the above illustration, the biasing of one stage has no effect on the
biasing of the next, because DC voltage is blocked from passing on to
the next stage.
Transformer coupling between amplifier
stages is also a possibility, but less often seen due to some of the
problems inherent to transformers mentioned previously. One notable
exception to this rule is in the case of radio-frequency amplifiers
where coupling transformers are typically small, have air cores (making
them immune to saturation effects), and can be made part of a resonant
circuit so as to block unwanted harmonic frequencies from passing on to
subsequent stages. The use of resonant circuits assumes that the signal
frequency remains constant, of course, but this is typically the case in
radio circuitry. Also, the "flywheel" effect of LC tank circuits allows
for class C operation for high efficiency:
Having said all this, it must be
mentioned that it is possible to use direct coupling within a
multi-stage transistor amplifier circuit. In cases where the amplifier
is expected to handle DC signals, this is the only alternative.
- REVIEW:
- Capacitive coupling acts like a
high-pass filter on the input of an amplifier. This tends to make the
amplifier's voltage gain decrease at lower signal frequencies.
Capacitive-coupled amplifiers are all but unresponsive to DC input
signals.
- Direct coupling with a series resistor
instead of a series capacitor avoids the problem of
frequency-dependent gain, but has the disadvantage of reducing
amplifier gain for all signal frequencies by attenuating the input
signal.
- Transformers and capacitors may be
used to couple the output of an amplifier to a load, to eliminate DC
voltage from getting to the load.
- Multi-stage amplifiers often make use
of capacitive coupling between stages to eliminate problems with the
bias from one stage affecting the bias of another.
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