Amplifier improvements for limiting clipping

A differential amplifier is connected to an amplifier for comparing an amplifier input signal with the signal fed back by the amplifier feedback circuit. The output of the differential amplifier is coupled to a full wave rectifier the output of which controls an attenuator connected between a source of electrical input signals and the amplifier. When the amplifier is operating in its linear range, the differential amplifier inputs are identical and it does not produce an output. Under this condition, the input signal is not attenuated. However, in the presence of amplifier clipping, the differential amplifier produces an output which, after full wave rectification, operates to increase the attenuation, and thus reduce the amplitude of the signal applied to the amplifier; clipping is limited.

FIELD OF THE INVENTION 
The present invention relates to improvements in high fidelity amplifiers 
to minimize clipping. 
BACKGROUND OF THE INVENTION 
In the last 20 to 30 years, there has been a veritable explosion of high 
fidelity equipment intended for home use. During this period of time, 
available equipment became more and more sophisticated; for example, 
stereophonic equipment has almost wholly displaced monophonic equipment. 
Other advances have been made both in the signal chain as well as in 
associated devices for control and/or protection purposes. 
As those skilled in the art are aware, the primary object of such equipment 
is to increase the signal level to the extent necessary to drive whatever 
transducers are included in the system, while at the same time, 
maintaining as linear a characteristic as is possible. Since signal source 
levels vary, and since desirable audio listening levels can also vary, the 
equipment includes, almost universally, volume controls to vary the power 
output. For many years it has been known that the amplifiers included in 
the equipment were subject to clipping as a result of setting the 
amplifier for a relatively high power output, providing the amplifier with 
a relatively high input signal, or a combination of these factors. 
Clipping, of course, introduces distortion and, to that extent, thus 
destroys the desired linearity. 
Ozawa et al in U.S. Pat. No. 3,761,775, illustrate a protective circuit 
which includes a device to compare the amplifier input with a modified 
form of the amplifier output which is adapted to open circuit the 
amplifier output if substantial differences exist. Ozawa et al teach that 
this arrangement can be employed to remedy clipping. As a device for 
minimizing clipping, this arrangement has at least two disadvantages. In 
the first place, an attenuation circuit is required to attenuate the 
output signal by approximately the same factor as the amplification factor 
of the amplifier. Since most of the amplifiers that are in use today in 
high fidelity equipment are feedback amplifiers, the attenuation circuit 
merely duplicates circuitry already included in the amplifier. Although 
the attenuation circuit, as illustrated in Ozawa is merely a voltage 
divider, in actual practice, phase correction would also be necessary 
requiring the addition of at least a capacitor. A second more significant 
disadvantage to the Ozawa arrangement is that in the presence of clipping, 
the output signal is removed from the loudspeaker. This appears to be an 
example of "cutting off your nose to spite your face". Particularly, the 
presence of clipping would distort the output signal and could disturb a 
listener. Removing the output signal entirely does not appear to be a 
desirable solution. Instead, what is desired is an arrangement to minimize 
the effect of clipping while retaining the desired output signal. 
Suzuki et al, in U.S. Pat. No. 3,891,933, teach an arrangement which 
eliminates the first disadvantage mentioned above with regard to Ozawa, 
but their arrangement is no better than Ozawa with regard to the second 
disadvantage above-mentioned. 
SUMMARY OF THE INVENTION 
The present invention provides a device for minimizing the effect of 
clipping of an amplifier which includes a negative feedback coupling 
circuit coupling the output signal of the amplifier back to an input of 
the amplifier which further includes a differential amplifier with a pair 
of inputs, one coupled to the input electrical signal and the other 
connected to the output of the negative feedback coupling circuit, the 
differential amplifier producing an output signal when the inputs are 
different, a full wave rectifier connected to the output of the 
differential amplifier and input attenuator means coupling a source of 
electrical input signals to the amplifier and controlled by the output of 
the full wave rectifier for reducing the amplitude of the input signal 
when the differential amplifier produces an output.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 illustrates a preferred embodiment of the present invention. More 
particularly, a signal source (not illustrated) provides a signal intended 
for amplification before application to a load (also not illustrated). The 
signal is provided as an input to a pre-amplifier 10. The output of the 
pre-amplifier 10 is provided to a power amplifier 20 which includes a 
feedback circuit 25 coupling the output of the power amplifier 20 to an 
input thereof. The inputs to the power amplifier including the output of 
the pre-amplifier 10 and the output of the feedback circuit 25, are 
coupled as inputs to a differential amplifier 30, whose output provides an 
input to a full wave rectifier 32. The output of full wave rectifier 32 
drives an attenuator 31 as well as a limit lamp driver 33, whose output 
energizes either a normal indicator 34 or a limit indicator 35. As shown 
in FIG. 1, the input signal is connected to the input of pre-amplifier 10 
through a resistance. Furthermore, the attenuator 31 is shown as being 
connected in parallel with the pre-amplifier 10 between the signal source 
and ground. As this description proceeds, those skilled in the art will 
understand that this pre-amplifier is not essential to the present 
invention and other forms of signal modifying means may be included in 
series with the pre-amplifier 10, or may be included between the 
pre-amplifier 10 and the power amplifier 20. Regardless of the specific 
connection and the specific type of device employed, the attenuator 31 
performs the function of controlling the signal level supplied to the 
power amplifier 20 as a function of the output of full wave rectifier 32. 
In normal operation, assuming that the input signal level and thus the 
power output of the amplifier 20 are such that clipping does not occur, 
the inputs to differential amplifier 30 will be substantially equal, and 
thus the differential amplifier 30 will produce substantially no output. 
There is no current to be rectified and therefore no output signal to the 
attenuator or limit lamp driver. As a result, the limit lamp driver 
energizes the normal indicator 34 and the attenuator 31 has no effect on 
the signal which is coupled through the pre-amplifier 10 to the power 
amplifier 20. 
On the other hand, in the presence of clipping, which may be due to an 
excessive input signal level, excessive power output, or a combination of 
these two factors, the inputs to differential amplifier 30 will not be 
substantially equal. As a result, the differential amplifier 30 will 
produce a significant output which, after full wave rectification, has the 
following effects. The limit lamp driver 33 de-energizes the normal 
indicator 34 and energizes the limit indicator 35 to indicate that signal 
limiting is occurring. Furthermore, the attenuator 31 receives a 
significant output from the full wave rectifier 32 and reduces the input 
signal level to thereby minimize clipping. 
As is known to those skilled in the art, clipping in an amplifier occurs as 
a result of an excursion of a signal to a control electrode of an 
amplifying device which exceeds the linear range of that device. Such an 
excessive excursion can be caused by excessive input signals to an 
amplifier or mismatched loads which exceed the linear output current 
capability of amplifier 20. The embodiment of the invention illustrated in 
FIG. 1 serves to minimize clipping by reducing the input signal level 
amplitude to the extent necessary to reduce the signal excursion at the 
amplifying device to the linear range. 
Reference is now made to FIG. 2 for a more detailed understanding of the 
construction and operation of a preferred embodiment of the invention. 
In FIG. 2, the signal source is connected through resistance R1 to a 
pre-amplifier 10 which includes a pair of transistors Q1 and Q2, and 
associated passive components coupling the signal from an unillustrated 
signal source to an input of amplifier 20. As has been mentioned above, 
the pre-amplifier 10 is not essential. One terminal of attenuator 31 is 
connected between the input to the pre-amplifier 10 and the base of 
transistor Q1. This terminal is connected to one terminal of a light 
dependent resistor (LDR) whose other terminal is connected to ground 
through a SPST switch. Attenuator 31 and R1 form a voltage divider which 
serves to reduce the input signal reaching pre-amplifier 10. The output of 
the pre-amplifier 10 is connected to the input of power amplifier 20 which 
includes a plurality of transistors Q3-Q10. Amplifier 20 is only 
illustrative of a variety of power amplifiers that can be employed, and, 
in fact, any power amplifier which includes feedback can be utilized. More 
particularly, the output of power amplifier 20 is taken at the junction of 
collectors of transistors Q6 and Q10. The junction of the collectors of Q6 
and Q10 is connected to a resistor R2 and to a capacitor C4. The other 
terminal of the parallel combination of capacitor C4 and resistor R2 is 
connected to the base of transistor Q4. The base of Q4 is also connected 
to ground through a series circuit consisting of resistor R3 and capacitor 
C3. The aforementioned circuit comprises a feedback circuit included 
within a portion of the amplifier 20 in accordance with conventional 
feedback amplifier design so as to provide the amplifier with an overall 
linear amplification characteristic. 
The inverting input of differential amplifier 30 is connected via resistor 
R7, to the base of transistor Q4; the non-inverting input of differential 
amplifier 30 is connected through resistor R8, to the input of amplifier 
20. The output of differential amplifier 30 is coupled, through capacitor 
C5, to a full wave bridge rectifier 32 comprising diodes D1-D3 and the 
base emitter junction of a transistor Q11. The cathode of diode D1 which 
is connected to the base of transistor Q11, is also connected to the anode 
of a light emitting diode (LED) D4 whose light output is optically coupled 
to the LDR in the attenuator unit 31. The cathode of the LED is coupled to 
the junction of the anodes of diodes D2 and D3. The collector of 
transistor Q11 is coupled to the emitter via a capacitor C6 and is also 
coupled via a resistor to the base of a transistor Q12, included in the 
lamp driver 33, which also includes transistor Q13. In lamp driver 33, the 
emitter of Q12 is connected to a negative supply which is also connected 
via a resistor to the base of transistor Q12. The base of transistor Q12 
is also coupled via another resistor to the collector of transistor Q13 
and to one terminal of an indicator light 34, whose other terminal is 
grounded. The emitter of Q13 is also coupled to a negative supply. The 
base of Q13 is connected via another resistor to the collector of Q12 and 
also, through a further resistor, to the same negative supply. The 
collector of Q12 is also connected to one terminal of another indicating 
light 35, whose other terminal is grounded. Finally, the collector of Q13 
is connected, via a still further resistor to the negative supply. 
In one embodiment of the invention, differential amplifier 30 uses a 
commercially available integrated circuit operational amplifier having the 
designation LM301A. In the same embodiment, the attenuator 31 uses a 
commercially available unit from Clairex, having the designation CLM6000. 
The operation of the inventive circuit will now be explained assuming that 
the switch coupled to the attenuation device 31 is in its "on" condition, 
connecting one terminal of the LDR to ground. When the switch is in its 
"off" position, operation of the circuit has no effect on the signal 
passing the pre-amplifier 10 and amplifier 20 but does allow normal 
function of the indicator lights. 
Under normal conditions, when the signal level to the amplifier is such 
that clipping does not occur, an input signal from the signal source 
passes through pre-amplifier 10, and is amplified in the amplifier 20 and 
made available at its output terminal. Amplifier 20 includes a feedback 
network including resistors R2-R3 and capacitors C3-C4. The output of this 
feedback network is provided to the base of transistor Q4, and is also 
provided as the inverting input to the differential amplifier 30. The 
differential amplifier also receives, on the non-inverting input, the same 
signal which is provided to the input of amplifier 20. Assuming that 
clipping is not occurring, these two signals will be substantially 
identical and the differential amplifier 30 will not produce an output. 
Under these conditions, there is no current to rectify and thus, the LDR 
in the attenuator 31 maintains its normally high resistance and the signal 
is passed to the preamplifier 10 unattenuated. At the same time, there is 
no drive from amplifier 30 to transistor Q11 and lack of conduction in 
transistor Q11 maintains transistor Q12 to cutoff and thus indicator 35 is 
not illuminated. At the same time, however, transistor Q13 is conducting. 
As a result, indicator 34 is illuminated by a current passing from ground 
through the indicator 34, through transistor Q13 to the negative supply. 
As indicated above, when the amplifier 20 operates in the linear range, 
the inputs to the differential amplifier 30 are substantially identical. 
Differential amplifier 30 is adjusted to have equal inverting and 
noninverting gains by proper selection of resistors R6 through R9. R6 and 
R7 determine the inverting gain. R8 and R9 adjust the gain of the 
non-inverting input to be equal in magnitude to that of the inverting 
input. Capacitor C7 across resistor R9 is used to adjust phase balance of 
the system at all operating frequencies. Optionally, the inverting and 
non-inverting differential amplifier inputs may be exchanged and 
equivalent system performance will be obtained. 
If clipping now occurs, because the input signal level rises, the inputs to 
differential amplifier 30 will not be equal, the differential amplifier 30 
will produce an output, which, when rectified by bridge rectifier 32 will 
produce a current flow through the LED in the attenuator 31, lowering the 
resistance of the LDR. The base-emitter junction of transistor Q11 is one 
diode of the bridge rectifier. Conduction of transistor Q11 will turn 
transistor Q12 on, and transistor Q13 off. This results in the indicator 
35 activating and indicator 34 extinguishing. 
Due to the forward breakdown voltage characteristic of the bridge rectifier 
diodes and LED D4, a finite signal is required from differential amplifier 
30 before conduction starts. This characteristic in combination with the 
gain of differential amplifier 30 determines the clipping distortion level 
at which attenuator 31 and indicator lights 34 and 35 operate. 
The reduced resistance of the LDR in attenuator 31 will reduce the signal 
amplitude applied to the base of transistor Q1 in the pre-amplifier 10 and 
thus have the effect of minimizing clipping. Since the current flowing in 
the LED is produced by reason of a difference between the inputs to 
differential amplifier 30 and the difference between these inputs is a 
function of the degree of clipping, the attenuator 31 will tend to reduce 
the input signal level to the extent necessary to minimize clipping. When 
the signal level from the signal source again decreases to the point where 
clipping would not ordinarily occur, the inputs to differential amplifier 
30 will approach equality reducing the current flowing in the LED and thus 
allowing the resistance of the LDR to approach its nominal value, until 
the point where the circuit is again in its nominal condition. 
In one embodiment, the attenuator 31 has a slow decay in attenuation to 
input current compared to a period of the signal to be amplified. 
Therefore, the attenuator 31 operates as an integrator of the output of 
differential amplifier 30. Thus, the complete wave form of the signal 
source is linearly attenuated. 
Since clipping can occur on positive or negative peaks, the rectifier (full 
wave) is chosen so that it will detect either positive or negative 
clipping (or both). Full wave rectification gives added advantages at low 
frequencies. Since the resistance of the LDR follows the illumination 
which is, in turn, derived from the extent of clipping, at low signal 
frequencies the attenuator will attempt to follow the signal introducing 
further distortion. The frequency doubling effect of the full wave 
rectifier however, reduces this distortion. The particular chosen LDR 
responds quickly (50 - 100 .mu. sec) to "turn on" illumination but slowly 
to "turn off" (50 msec) conditions. This further tends to minimize the 
distortion introduced by operation of the inventive circuit. 
Since signal levels can change rapidly, so rapidly that the eye cannot 
respond to the illumination of an indicator for that period, capicator C6 
is provided. The capacitor C6 is normally charged to the negative supply 
potential through a pair of resistors. When transistor Q11 conducts, 
however, a portion of this charge is dissipated. When transistor Q11 
ceases to conduct, the capacitor C6 will have been discharged, at least to 
some extent. Even after transistor Q11 is turned off, therefore, the 
capacitor C6 will provide a supply of current to maintain transistor Q12 
conducting, and transistor Q13 turned off, increasing the period of time 
during which indicator 35 is energized and indicator 34 is de-energized 
even though the output of differential amplifier 30 may be reduced to 
zero. As a result, even though the clipping may occur for a short 
duration, the indicator is maintained energized for sufficient period so 
that it is visible. 
FIG. 3 illustrates two (2) curves, showing the output of amplifier 20 in 
the presence of an input signal having an amplitude sufficiently large to 
cause clipping. Wave form B shows the output of the amplifier when the 
switch is in the "off" condition. Under these conditions, it is apparent 
that the output is subject to severe clipping. Wave form A, on the other 
hand, illustrates the output of the amplifier, under the presence of the 
same input signal, when the switch is in the "on" condition. Under these 
circumstances, it will be seen that clipping has been minimized by 
operation of the inventive circuit. 
In an embodiment which has been built the indicators switch at a clipping 
level equivalent to 0.5% total harmonic distortion while the LDR responds 
at a level of clipping approximating 1% total harmonic distortion. 
One significant advantage of the inventive circuit is that it can be used 
as an "add-on" to existing equipment. Furthermore, application of the 
invention does not restrict the amplifier design as would the Suzuki 
arrangement. Finally, since internal amplifier connections are not 
required, a conventional IC amplifier can be employed. The only amplifier 
requirement is that it have a differential input configuration and employ 
a negative feedback system.