Amplifiers – With semiconductor amplifying device – Including differential amplifier
Reexamination Certificate
2002-09-11
2004-03-30
Choe, Henry (Department: 2817)
Amplifiers
With semiconductor amplifying device
Including differential amplifier
C330S257000, C327S359000
Reexamination Certificate
active
06714074
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a power amplifier circuit, and more particularly, to a power amplifier clipping circuit that smoothes a sudden signal change when an output waveform is clipped, and generates the same output waveform for unclipped portions as the original waveform.
2. Description of the Related Art
A power amplifier, which generally uses a plurality of transistors or integrated circuit (“IC”) devices, is an apparatus that allows an input to control a power source to produce some useful output. For example, using a power amplifier, a voice signal output from an audio apparatus may have a higher gain and better sound quality.
The power amplifier circuit has a feedback loop which feeds an output signal back to the power amplifier circuit. When an excessive input voltage is provided to the power amplifier circuit, or the power supply circuit becomes inoperable due to a short circuit, the output waveform of the power amplifier circuit is distorted. This distortion is usually referred to as clipping. For the interval when the output waveform is clipped, the feedback loop of the power amplifier circuit cannot operate.
FIG. 1
is a waveform diagram, generally indicated by the reference numeral
100
, showing the input waveform
110
and the output waveform
112
of a typical power amplifier clipping circuit. Referring to
FIG. 1
, the dotted lines
110
outside the dynamic range show the original waveform when the output waveform was not distorted, while the solid lines
112
show the output waveform after clipping. It is shown that when an excessive input is provided, the portions that exceed the dynamic range of the power amplifier are clipped. High-frequency noise
114
is introduced at the clipping transitions. In particular, this is a serious problem in the case of a high-gain amplifier connected to a high inductance external speaker, such as when high-frequency noise
114
as shown in
FIG. 1
appears while the output waveform is clipped and annoys the listener.
FIG. 2
is a circuit diagram showing a typical power amplifier clipping circuit for reducing output clipping. Referring to
FIG. 2
, the typical power amplifier clipping circuit
200
comprises the resistors R
1
and R
2
which are serially connected between a positive external power voltage +VCC and ground GND, and between ground GND and a negative external power voltage −VCC, respectively; a first comparator I
1
which receives a predetermined first voltage VP
1
that is divided by the resistors R
1
and R
2
, and an input voltage VI, compares the two voltages and outputs the larger result; a second comparator
12
, which receives a predetermined second voltage VP
2
that is divided by the resistors R
1
and R
2
and the input voltage VI, compares the two voltages and outputs the larger result; a first diode D
1
which receives the output of the first comparator I
1
; a second diode D
2
which receives the output of the second comparator I
2
; a capacitor C
1
which is charged by the output of the first diode D
1
and the second diode D
2
; a transistor J
1
which operates charged by the capacitor C
1
; resistors R
0
, R
3
, and R
4
; and an inductor L of an external speaker.
FIG. 3
is a waveform diagram
300
showing the output waveform
312
of the power amplifier clipping circuit of FIG.
2
. The original waveform is indicated generally by the reference numeral
310
for comparison. Although clipping takes place in the regions
314
, additional distortion between the original waveform
310
and the output waveform
312
can be seen between the respective plots in other regions as well. The power amplifier clipping circuit
200
is to improve the high frequency noise shown in the output waveform of FIG.
1
. Although the high-frequency noise may be reduced by the power amplifier clipping circuit
200
of
FIG. 2
, the capacitor C
1
causes the additional distortion as shown by the differences between the output waveform
312
and the input waveform
310
.
The operation of the typical power amplifier clipping circuit
200
will now be explained. If the gain of the amplifier
210
is AV, and the values of resistors R
1
and R
2
are set so that (R
1
+R
2
)/R
2
=AV, the first voltage VP
1
and the second voltage VP
2
have the same value as the value of the input voltage VI at which the output V
0
of the amplifier
210
is clipped. In this case, it is assumed that the dynamic range of the amplifier
210
is given by the interval defined by the positive external power source voltage +VCC and the negative external power source voltage −VCC. Although the devices inside the amplifier
210
limit this dynamic range, this deviation can be compensated by adjusting the values of the resistors R
1
and R
2
. Thus, taking the dynamic range as above is a reasonable assumption for convenience of explanation.
When the input voltage VI is less than the first voltage VP
1
and greater than the second voltage VP
2
, that is, in a normal operation interval, the outputs of the first comparator
11
and the second comparator
12
have negative values, and therefore, the outputs of the first comparator I
1
and the second comparator I
2
are blocked by the first diode D
1
and the second diode D
2
, and the capacitor C
1
is not charged. Accordingly, the electric potential of the capacitor C
1
is 0V and the transistor J
1
is turned off. Since current does not flow through the resistor R
0
that is serially connected to the input voltage VI, a voltage drop does not occur, and the entire input voltage VI is provided to the amplifier such that a normal operation is performed.
However, when the input voltage VI is greater than the first voltage VP
1
, that is, when an excessive input voltage is provided to the power amplifier clipping circuit
200
, the output of the second comparator
12
is still at a negative value, but the output of the first comparator I
1
is at a positive value, the first diode D
1
is turned on and the capacitor C
1
is charged. Therefore, the transistor J
1
is turned on, and the input voltage VI reaches a value which is divided by the resistor R
0
and the turn-on resistance of the transistor J
1
, and since the value is less than the original input voltage, VI is provided to the amplifier
210
. Here, the transistor J
1
is a junction field effect transistor (“JFET”). From the aspect of the amplifier
210
, the feedback loop (not shown) is continuously maintained while generation of the high frequency noise is curbed as shown in
FIG. 3
, and the output waveform is clipped.
It shall be understood that when the input voltage VI is less than the second voltage VP
2
, the result is similar to the case when the input voltage VI is greater than the first voltage VP
1
. Accordingly, the duplicate explanation will be omitted.
However, when the power amplifier clipping circuit
200
having the circuit structure as shown in
FIG. 2
is used, the capacitor C
1
should be in the circuit. In addition, if, as shown in
FIG. 3
, the input voltage VI, which is greater than the first voltage VP
1
, is provided to the power amplifier clipping circuit
200
and the capacitor C
1
is charged by the resistor R
4
, the transistor J
1
is turned on until the current is completely discharged from the capacitor Cl, and the input voltage VI is divided. Accordingly, even though a normal input voltage VI is provided again after the excessive input voltage VI is provided, the output waveform V
0
of the amplifier
210
is smaller than the normal output waveform V
0
, as indicated by dotted waveform in
FIG. 3
, for a predetermined interval or until the current in the capacitor C
1
is completely discharged, and the output signal becomes different from the original one. Thus, the added distortion
In addition, the device presented in the European Patent Application No. 88108772.0, which displays a circuit structure different from that of the power amplifier clipping circuit
200
of
FIG. 2
, but that has a clipping detector and a voltage
Choe Henry
F. Chau & Associates LLC
Samsung Electronics Co,. Ltd.
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