Amplifiers – Sum and difference amplifiers
Reexamination Certificate
2000-03-17
2001-10-16
Mottola, Steven J. (Department: 2817)
Amplifiers
Sum and difference amplifiers
C330S084000
Reexamination Certificate
active
06304138
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to audio-frequency power amplifiers and, more particularly, to an audio-frequency power amplifier that utilizes a bridged amplifier configuration.
2. Description of the Related Art
An audio-frequency power amplifier is a device that delivers power to a load, such as a speaker. Audio-frequency power amplifiers are commonly implemented in a number of ways. The efficiency by which these power amplifiers deliver power to the load, which is measured as the ratio of the power delivered to the load divided by the total power input to the amplifier, varies greatly among the different implementations.
One type of implementation, known as a Class A amplifier, is the most inefficient at delivering power to a load. Class A amplifiers have a linear region of operation, and are biased to operate from the center of the linear region of operation. When the input signal has no amplitude, a current that corresponds with the center of the linear region flows through the amplifier. When the amplitude of the input signal increases, the current increases, and when the amplitude decreases, the current decreases. As a result, a Class A amplifier consumes power throughout the entire cycle of the input signal, i.e., consumes power regardless of the amplitude of the input signal.
One example of a Class A amplifier is a power field-effect transistor (FET). A power FET has a source, a drain, a gate, and a linear region of operation. When configured as a Class A amplifier, a bias voltage is applied to the gate which, in turn, causes a current to flow through the FET which corresponds with the center of the linear region of operation.
When the input signal makes a positive excursion, the voltage on the gate is increased which causes the magnitude of the current to increase. When the input signal makes a negative excursion, the voltage on the gate is decreased which causes the magnitude of the current to decrease. Although inefficient from a power standpoint, Class A amplifiers provide a minimum amount of waveform distortion, and are thus widely used in audio systems.
Another implementation, known as a Class B amplifier, is more efficient at delivering power to a load than a Class A amplifier, but adds significantly more distortion to the output signal than a Class A amplifier. Class B amplifiers have a linear region of operation that ideally passes through the turn off point of the amplifier, and are biased to operate from the turn off point. Thus, when the input signal has no amplitude, no current flows through the amplifier. When the amplitude of the input signal increases, the current increases, and when the amplitude decreases, the current decreases.
One example of a Class B amplifier is an n-channel power FET connected in series with a p-channel power FET where the gates of the FETs form the input and the sources of the FETS form the output. When the input signal is equal to zero, both FETs are turned off. When the input signal makes a positive excursion, the n-channel FET turns on to source a current while the p-channel turns off. On the other hand, when the input signal makes a negative excursion, the p-channel FET turns on to sink a current while the n-channel turns off. Thus, since neither of the FETs in on when the input signal has no amplitude, and only one of the FETs is on when a positive or negative amplitude is present, a Class B amplifier is more efficient than a Class A amplifier.
As noted above, a Class B amplifier adds significantly more distortion to the signal than a Class A amplifier. This result occurs because power FETs do not have a linear region of operation that extends down to the turn off point. Instead, power FETs have a small non-linear region that lies between the turn off point and the linear region of operation. Thus, each time the input signal transitions by the turn off point, the signal is distorted.
Another amplifier, known as a Class A/B, is more efficient at delivering power to a load than a Class A (although less efficient than a Class B), and adds less distortion than a Class B (although more distortion than a Class A). Amplifiers that are categorized as Class A/B amplifiers utilize features taken from both Class A amplifiers and Class B amplifiers.
The above described example of a Class B amplifier can be converted into a Class A/B amplifier by applying a positive bias voltage to the gate of the n-channel FET and a negative bias voltage to the gate of the p-channel FET. The positive bias voltage is sufficient to place the n-channel FET at lower end of the linear region of operation, while the negative bias voltage is sufficient to place the p-channel FET at the upper end of the linear region of operation.
In this configuration, when the input signal has no amplitude, the n-channel sources a current which is sunk by the p-channel transistor. As the amplitude of the input signal increases, the n-channel FET linearly increases the current being sourced, while the p-channel non-linearly decreases the current being sunk, and then stops. Thus, the effect of the non-linear region of the p-channel FET is reduced by the stronger effect of the linear region of the n-channel FET.
Similarly, as the amplitude of the input signal decreases, the p-channel FET linearly increases the current being sunk, while the n-channel non-linearly decreases the current being sourced, and then stops. Thus, the effect of the non-linear region of the n-channel FET is reduced by the stronger effect of the linear region of the p-channel FET.
In this example, the Class A/B amplifier is less efficient than a Class B in that the n-channel FET is turned on during the positive excursions of the input signal and a portion of the negative excursions. In the Class B example, the n-channel FET was not turned on at all during the negative excursions of the input signal. Similarly, the p-channel FET is turned on during the negative excursions of the input signal and a portion of the positive excursions. In the Class B example, the p-channel FET was not turned on at all during the positive excursions of the input signal.
A Class A/B amplifier can also be implemented differentially as a bridged amplifier.
FIG. 1
shows a schematic diagram that illustrates a conventional bridged amplifier
100
. As shown in
FIG. 1
, amplifier
100
has a first operational amplifier (op amp)
110
. First op amp
110
, in turn, has a positive input connected to an input node N
IN
, and a negative input connected to a first intermediate node N
1
. In addition, op amp
110
also has an output connected to a first output node N
out1
. further, op amp
110
is connected to an upper supply rail VCC, and a lower supply rail VEE.
As additionally shown in
FIG. 1
, amplifier
100
also has a second operational amplifier (op amp)
112
. Second op amp
112
has a positive input connected to a reference voltage V
REF
, and a negative input connected to a second intermediate node N
2
. In addition, op amp
112
also has an output connected to a second output node N
OUT2
. Further, op amp
112
is connected to the upper supply rail VCC, and the lower supply rail VEE.
Amplifier
100
additionally has a pair of feedback resistors RF
1
and RF
2
, and a pair of input resistors RIN
1
and RIN
2
. Feedback resistor RF
1
is connected between the output and the negative input of op amp
110
, while feedback resistor RF
2
is connected between the output and the negative input of op amp
112
.
Further, input resistor RIN
1
is connected between the negative input of op amp
110
and the reference voltage V
REF
, while input resistor RIN
2
is connected between the negative input of op amp
112
and the input node N
IN
. An input signal V
IN
can be directly applied to input node N
IN
if a common reference exists between the input voltage V
IN
and the reference voltage V
REF
, or can be applied via a capacitor C as shown in FIG.
1
.
In operation, op amp
110
and resistors RF
1
and RIN
1
are connected together to form a non-inverting negative feedback circuit t
Mottola Steven J.
National Semiconductor Corporation
Pillsbury & Winthrop LLP
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