Patent Application
20090274319
1. Field of Invention
The present invention relates to a power amplifier. More particularly, the present invention relates to a power amplifier for amplifying an audio signal.
2. Description of Related Art
As the name suggests, the audio amplifier amplifies audio signals. These audio amplifiers typically include an audio input stage and an audio output stage. The audio input stage is connected to some type of audio source and the audio output stage is connected to the audio device such as a speaker. These audio amplifiers receive audio signals from the audio source, amplify those audio signals, generate audio current signals based on those amplified signals, and output the audio current signals to the speaker. The audio current signals drive the speaker and cause the speaker to reproduce the audio signals that are generated by the audio source to create sound.
To amplify the audio signal, the amplifying circuit of the audio amplifier typically requires a bias circuit to provide the bias voltages for biasing the amplifying circuit. However, when the audio amplifier just powers on, the power source, such as the supply voltage, is suddenly applied to the bias circuit, which causes the bias voltage generated by the bias circuit vibrates a lot. As a result, the output audio signal generated according to the bias voltage vibrates as well, which might damage the audio amplifier and produce unpleasant pop noises.
Therefore, there is a need for a new audio amplifier which can prevent the pop noise, and prevent the amplifier circuit from being damaged when the audio power amplifier powers on or powers down.
An audio power amplifier is disclosed. According to one embodiment of the present invention, an audio amplifier includes an amplifying circuit, a bias control circuit, and a decoupling device. The amplifying circuit amplifies an audio signal. The bias control circuit provides at least one bias voltage for the amplifying circuit according to a power down signal, in which the power down signal represents that the audio amplifier is powered on or powered down. The decoupling device reduces the damping phenomenon of the bias voltage caused by powering on or powering down the audio amplifier.
It is to be understood that both the foregoing general description and the following detailed description are by examples, and are intended to provide further explanation of the invention as claimed.
These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, appended claims, and accompanying drawings where:
Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts.
The decoupling device of the embodiments shown below can stabilize the bias voltage of the audio amplifier, such that the pop noise of the output audio signal can be eliminated.
When the audio amplifier is just powered on/powered down, the power source is suddenly passed to the bias control circuit 101, the bias control circuit 101 suddenly begins/stops generating the bias voltage VB and passes the bias voltage VB to the amplifying circuit 105. The sudden appearance of the power source produce the damping phenomenon of the bias voltage VB. As a result of the damping phenomenon of the bias voltage VB, the output audio signal vibrates, which causes the undesired pop noise.
To eliminated the undesired pop noise, the decoupling device 103 is added to reduce the damping phenomenon of the bias voltage VB when the audio amplifier powers on or powers down, which in turn eliminates the undesired pop noise of the output audio signal.
The decoupling device 203 includes a decoupling capacitor 213 and a decoupling capacitor 215. The decoupling capacitor 213 has one end coupled to the output terminal outputting the bias voltage VB1, and the other end receiving a ground voltage. The decoupling capacitor 215 has one end coupled to the output terminal outputting the bias voltage VB2, and the other end also receiving a ground voltage. The decoupling capacitor 213 and the decoupling capacitor 215 keep the bias voltage VB1 and the VB2 stable, such that the damping phenomenon of the bias voltage VB1 and the bias voltage VB2 can be reduced.
The amplifying circuit 205, amplifying the audio signal VI, includes a first operation amplifier 207 and a second operation amplifier 209. The first operation amplifier 207 generates an inverting output voltage Von according to the audio signal VI. The second operation amplifier 209 generates a non-inverting output voltage Vop according to the inverting output voltage Von and the audio signal VI. The output voltage Vop and the output voltage Von drive the speaker 211 to reproduce the audio signal VI.
The first operation amplifier 207 and the second operation amplifier 209 can be class AB amplifiers, class B amplifiers, or class A amplifiers. The class A amplifier has its power transistors been in conductive states all the time, which means that the power transistors continuously dissipate power. Thereby, the class A amplifier has low power efficiency.
The class B amplifier has its power transistors successively driven between conductive and non-conductive states, therefore, the power transistors of the class B amplifier are operated only 50% of the time. The class AB amplifier has its power transistors been in the conductive state for time period greater than one-half of the total period. Thereby, the power transistors of the Class AB amplifier are operated somewhere between 50% and all of the time period. As a result, because Class AB amplifier and Class B amplifier have better power efficiency, they are used more often in the audio amplifier than class A amplifier.
The amplifying circuit 205 further includes a first resistor R1 a second resistor R2, a third resistor R3, and the fourth resistor R4. The first resistor R1 has one end receiving the audio signal VI, and the other end connected to a negative terminal of the first operation amplifier 207. The second resistor R2 connected between the negative terminal and an output terminal of the first operation amplifier 207. The third resistor R3 has one end connected to the output terminal of the first operation amplifier 207 and the other end connected to a negative terminal of the second operation amplifier 209. The fourth resistor R4 connected between the negative terminal and an output terminal of the second operation amplifier 209. Tuning the resistance of the resistor R1\R2\R3\R4 changes the value of the voltage gain Von/VI and the voltage gain Vop/VI.
According to the above embodiments, the decoupling device can reduce the damping phenomenon of the bias voltage of the audio amplifier, such that the pop noise of the output audio signal can be eliminated.
It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.
