The present invention relates generally to electronic circuit techniques. More specifically, embodiments of the present invention relate to techniques for cost effective large time-constant generation. Merely by way of example, embodiments of the invention have been applied to audio amplifier and systems in applications, such as pop-noise suppression. But it would be recognized that the invention has a much broader range of applicability.
Amplifier circuits are prevalent in modern electronic devices. An electronic amplifier is a device for increasing the power and/or amplitude of a signal. In particular, power amplifier circuits are used at the output stage of a system to drive an external device, such as a speaker. Power amplifier circuits output stages can be classified as A, B, AB and C for analog designs, and class D and E for switching designs. This classification is based on the portion of the input signal cycle during which the amplifying device conducts.
A Class A amplifier operates over the whole of the input cycle such that the output signal is an exact magnified replica of the input with no clipping. Class A amplifiers are the usual means of implementing small-signal amplifiers. In a Class A circuit, the amplifying element is biased so the device is always conducting to some extent, and is operated over the most linear portion of its characteristic curve. Because the device is always conducting, even if there is no input at all, power is drawn from the power supply. Accordingly, class A amplifiers tend to be relatively in efficient. For large powers this means very large and expensive power supplies and heat sinking.
Class B amplifiers only amplify half of the input wave cycle. As such they create a large amount of distortion, but their efficiency is greatly improved and is much better than Class A. This is because the amplifying element is switched off altogether half of the time, and so cannot dissipate power. A practical circuit using Class B elements is the complementary pair or “push-pull” arrangement. Here, complementary or quasi-complementary devices are used to each amplify the opposite halves of the input signal, which is then recombined at the output. This arrangement gives excellent efficiency, but can suffer from the drawback that there is a small mismatch at the “joins” between the two halves of the signal. This is called crossover distortion. An improvement is to bias the devices so they are not completely off when they're not in use. This approach is called Class AB operation.
In Class AB operation, each device operates the same way as in Class B over half the waveform, but also conducts a small amount on the other half. As a result, the region where both devices simultaneously are nearly off (the “dead zone”) is reduced. The result is that when the waveforms from the two devices are combined, the crossover is greatly minimized or eliminated altogether. Here the two active elements conduct more than half of the time as a means to reduce the cross-over distortions of Class B amplifiers. In the example of the complementary emitter followers a bias network allows for more or less quiescent current thus providing an operating point somewhere between Class A and Class B.
Class C amplifiers conduct less than 50% of the input signal and the distortion at the output is high, but high efficiencies are possible. Some applications (for example, megaphones) can tolerate the distortion. A much more common application for Class C amplifiers is in RF transmitters, where the distortion can be vastly reduced by using tuned loads on the amplifier stage. The input signal is used to roughly switch the amplifying device on and off, which causes pulses of current to flow through a tuned circuit.
An audio amplifier is an electronic amplifier that amplifies low-power audio signals to a level suitable for driving loudspeakers. Audio signals generally refer to signals composed primarily of frequencies between 20 hertz to 20,000 hertz, the human range of hearing. An audio output amplifier is often the final stage in a typical audio playback chain. In a typical audio system, the audio amplifier is usually preceded by low power audio amplifiers which perform tasks like pre-amplification, equalization, tone control, mixing/effects, or audio sources like record players, CD players, and cassette players. Important applications include public address systems, theatrical and concert sound reinforcement, and domestic sound systems. The sound card in a personal computer contains several audio amplifiers (depending on number of channels), as does every stereo or home-theatre system. Most audio amplifiers require these low-level inputs to adhere to line levels. While the input signal to an audio amplifier may measure only a few hundred microwatts, its output may be tens, hundreds, or thousands of watts.
Class AB push-pull circuits are the most common design type found in audio power amplifiers. Class AB is widely considered a good compromise for audio amplifiers, since much of the time the music is quiet enough that the signal stays in the “class A” region, where it is amplified with good fidelity, and by definition if passing out of this region, is large enough that the distortion products typical of class B are relatively small. The crossover distortion can be reduced further by using negative feedback. Class B and AB amplifiers are sometimes used for RF linear amplifiers as well. Class B amplifiers are also favored in battery-operated devices, such as transistor radios.
Even though conventional audio amplifiers are widely used, they suffer from many limitations. One of the limitations is pop noise or click noise that can be produced in transient states of the amplifier. For example, a pop noises can often be heard during power-on of an audio amplifier. Conventional circuit techniques have been proposed, but they tend to be expensive and are often ineffective.
Accordingly, it is desirable to provide a simple and cost-effective techniques for improving amplifier circuit.
As noted above, conventional amplifier circuits often suffer from transient related problems, such as pop noise during power-up or power-down. According to embodiments of the present invention, a large RC time-constant generation circuit can be used in arranging the sequence of circuit events in different stages of the amplifier circuit. In other applications, it may also be desirable to have cost-effective circuits to generate large RC time-constant.
The present invention relates generally to electronic circuit techniques. More specifically, embodiments of the present invention relate to techniques for cost-effective circuits for generating RC time-constants. In a specific embodiment, a large RC time-constant circuit includes a capacitor coupled in series with an MOS transistor configured to operate in saturation mode. Merely as an example, such a large RC time-constant circuit has been implemented in an audio amplifier to minimize pop noise generation in an audio system. But it is recognized that the invention can be used in other circuits or systems in which large time-constants are needed, e.g., in delay generation circuits or compensation circuit, etc., in analog or digital systems.
According to an embodiment of the present invention, a circuit for generating a large RC time-constant includes an input node for receiving an input signal making a transition from a first state to a second state characterized by a first time-constant, and an output node for providing an output signal making a transiting from the first state to the second state in response to the input signal. The circuit also includes a first MOS field effect transistor coupled between the input node and the output node. The circuit further includes a first capacitor coupled between the output node and a ground node. A switch circuit is connected to a gate of the first MOS field effect transistor. The switch circuit is configured to bias the MOS field effect transistor to operate in saturation mode during substantially the entire time when the input signal makes the transition from the first state to the second state. For example, for an input signal making a transition from a low state to a high state, the MOS transistor is an NMOS transistor having a gate and a source connected to the output node. In another example, for an input signal making a transition from a high state to a low state, the MOS transistor can be an NMOS transistor having a gate and a source connected to the input node. As a result, the MOS transistor is configured to operate in saturation mode, i.e., the gate-to-drain voltage is smaller than or approximately equal to the threshold voltage. In the saturation mode, the transistor has a large output resistance. Therefore, the transition of the output signal is characterized by a time-constant associated with this large output resistance and the capacitor coupled to the output node. This time-constant can be substantially longer than the first time-constant of the transition of the input signal. Consequently, a cost-effective circuit for generating a large RC time-constant can be realized in an integrated circuit without the chip area penalty of a large on-chip resistance.
In alternative embodiments of the circuit for generating a large RC time-constant described above, a PMOS transistor can be used, instead of the NMOS transistor. In this case, the connections may vary depending on the transistor and signal transition. For example, when the input signal makes a transition from a low state to a high state, the PMOS transistor is configured to have a gate and a source connected to the input node. In some embodiments, the transistor in the RC time-constant circuit can have a threshold voltage of approximately 0V. In these embodiments, when the gate and drain are connected together the gate-to-drain voltage is approximately equal to the threshold voltage. In some embodiments, each of the MOS transistors can be a native transistor. That is, the MOS transistors with low threshold voltage or nearly 0V threshold can be formed using their respective well doping and without additional threshold implant.
In some embodiments of the circuit described above, the capacitor can include an MOS capacitor. For example, the capacitor can include an MOSFET having a drain and a source connected together. In some embodiments the MOS transistor and the capacitor are included in a single integrated circuit chip, whereas in other embodiments, the circuit can also be implemented using discrete components.
In a specific embodiment of the circuit for generating a large RC time-constant described above, the circuit can also include one or more RC time-constant circuit cells coupled between the input node and the first MOS field effect transistor. Each cell has an MOS field effect transistor, a capacitor coupled to the MOS field effect transistor, and a switch circuit coupled to a gate of the MOS field effect transistor for biasing the MOS field effect transistor to operate in saturation mode when the input signal makes the transition from the first state to the second state.
According to another embodiment of the present invention, a circuit for providing a large RC time-constant circuit includes an input terminal for receiving a input signal making a transition from a first state to a second state characterized by a first time-constant; an output terminal for providing an output signal making a transiting from the first state to the second state in response to the input signal, and a plurality of RC time-constant circuit cells connected in series between the input terminal and the output terminal. Each cell includes an MOS field effect transistor, a capacitor coupled to the MOS field effect transistor, and a switch circuit coupled to a gate of the MOS field effect transistor. The switch circuit is configured to connect the gate of the MOS field effect transistor to the output terminal when the input signal makes the transition from a low state to a high state. The switch circuit is also configured to connect the gate of the MOS field effect transistor to the input terminal when the input signal makes a transition from a high state to a low state. In embodiments of the invention, the MOS field effect transistor in each cell is configured to operate in saturation mode during at least a portion of the time period when the input signal makes the transition and the output signal exhibits a time-constant that is substantially longer than the first time-constant of the input signal.
According to yet another embodiment of the present invention, a circuit for providing a large RC time-constant includes an input node for receiving a input signal that is capable of making a transition from a first state to a second state, an output node for providing an output signal capable of making a transiting from the first state to the second state in response to the input signal. The circuit also includes a capacitor coupled between the output node and a ground node and an MOS field effect transistor coupled between the input node and the output node. The MOS field effect transistor is biased to operate in saturation mode during at least a portion of the time when the input signal makes the transition from the first state to the second state. In such configuration, the MOS field effect transistor exhibits a saturation mode drain resistance and the output signal is characterized by a time-constant that is substantially longer than that of the input signal.
In a specific embodiment of the RC time-constant generation circuit described above, the MOS field effect transistor is biased to operate in saturation mode during substantially the entire time when the input signal makes the transition from the first state to the second state. In an embodiment wherein the input signal is input signal is configured to make a transition from a low state to a high state, the MOS transistor can be an NMOS transistor having a gate and a source connected to the output node. In another embodiment wherein the input signal is configured to make a transition from a high state to a low state, the MOS transistor can be an NMOS transistor having a gate and a source connected to input node.
In another specific embodiment of the RC time-constant generation circuit described above, the input signal is configured to make a transition from a low state to a high state, the MOS transistor can be a PMOS transistor having a gate and a source connected to the input node. In yet another embodiment wherein the input signal is configured to make a transition from a high state to a low state, the MOS transistor can be a PMOS transistor having a gate and a source connected to output node. In an embodiment, the capacitor can include an MOS capacitor. Alternatively, the capacitor can include a second MOS transistor having a drain and a source connected together. In an embodiment, the MOS transistor and the capacitor are included in a single integrated circuit chip. In some embodiment, each of the MOS transistor can be a native transistor. In certain embodiment, the circuit can also includes a switch that connects the gate of the transistor to the drain or the source depending on the direction of the transition.
According to an alternative embodiment, the invention provides an integrated circuit that includes a power supply terminal for connecting to a power supply, an output terminal for providing an audio frequency output signal, a large RC time-constant circuit having an input node coupled to the power supply terminal and an output node for providing an operating voltage, and an amplifier circuit coupled to the output node of the large RC time-constant circuit for receiving the operating voltage. The amplifier circuit is configured for providing the audio frequency output signal to the output terminal. In this embodiment, the large RC time-constant circuit includes one or more RC time-constant circuit cells, each cell having a capacitor coupled to an MOS transistor configured to provide a saturation mode output resistance for generating a large time-constant. In some embodiments, each cell further includes a switch circuit that is configured to bias the MOS transistor in the saturation mode during a transition in the power supply voltage. In a specific embodiment, the switch circuit in each cell is configured to connect a gate terminal of the MOS transistor to a drain terminal or a source terminal thereof in response to a change in the power supply voltage.
According to another alternative embodiment, the present invention provides an audio system that includes an input for receiving an audio frequency input signal, a power supply terminal for connecting to a power supply, and an output terminal for providing an audio frequency output signal. The audio system also has a large RC time-constant circuit having an input node coupled to the power supply terminal and an output node for providing an operating voltage. Moreover, the audio system also includes an amplifier circuit coupled to the output node of the large RC time-constant circuit for receiving the operating voltage. The amplifier circuit is configured to provide the audio frequency output signal to the output terminal, which is coupled to a speaker. In an embodiment the large RC time-constant circuit includes one or more RC time-constant circuit cells, each of the cells having a capacitor coupled to an MOS transistor that is configured to provide a saturation mode output resistance for generating a large time-constant. In some embodiments, each of the RC time-constant circuit cells further comprises a switch circuit that is configured to bias the MOS transistor in the saturation mode during a transition in the power supply voltage. In a specific embodiment, the switch circuit in each cell is configured to connect a gate terminal of the MOS transistor to a drain terminal or a source terminal thereof in response to a change in the power supply voltage.
Many benefits are achieved by way of the present invention over conventional techniques. For example, the present technique provides an easy to use design that that is compatible with conventional integrated circuit design and fabrication process technologies. In certain embodiments, the invention provides techniques for generating large RC time-constants. In a specific embodiment, the circuit includes an MOS transistor configured to operate in the saturation mode and provide a large time-constant during signal transition without the penalty of having to use a large on-chip resistance. Merely as an example, an embodiment of the invention is applied to an audio system for suppressing transient noise such as pop noise in an audio amplifier. It is understood, however, the technique can be easily adopted for other applications, such as providing a long delay time between different stages of a circuit. Depending upon the embodiment, one or more of these benefits may be achieved. These and other benefits will be described in more detail throughout the present specification and more particularly below.
Various additional objects, features and advantages of the present invention can be more fully appreciated with reference to the detailed description and accompanying drawings that follow.
In
In the embodiment of the RC time-constant generation circuit described above, the MOS field effect transistor is biased to operate in saturation mode during substantially the entire time when the input signal makes the transition from the first state to the second state. In a specific embodiment, transistor 320 can have a threshold voltage of approximately 0V. In this case, transistor 320 stays turned on during signal transition with its gate 303 and source 304 tied together. In another embodiment, the threshold voltage of transistor 320 can be positive and transistor 320 may be operating in subthreshold region with reduced current and can have an even longer time-constant. A low threshold voltage or nearly zero volt threshold voltage can be obtain by using a native transistor structure, which can be formed by maintaining a background well doping concentration in the channel region and without a threshold voltage ion implantation step.
In the embodiment wherein the input signal is configured to make a transition from a low state to a high state, such as illustrated in
Accordingly to embodiments of the present invention, a large resistance and, hence, can be obtained with a transistor configured to operate in saturation mode. The transistor can also be a PMOS transistor. In a specific embodiment of the RC time-constant generation circuit, in which the input signal is configured to make a transition from a low state to a high state, the MOS transistor can be a PMOS transistor having a gate and a source connected to the input node. In yet another embodiment wherein the input signal is configured to make a transition from a high state to a low state, the MOS transistor can be a PMOS transistor having a gate and a source connected to output node. In an integrated circuit, the capacitor can include an MOS capacitor. Alternatively, the capacitor can include a second MOS transistor having a drain and a source connected together. In some embodiments, the MOS transistor can be a native transistor, as noted above.
In an embodiment, the large time-constant generation circuit can be implemented in a single integrated circuit chip to provide a large time-constant without the penalty of a large on-chip resistance. As a example, a time-constant of 8 msec can be achieved with an on-chip capacitor of 10 pF and a resistance of 800 MΩ. Such a resistance can be provided by, e.g., a transistor of W=0.42 μm by L=20 μm cascaded 256 times for total area of 2150 μm2 according to an embodiment of the present invention. In contrast, a convention on-chip diffusion resistor of 800 MΩ can take a chip area of as much as W=0.42 μm by L=20 μm cascaded 285,000 times, totaling 1000 times larger area. Therefore, a cost-effective large time-constant circuit can be provided using the embodiments described in
In some applications, it may be desirable to allow signal transition from both directions, i.e., from a low state to a high state, and from a high state to a low state. In some embodiments, the present invention provides a large time-constant generation circuit that is configured to operate with signal transitions in both directions. In a specific embodiment, the circuit can include a switch circuit that connects the gate of the transistor to either the drain or the source depending on the direction of the transition.
For example, for an input signal making a transition from a low state to a high state, the MOS transistor is an NMOS transistor having a gate and a source connected to the output node. In another example, for an input signal making a transition from a high state to a low state, the MOS transistor can be an NMOS transistor having a gate and a source connected to the input node. As a result, the MOS transistor is configured to operate in saturation mode, i.e., the gate-to-drain voltage is smaller than or approximately equal to the threshold voltage. In the saturation mode, the transistor has a large output resistance. Therefore, the transition of the output signal is characterized by a time-constant associated with this large output resistance and the capacitor coupled to the output node. This time-constant can be substantially longer than the first time-constant of the transition of the input signal. Consequently, a cost-effective circuit for generating a large RC time-constant can be realized in an integrated circuit without the chip area penalty of a large on-chip resistance.
In alternative embodiments of the circuit for generating a large RC time-constant described above, a PMOS transistor can be used, instead of the NMOS transistor. In this case, the connections may vary depending on the transistor and signal transition. For example, when the input signal makes a transition from a low state to a high state, the PMOS transistor is configured to have a gate and a source connected to the input node. In some embodiments, the transistor in the RC time-constant circuit can have a threshold voltage of approximately 0V. In these embodiments, when the gate and drain are connected together the gate-to-drain voltage is approximately equal to the threshold voltage. In some embodiments, each of the MOS transistors can be a native transistor. That is, the MOS transistors with low threshold voltage or nearly 0V threshold can be formed using their respective well doping and without additional threshold implant.
In some embodiments of the circuit described above, the capacitor can include an MOS capacitor. For example, the capacitor can include an MOSFET having a drain and a source connected together. In some embodiments the MOS transistor and the capacitor are included in a single integrated circuit chip, whereas in other embodiments, the circuit can also be implemented using discrete components.
In a specific embodiment of the circuit for generating a large RC time-constant described above, the circuit can also include one or more RC time-constant circuit cells coupled between the input node and the first MOS field effect transistor. Each cell has an MOS field effect transistor, a capacitor coupled to the MOS field effect transistor, and a switch circuit coupled to a gate of the MOS field effect transistor for biasing the MOS field effect transistor to operate in saturation mode when the input signal makes the transition from the first state to the second state.
In
According to embodiments of the invention, each of the embodiments in
In an embodiment, output amplifier 930 includes an upper stage 932 and a lower stage 934. In a specific embodiment, output driver circuit 930 may include a CMOS output driver circuit. In an embodiment, large RC time-constant generation circuit 925 is coupled to a power supply voltage VDD and provides a bias voltage to the circuits in the amplifier. Depending on the embodiment, the large RC time-constant generation circuit 925 may be similar to one of the large time-constant circuits described above with reference to
Thus, according to an embodiment, the present invention provides an audio system that includes an input for receiving an audio frequency input signal, a power supply terminal for connecting to a power supply, and an output terminal for providing an audio frequency output signal. The audio system also has a large RC time-constant circuit having an input node coupled to the power supply terminal and an output node for providing an operating voltage. Moreover, the audio system also includes an amplifier circuit coupled to the output node of the large RC time-constant circuit for receiving the operating voltage. The amplifier circuit is configured to provide the audio frequency output signal to the output terminal, which is coupled to a speaker. In an embodiment the large RC time-constant circuit includes one or more RC time-constant circuit cells, each of the cells having a capacitor coupled to an MOS transistor that is configured to provide a saturation mode output resistance for generating a large time-constant. In some embodiments, each of the RC time-constant circuit cells further comprises a switch circuit that is configured to bias the MOS transistor in the saturation mode during a transition in the power supply voltage. In a specific embodiment, the switch circuit in each cell is configured to connect a gate terminal of the MOS transistor to a drain terminal or a source terminal thereof in response to a change in the power supply voltage.
While the preferred embodiments of the invention have been illustrated and described, it will be clear that the invention is not limited to these embodiments only. Numerous modifications, changes, variations, substitutions and equivalents will be apparent to those skilled in the art without departing from the spirit and scope of the invention as described in the claims.