Patent Application
20110274290
Embodiments of the present invention relates generally to electronic circuit techniques. More specifically, embodiments of the present invention provide techniques for suppressing transients in amplifier circuits. Merely by way of example, some embodiments of the invention have been applied to audio power amplifiers for suppressing power up transients (e.g., popping noises) while also providing fast startup when the power amplifiers are turned on. But it would be recognized that the invention has a much broader range of applicability.
Amplifier circuits are prevalent in modern electronic devices. For example, an audio amplifier is an electronic amplifier that amplifies low-power audio signals to a level suitable for driving sound producing devices, such as loudspeakers or headphones. When audio power amplifiers use a single power supply, their output is usually biased at the mid-point of the power supply voltage. A large AC coupling capacitor may be connected between the output of the amplifier and a loudspeaker. The capacitor is used to block any DC current from flowing through a loudspeaker that has low impedance.
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 are expensive and often ineffective.
Therefore, new circuits and methods are needed that eliminate pops and clicks without significantly impacting the performances of audio amplifiers.
When an amplifier is powered on, its internal nodes can be charged at different speeds causing transient currents to flow in an uncontrollable manner and may also result in a long settling time before the amplifier is ready. Transient currents may flow through the AC coupling capacitor to the loudspeaker, which generates pop or click sound. In the following description, the term “pop” will be used for unwanted sound generated at power up and power down of an audio amplifier.
In an embodiment, a driver device for suppressing audible transients of an audio amplifier includes an amplifier for receiving an audio signal and a bias circuit configured to generate a stable voltage level for biasing the amplifier, wherein the voltage level is maintained even if the driver device is in an idle state or is powered off. In an embodiment, the bias circuit includes a CMOS circuit having minimum-geometry transistors such that the bias current is kept low when the bias voltage level is about half the supply voltage. In a specific embodiment, the bias circuit includes a CMOS inverter with an output tied to its input for providing a bias level near the mid point of the voltage supply range. In an embodiment, a buffer circuit is coupled between the bias circuit and a capacitor. The buffer circuit is configured to charge up the capacitor to the bias level quickly when the device is turned on, and, when the power is turned off, the buffer circuit is in a high impedance state to prevent leakage from the capacitor. In an embodiment, the buffer circuit is configured to maintain the capacitor voltage at around the mid point of the voltage supply. In a specific embodiment, the buffer circuit is implemented as a CMOS source follower with a transistor having substantially zero threshold voltage.
In another embodiment, a voltage reference circuit includes a bias circuit for generating a reference voltage. The voltage reference circuit further includes a capacitor coupled to the bias circuit for storing and maintaining the reference voltage. The voltage reference circuit also includes a buffer circuit for charging the capacitor, wherein the buffer circuit is in a high-impedance state when the circuit is in an idle state or powered off.
In yet another embodiment, a method of suppression audible transients includes generating a reference voltage using a self-biased circuit and charging a capacitor using a buffer circuit, wherein the buffer circuit is in a high impedance state when a power supply source is disconnected or powered off.
The device, circuit, and methods according to the present invention can be applied to conventional audio amplifiers. Parts and functions of the present invention include a native NMOS transistor as output driver for charging an external capacitor. In an embodiment, parts and functions of the present invention include a CMOS inverter having the output electrically coupled to the input for forming a self biased circuit that generates a common-mode voltage substantially independent of the power supply voltage and temperature.
These and other features and advantages of embodiments of the present invention will be more fully understood and appreciated upon consideration of the detailed description of the preferred implementations of the embodiments, in conjunction with the following drawings.
As noted above, conventional amplifier circuits can be susceptible to transient settling and noises. Specifically, in an audio amplifier, power-on transient can create pop noises and long turn-on times which are undesirable. As an example,
Audio amplifier 100 also includes a capacitor C 160 for keeping the common mode voltage stable. Capacitor C 160 needs to be large in order to get a high PSRR value (power supply rejection ratio). But a large capacitor will slow down the start-up time of audio amplifier 100 because of the long charging time period until the capacitor is fully charged. One known solution is to have a low initial impedance value for resistor R3113 for rapidly charging capacitor C 160, then switch to a higher resistive value of R3113 once capacitor C 160 reaches a predetermined voltage level. That is, the solution uses a two-step approach to charge capacitor 160: a fast charging step and a slow charging step that are controlled by a comparator that compares the charging voltage level with the predetermined voltage level. However, this two-step approach does not fully suppress or eliminate pops and clicks because it does not guarantee that the voltage levels at the positive and negative inputs of amplifier A1110 will have the same value during the start-up time. For example, voltage signal 104 may arrive later than the built-up of VMID 111 due to the delay in the low-pass filter. Or voltage reference VMID 111 may arrive later than signal 104 due to the large time constant determined by R3113 and C 160.
Various embodiments of the present invention will be described in detail with reference to the drawings. These diagrams are merely examples, and should not unduly limit the scope of the claims herein. In connection with the various aspects illustrated and described, one of ordinary skill in the art would recognize other variations, modifications, and alternatives.
Throughout the specification and claims, the term “circuit” means at least a single component or multiple components; the terms “bias voltage,” “reference voltage,” “common mode voltage” mean an operating voltage level of the audio amplifier; the term “audio driver” means an “amplifier circuit” for driving a speaker. The term “signal” means at least a current, voltage, or other signal. The term “powered off” means at least that an internal or external power supply to the audio driver is electrically disconnected from the audio driver or turned off.
In an embodiment, the relative output level of the inverter needs to be made to approximate one half of the voltage supply. A CMOS inverter solution for achieving that involves setting a width to length ratio for the two transistors. For example, the width to length (W/L) ratio of a P-channel transistor can be made much larger than the W/L ratio of the N-channel transistor.
A small signal analysis of the inverter can be undertaken about the self-bias point, in which the input and output are coupled together without any other extra load. The first order of approximation of the MOS transistor's source-drain current in the saturated region is given by the equation:
Ids=(K*W/L)*(Vgs−Vt)2 (1)
for Vds>(Vgs−Vt) (i.e., in saturation region), where
The P-channel MOS transistor 412 operates as a current source for providing current to the N-channel MOS transistor 414, which also operates as current source. The on-resistance of transistor 412 is very high and the current flowing across it is very low. Transistors 412 and 414 behave as a high-resistive voltage divider where the divide ratio is a function of their respective on-resistive values. Transistors 412 and 414 can be sized so that their interception point, i.e., node 413, will have a voltage level substantially equal to the half of the power supply voltage. In an embodiment, the dc bias circuit 350 is implemented using a MOS process. P-channel transistor 412 has a width of 0.5 micron and a length of 450 micron. And the current Ip 415 flowing across transistor 412 is less than 100 nA with the power supply of about 5.0V.
Buffer circuit 450 in
Vgs=Vin−Vout≈Vt (2)
where Vt is the threshold voltage of the N-channel MOS transistor 452. In an embodiment, MOS transistor 452 is a native N-channel MOS transistor having a threshold voltage substantially equal to 0V so that transistor 452 can deliver a large pull-up current.
The source of transistor 452 is further connected to a source of transistor 454 that represents a resistive load Rload. In an embodiment, the interconnection of transistor 452 and 454 is electrically connected to a bonding pad 453. Pad 453 has a voltage level that follows the voltage level Vref 413 at the input of buffer circuit 450. Reference voltage circuit 400 further includes a capacitor Cext 460 that is coupled to pad 453. In an embodiment, pad 453 is a bonding pad that constitutes the external connection terminal. Thus, capacitor Cext 460 is external to reference voltage circuit 400. Capacitor Cext 460 has a capacitance value that is adequate to maintain the voltage at pad 453 for a long period of time when the power supply source of reference voltage circuit 400 is switched off or disconnected. In order to rapidly charge capacitor Cext 460, it is preferable to obtain a sufficient large W/L ratio of N-channel MOS transistor 452.
If the power supply is switched off or disconnected, both transistors 452 and 454 will change to the off-state so that their impedance rapidly reach a very high value, i.e., buffer circuit 450 is in a high-impedance state. Capacitor Cext 460 thus keeps the stored voltage level for a time duration that is determined by the values of the Ron, Rload and Cext. As the value of Ron, Rload of transistors 452, 454, and Cext is very high, the discharge time of capacitor 460 is very long. Thus, the voltage reference Vref is maintained when the power supply is switched off or electrically disconnected from the audio driver. When the power supply is switched on again or electrically connected back to the audio driver, the reference voltage is already available to the audio driver without any significant delay so that the voltage difference between the speaker terminals is minimal and pops noise is suppressed. It is noted that the buffer circuit is in a high-impedance state if the voltage is about half the supply voltage, VDD. If the voltage deviates from half VDD due to, for example, too much by leakage or some external influence, the buffer circuit becomes very low impedance to bring the voltage back to half VDD. Thus, the buffer circuit is configured to maintain the voltage around half VDD.
In an embodiment, PMOS transistor 412 of self-biased circuit 410 has been implemented with a channel width of 0.5 micron and a channel length of 450 micron; NMOS transistor 414 has been implemented with a channel width of 0.5 micron and a channel length of 900 micron. The standby current is less than 100 nA with a power supply of 5V.
The above described embodiments of the present invention not only provide pops suppression, but also allow a fast start-up operation. Although the embodiments are described for audio driver applications, they may be used for other applications where a fast start-up time is necessary or desired.
Many modifications may be made to the above described embodiments without departing from the scope of the invention as claimed below. For example, additional devices may be inserted between various nodes, terminals, and devices in the above embodiments without materially changing their overall function. For example, voltage drops may be introduced by diodes, or transistors configured as diodes, to change various voltage levels, or buffers may be inserted between various nodes, transistors, and devices.
