[0046] Generally speaking, in the example of the present invention, in the standby mode, the amplifier or buffer output in the circuit (or the output of a filter or other signal processing circuit) is disconnected from the circuit. Although in general, the output node voltage of the circuit will be pulled to the main supply voltage by one or more resistive elements in the circuit (for example, pulled down), but because the design of the present invention adds elements matching these resistive elements , So the output node voltage will be pulled back to the static voltage or non-signal voltage. The resistance element that pulls the output voltage to the main supply voltage is usually the element of the feedback circuit, which may be one or more resistors; the elements that match these resistance elements are preferably matched resistors. Those with experience in the industry can understand what is said here. The "matched" does not necessarily mean "equal", but refers to the use of matched resistors to provide a precise resistance ratio.
[0047] Generally speaking, in the example of the present invention, one or more switch resistors are used to form a voltage divider, and at least a part of the voltage divider is formed by one or more resistors (or resistance elements). These resistors are used when the circuit is in the working mode. Play a certain role, such as setting the operating point, parameters, or characteristics of the circuit. This design provides an accurate and inexpensive method for setting the quiescent output voltage in standby mode. The matching resistance and amplifier bias should preferably be more accurate, so as to provide a low voltage stage when switching between working mode and standby mode, for example, less than 30mV, preferably less than 10mV.
[0048] In the embodiment of the present invention, when the amplifier or buffer has two working stages, if the output of one stage (for example, the second stage) in the signal path is pulled to the main by the feedback elements of the two stages through the DC signal path Power supply voltage, then the present invention can pull the output node voltage of any stage back to the static voltage. For example, (in the signal path) the output of the second stage can be pulled back to the above-mentioned static voltage, and the output of the first stage can also be pulled back to a certain voltage value (especially the static voltage), which can make the second stage's output The output is pulled back to the quiescent voltage. In the standby mode, it is better to disconnect the working output of the above two stages from the feedback circuit in order to fix the output voltage of one of the stages to a static level.
[0049] An amplifier system implemented according to the present invention may have more than two modes, including a working mode and a standby mode. An output node of the system is connected to a voltage different from its static "operating" voltage in a resistive manner; in the operating mode, this node is driven to the "correct" static voltage value by the amplifier; in the standby mode, the amplifier is Disconnected or disabled, so the aforementioned resistive connection to the output node tends to pull the output node voltage toward the (different) voltage connected to the output node. This trend can be counteracted by a resistor connected to a voltage source through a switch. The value of this resistance should be selected to be able to restore the output node voltage in standby mode to the quiescent voltage.
[0050] In many cases (not necessary), the output node is AC coupled to the load through a capacitor. By providing the same static DC voltage on the working side of the capacitor, the switching transient can be suppressed or eliminated, thereby controlling or eliminating the output "thumping sound". It is best to use existing methods commonly used in the industry to control the sound of power-on heavy blows, such as limiting the value of power-on working current, or slowing down the rate of change of Vmid (dV/dt).
[0051]The switch that connects the resistor to restore the static voltage on the output node can be a MOS transistor, a strobe circuit, or a two-pole transistor. In order to be able to disconnect the amplifier, a similar switch can be connected in series with its output, or the output terminal can be closed to achieve zero output current (that is, put in a high-impedance state). The connection between Vss and Vdd may or may not be an external ground potential. The quiescent voltage can be Vdd/2 or other voltages.
[0052] The switch resistance can be at the output end, or at the fixed voltage end of the switch, or the switch can be connected between a pair of resistors. The resistance of this pair of resistors can restore the static voltage at the output node. If a switch is connected between the above-mentioned pair of resistors, then a capacitor should be connected between the switch end of the top resistor (connected to the reference voltage or supply voltage) and the ground to filter out AC power supply noise.
[0053] The above-mentioned amplifier system may have multiple channels, and it is better to provide an independent switch resistance for each channel in order to restore the voltage of the output node of each channel to a static voltage value. In this design, multiple channels can be switched independently, but can share a decoupling capacitor and top resistance (connected to the reference voltage or supply voltage).
[0054] Please refer to image 3 As shown, image 3 In addition to having an audio transient suppression circuit of an embodiment of the present invention, the audio system 300 in figure 1 The audio system in is similar. In particular, the resistor R6 304 and the switch S4 302 are connected in series between Vdd102 and the output node X128.
[0055] When the system is in working mode, switch S4 302 is open and switch S1 126 is closed. When the system is in the standby mode, the switch S1 126 is opened to disconnect the output of the working amplifier 120; the switch S4 302 is closed, and the output node 128 is connected to Vdd through the resistor R6 304. The selected value of R6 304 (and the resistance value of switch S4 302 if necessary) should be guaranteed: in standby mode, when R6 304 is connected in series with R1 122 and R2 124 between Vdd102 and Vss104, the value of R6 304 The voltage value of node X128 should be fixed to Vmid (ie, static voltage). If the quiescent voltage is equal to Vmid, then the value of R6 304 is actually equal to the resistance value of R1 122 and R2 124 connected in series.
[0056] image 3 The design has many advantages:
[0057] In an integrated circuit system, the resistors R1 122, R2 124 and R6 304 can be designed to be precisely matched, so that Vmid will be its nominal resistance, so when the system is switched from working mode to standby mode, it can be minimized Transient on node X128. In the working mode, there is no need to connect any transistor switches in series with R1 122 and R2 124, thus avoiding any bias or distortion that may be introduced. Moreover, in a multi-channel system, since the switch S4 302 connects the resistor R6 304 to a low-impedance, well-decoupled power supply (Vdd122), the possibility of transient coupling to other channels is reduced.
[0058] Figure 4 is based on image 3 Schematic diagram of the design concept of mid-range audio transient suppression.
[0059] in Figure 4 Here, the amplifier 120 is represented as a Thevenin voltage source 402 that provides a voltage output Va. in Figure 4 In the schematic diagram with image 3 The components are similar to those used image 3 Reference number. The voltage source 402 drives the output node 128 through the switch S1 126. When the circuit is in the working mode, the switch S1 126 is normally closed, and when the circuit is in the standby mode, it is in an open state. in Figure 4 in, image 3 The feedback resistance pair R1 122 and R2 124 in the circuit is represented by the resistance Ra408, the resistance R6 304 is represented by the resistance Rb406, and Rb406 is coupled to Vdd102 through the switch S4 404 (normally open in the working mode). The output node X128 is coupled to the load R3 through the AC coupling capacitor 130. in Figure 4 In the example shown, Ra=R1+R2, but those experienced in the industry may understand that Ra can represent different combinations of resistance elements in other signal processing circuits.
[0060] Generally speaking, Va is:
[0061] Va=Vss+(Vdd-Vss)*y (Formula 1)
[0062] Where y is a fixed value between 0 and 1. In this example, it is usually 0.5.
[0063] Since S1 126 is in the feedback loop, its resistance can be ignored. In the standby mode, the switch S1 126 is open and the switch S4 302 is closed. Resistor Rb406 represents R6 304, which is preferably connected in series with any parasitic resistance of switch S4 302. Therefore, in the standby mode, the voltage Vx of the output node X128 is:
[0064] Vx=Vss+(Vdd-Vss)/(1+Rb/Ra) (Formula 2)
[0065] The condition that the node X128 has the same voltage value in working mode and standby mode is:
[0066] Vss+(Vdd-Vss)*y=Vss+(Vdd-Vss)/(1+Rb/Ra) (Formula 3)
[0067] therefore:
[0068] Rb/Ra=(1/y)-1 (Equation 4)
[0069] If y=0.5, then Rb=Ra.
[0070] FIG. 5 is an extension of the audio transient suppression design of the two-stage amplifier circuit 500. The design in Figure 5 and image 3 The design in is similar, but an inverting amplifier stage is added between the output node 128 and the output coupling capacitor 130. This stage consists of a working amplifier 310 with a non-inverting input connected to Vmid122 and feedback resistors R8 306 and R7308 to set the gain. Like the working amplifier 120, the switch S5 122 is used to remove the output coupling of the amplifier 310 from the output node Z314 connected to the coupling capacitor 130. Switch S5 122 is closed in working mode and open in standby mode.
[0071] As shown in FIG. 5, the voltage of the output node Z314 is pulled to Vss104 by a resistance path including R8 306, R7 308, R1 122, and R2 124 (when the system 500 is in the standby mode). The switch resistor R6 304 is connected to the node Z314. From Figure 4 We can see from the principle that Ra=R8+R7+R1+R2. In order to set the voltage of the output node Z314 at Vmid in the standby mode, the sum of the parasitic resistance of the resistor R6 304 and the switch S4 302 should be equal to the combined value of R8 306, R7 308, R1 122 and R2 124.
[0072] In another design of the present invention, the resistor R6 304 may be coupled to the node X128 instead of the node Z314, as shown by the dashed line 316 in the figure. In this case, Ra=R1+R2. In order to set the voltage value of node X128 to Vmid, the combined value of R6 304 and any on-resistance of switch 302 should be equal to the series combined value of R1 122 and R2 124. We can know that in this design, when the switch S5 312 is open, the voltage on the node Z314 will be pulled to the static voltage value of the node X128 through the resistors R8 306 and R7 308, and will eventually be the same as the voltage value of the node X128.
[0073] Those of ordinary skill in the art can understand that in the above example, the switch S1126 (or S5 312) may not exist in actual applications, but is actually an output driver stage, such as a working amplifier. The high-impedance output state realizes the disconnection, in this case, the driver actually provides a zero output circuit.
[0074] The above-mentioned circuits and methods are not limited to systems with AC coupling capacitors in the signal path. For a full DC coupling system, it is also very important to avoid transients when switching between standby mode and working mode. Those of ordinary skill in the art can know that the above-mentioned circuit and method are also applicable to this type of DC coupling system, such as a system in which the above-mentioned capacitor C1 130 is replaced by a short circuit.
[0075] Although the example of the present invention is described with an amplifier circuit, the present invention is also applicable to other audio signal processing systems, such as preamplifier circuits or filter circuits. Those of ordinary skill in the art can easily make changes on the basis of the present invention. However, it should be noted that the scope of application of the present invention is not limited to the above-mentioned examples. Therefore, any application obtained by equivalent modification or modification according to the present invention should belong to the scope of the present invention.
