Automatic input-gain control circuit and method thereof

Abstract

An automatic input-gain control circuit of an audio amplifier and a method thereof are provided. If an acoustic signal exceeds a dynamic range, a gain of the acoustic signal can be automatically attenuated based on a pre-set limit level to minimize a distortion of the acoustic signal caused by clipping of the acoustic signal and limit an over-input so as to input the acoustic signal within the dynamic range. Thus, an original form of an over-input signal on a specific level or more can be maintained but only a gain of the over-input signal can be attenuated. As a result, a distortion of an output waveform caused by clipping of the output waveform can be minimized, and a harmonic distortion and a stepped high frequency noise occurring during clipping can be simultaneously removed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

The above aspects and features of the present invention will be more apparent by describing certain embodiments of the present invention with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating a structure of a conventional D-class amplifier circuit;

FIGS. 2A and 2B are circuit diagrams illustrating a conventional limiter circuit;

FIGS. 3A and 3B are graphs illustrating clipping of an output voltage occurring when a peak vertex Vp of the output voltage exceeds a power supply voltage VCC;

FIG. 4 is a graph illustrating an internal operation waveform of an audio amplifier circuit;

FIG. 5 is a circuit diagram illustrating an automatic input-gain control circuit of an audio amplifier according to an embodiment of the present invention;

FIG. 6 is a circuit diagram illustrating an automatic input-gain control circuit according to another embodiment of the present invention;

FIG. 7 is a circuit diagram illustrating an equivalent circuit of an automatic input-gain control circuit according to an embodiment of the present invention; and

FIG. 8 is a graph illustrating a waveform of an output voltage output within a reference voltage according to an embodiment of the present invention.

Throughout the drawings, the same drawing reference numerals will be understood to refer to the same elements, features, and structures.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The matters defined in the description such as a detailed construction and elements are provided to assist in a comprehensive understanding of the embodiments of the invention and are merely exemplary. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, descriptions of well-known functions and constructions are omitted for clarity and conciseness.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

Certain embodiments of the present invention are characterized in that an over-input signal is attenuated to prevent a clipping distortion D1 caused by an over-input and an over-output and a stepped high frequency distortion D2 caused by a saturation of an internal block of the audio amplifier, so that an output signal does not approach a power supply voltage VCC of the audio amplifier but is within a threshold voltage Vth regardless of variations in a gain of the audio amplifier and the power supply voltage VCC. Also, a whole gain may be attenuated instead of cutting over-input components to maintain an original form of the input signal and minimize a distortion of the input signal so as to limit an input level. In addition, the threshold voltage Vth may be adjusted with respect to the power voltage VCC to vary an output limited level so as to design an amplifier having various output powers depending on products. Thus, the amplifier may be designed to have the same speaker impedance and power supply voltage as a previously designed amplifier so as to share a speaker and a power source block.

FIG. 5 is a circuit diagram illustrating an automatic input-gain control circuit of an audio amplifier according to an embodiment of the present invention. Referring to FIG. 5, the automatic input-gain control circuit of the audio amplifier according to the present embodiment includes an output comparator 510, an input gain adjuster 520, a switching amplifier 530, and a speaker SP.

The automatic input-gain control circuit may further includes the comparator 120, the gate driver 130, the low pass filter (LPF) 140, and the triangular wave generator 150 as shown in FIG. 1. That is, the automatic input-gain control circuit of the present invention may be incorporated in to a prior art device such as the one shown in FIG. 1. The structures of the comparator 120, the gate driver 130, the LPF 140, and the triangular wave generator 150 have been described above and thus, their detailed descriptions will be omitted herein.

The output comparator 510 compares a voltage of an output signal Vo (hereinafter called “output voltage”) with a reference voltage for inputting an acoustic signal of the audio amplifier within a power supply voltage VCC. The output comparator 510 includes first and second comparators 512 and 514 as shown in FIG. 5. The first comparator 512 compares the output voltage with a threshold voltage Vth of a plus power supply voltage +VCC. The second comparator 514 compares the output voltage with a threshold voltage Vth of a minus power supply voltage −VCC. Here, the threshold voltages Vth mean margin voltages for disallowing an output signal to reach the power supply voltage VCC.

The input gain adjuster 520 automatically adjusts a voltage of the acoustic signal to input the acoustic signal within the power supply voltage VCC according to the comparison result of the output comparator 510. In other words, the input gain adjuster 520 attenuates a whole gain of the acoustic signal so as to maintain an original form of the acoustic signal and minimum a distortion of the acoustic signal. For this purpose, the input gain adjuster 520 operates as a voltage control variable resistor Rds for adjusting an input impedance of the acoustic signal.

The switching amplifier 530 amplifies the acoustic signal.

In the automatic input-gain control circuit of the audio amplifier having the above-described structure shown in FIG. 5, the acoustic signal generated by an operation power source is applied to the switching amplifier 530 through an input resistor Rin. The output comparator 510 and the speaker SP are connected to an output port of the switching amplifier 530, and the input gain adjuster 520 for adjusting the acoustic signal is connected to an input port of the switching amplifier 530. The first comparator 512 of the output comparator 510 is connected to the plus power supply voltage +VCC above the switching amplifier 530, and the second comparator 514 of the output comparator 510 is connected to the minus power supply voltage −VCC under the switching amplifier 530.

FIG. 6 is a circuit diagram illustrating an automatic input-gain control circuit according to another embodiment of the present invention. The automatic input-gain control circuit according to the present embodiment includes a detection transmitter 610 and an input adjuster 620. Here, the detection transmitter 610 and the input adjuster 620 are exemplary implementations of the output comparator 510, and the input gain adjuster 520 of FIG. 5 respectively.

The detection transmitter 610 includes a detector and a transmitter. The detector operates when an output voltage exceeds a reference voltage. The transmitter is turned on depending on the operation of the detector to transmit the output voltage to the input adjuster 620. Here, the reference voltage is a voltage obtained through a subtraction of a threshold voltage Vth from a power supply voltage VCC. In other words, the reference voltage corresponds to a difference between the power supply voltage VCC and the threshold voltage Vth.

As shown in FIG. 6, the detector may be a constant voltage element including a zener diode DZ1, and the transmitter may be a switching element including a transistor Q1.

In more detail, in the detection transmitter 610, the zener diode DZ1 is connected to a resistor R in series between the power supply voltage VCC and a ground voltage, and a base of the transistor Q1 is connected to a connection point between the zener diode DZ1 and the resistor R. A diode D1 is connected to a resistor R1 in series between an emitter of the transistor Q1 and an output port, and a collector is connected to the input adjuster 620. Here, the transistor Q1 may be a plug-and-play (PNP) (?) type transistor.

A capacitor C1 is connected to an input resistor Rin in series between a voltage of an input signal Vin (hereinafter called “input voltage”) and an input port INPUT. The input adjuster 620 including a junction field effect transistor (J-FET) J1 is connected to a connection point between the input port INPUT and the input resistor Rin. In other words, the input adjuster 620 may include an element operating as a variable resistor or may include the J-FET J1 as shown in FIG. 6.

A drain of the J-FET J1 of the input adjuster 620 is connected to the connection point between the input port INPUT and the input resistor Rin. A source of the J-FET J1 is grounded, and a capacitor C2 is connected to a resistor R in series between a gate and a ground. A minus power supply voltage −VCC is connected to a connection point between the capacitor C2 and the resistor R through a resistor R2, and a collector of the transistor Q1 of the detection transmitter 610 is connected to the connection point between the capacitor C2 and the resistor R.

An equivalent circuit of the automatic input-gain control circuit shown in FIG. 6 is shown in FIG. 7. In other words, the equivalent circuit includes an input voltage, an input resistor Rin, and a variable resistor Rds and adjusts an intensity of the input voltage through the variable resistor Rds.

The operation of the automatic input-control circuit of the audio amplifier according to the present embodiment will now be described.

For the description of the operation of the automatic input-gain control circuit, it is supposed that the automatic input-gain control circuit shown in FIG. 6 is constituted as an input part of the audio amplifier shown in FIG. 1.

Here, the automatic input-control circuit operates when the output voltage exceeds the threshold voltage Vth defined as a margin with the power supply voltage VCC. Here, the threshold voltage Vth is defined as in Equation 2:

V _th =V _DZ1−2V_D1  (2)

wherein V_DZ1 denotes a backward voltage of the zener diode DZ1, and V_D1 denotes a forward voltage between the emitter and the base of the transistor Q1 or a forward voltage of the diode D1. Thus, if the output voltage is greater than the difference between the power supply voltage VCC and the threshold voltage Vth, the transistor Q1 operates so as to increase a gate voltage of the J-FET J1 of the input adjuster 620. If the increased gate voltage is greater than or equal to a turn-on voltage −V_G.ON of the J-FET J1, an impedance between the drain and source of the J-FET J1 varies and decreases. In this case, the input adjuster 620 may be equalized as shown in FIG. 7. Thus, a voltage V_INPUT substantially input to the input port INPUT of the audio amplifier is expressed with respect to the voltage Vin of the input signal as in Equation 3:

$\begin{array}{cc}{V}_{\mathrm{INPUT}}=\left(\frac{R_{\mathrm{ds}}}{R_{in}+R_{\mathrm{ds}}}\right)*V_{in}& \left(3\right)\end{array}$

As shown in Equation 3, variable resistance Rds decreases with an increase in an over-input state. Thus, the voltage V_INPUT is attenuated. As a result, a peak value of a substantially output voltage is fixed to and does not exceed the power supply voltage VCC and the threshold voltage Vth. A gain of the input signal is attenuated as in Equation 3 to satisfy the conditions of Equation 3. Thus, only a gain of the output signal is attenuated with an original form of the output signal maintained. If a level of the input signal is in a normal state not in the over-input state, the transistor Q1 and the J-FET J1 are turned off. Thus, Rds is greater than the input resistor Rin, and the voltage V_INPUT is equal to the input voltage. As a result, the input signal is attenuated to be equal to an original acoustic signal and then input to the input port of the audio amplifier.

As described, in an audio apparatus including an acoustic amplifier and a switching amplifier according to the present invention, if an input acoustic signal exceeds a dynamic range, a gain of the input acoustic signal is automatically attenuated based on a pre-set limit level so as to input the input acoustic signal within the dynamic range. Thus, an acoustic signal equal to an original signal is output, a distortion of the acoustic signal is minimized when the acoustic signal is clipped, a stepped distortion is removed, and an over-input is limited.

As described above, in an automatic input-gain control circuit and a method thereof according to the present invention, only a gain of an over-input signal more than or equal to a specific level can be attenuated instead of cutting the over-input signal. Thus, a distortion of an output waveform caused by clipping of the output waveform can be minimized. In particular, a switching amplifier can prevent a saturation of an internal block to simultaneously remove a harmonic distortion and a stepped high frequency noise occurring during clipping. Also, a linear analog amplifier can attenuate only the gain with maintaining an original form of the over-input signal instead of simply clipping the over-input signal so as to minimize a distortion of an output signal.

Electronic appliance having an audio function can use an identical power supply voltage to arbitrarily adjust a reference voltage of an output comparator circuit operating as a gain attenuating circuit so as to limit an output level. Thus, an effect of varying the power source or a speaker impedance can be obtained without actually doing so. Accordingly, a power supply voltage and a speaker impedance of a product having various output specifications can be standardized.

As aforementioned, the exemplary embodiments of the present invention are shown and described, but the present invention is not limited to the specific embodiments described above, and can be implemented in various modifications by those skilled in the art to which the present invention pertains without departing from the scope of the invention as defined by the appended claims and the full scope of equivalents thereof.

Claims

1. An automatic input-gain control circuit of an audio amplifier comprising: an output comparator comparing a voltage of an output signal of the audio amplifier with a reference voltage generated based on a threshold voltage Vth; andan input gain adjuster automatically adjusting a voltage of an input signal of the audio amplifier according to the comparison result of the output comparator so that the voltage of the output signal is within the reference voltage.

2. The automatic input-gain control circuit of claim 1, wherein the reference voltage is a voltage corresponding to a difference between a power supply voltage VCC and the threshold voltage Vth.

3. The automatic input-gain control circuit of claim 1, wherein the threshold voltage Vth is calculated using Equation “Vth=VDZ1−2VD1”, wherein VDZ1 denotes a backward voltage of a zener diode DZ1, and VD1 denotes a forward voltage between an emitter and a base of a transistor Q1 or a forward voltage of a diode D1.

4. The automatic input-gain control circuit of claim 1, wherein the output comparator comprises: a detector detecting whether the voltage of the output signal exceeds the reference voltage; anda transmitter transmitting the voltage of the output signal to the input gain adjuster according to the detection result of the detector.

5. The automatic input-gain control circuit of claim 4, wherein the detector is a constant voltage element.

6. The automatic input-gain control circuit of claim 5, wherein the constant voltage element comprises a zener diode.

7. The automatic input-gain control circuit of claim 4, wherein the transmitter is a switching element.

8. The automatic input-gain control circuit of claim 7, wherein the switching element comprises a transistor.

9. The automatic input-gain control circuit of claim 4, wherein the input gain adjuster is a variable resistor element adjusting an input impedance to attenuate an input gain so that the voltage of the output signal is within the reference voltage if the voltage of the output signal exceeds the reference voltage.

10. The automatic input-gain control circuit of claim 9, wherein the variable resistor element comprises a J-FET (junction field effect transistor).

11. The automatic input-gain control circuit of claim 4, wherein if the detector detects that the voltage of the output signal exceeds the reference voltage, the output comparator transmits the output signal to the input gain adjuster through the transmitter.

12. The automatic input-gain control circuit of claim 1, wherein the output comparator comprises: a first comparator comparing the voltage of the output signal with a plus reference voltage; anda second comparator comparing the voltage of the output signal with a minus reference voltage.

13. A method of automatically controlling an input gain of an audio amplifier, comprising: comparing a voltage of an output signal of an audio amplifier with a reference voltage generated based on a threshold voltage Vth; andautomatically adjusting a voltage of an input signal of the audio amplifier according to the comparison result so that the voltage of the output signal is within the reference voltage.

14. The method of claim 13, wherein the reference voltage is a voltage corresponding to a difference between a power supply voltage VCC and the threshold voltage Vth.

15. The method of claim 13, wherein the comparing of the voltage of the output signal of the audio amplifier with the reference voltage generated based on the threshold voltage Vth comprises: detecting whether the voltage of the output signal exceeds the reference voltage; andoutputting the output signal according to the detection result.

16. The method of claim 15, wherein the detecting of whether the voltage of the output signal exceeds the reference voltage is performed using a constant voltage element.

17. The method of claim 15, wherein the output signal of the audio amplifier is output using a switching element.

18. The method of claim 13, wherein if the voltage of the output signal exceeds the reference voltage, an input impedance is adjusted using a variable resistor element to attenuate an input gain so that the voltage of the output signal is within the reference voltage.

19. The automatic input-gain control circuit of claim 2, wherein the threshold voltage Vth is calculated using Equation “Vth=VDZ1−2VD1”, wherein VDZ1 denotes a backward voltage of a zener diode DZ1, and VD1 denotes a forward voltage between an emitter and a base of a transistor Q1 or a forward voltage of a diode D1.