CN102332869B - Highly integrated BTL audio power amplifier - Google Patents

Abstract

The invention discloses a highly integrated BTL audio power amplifier, including 4 amplifiers, 2 transconductance amplifiers, first to ninth resistors, internal compensation capacitors, and a reference current module, wherein the first and second amplifiers provide the main path And as the upper and lower drive units of the audio power amplifier, the third and fourth amplifiers, the first transconductance amplifier, and the fifth to eighth resistors together form an output offset compensation module. The first transconductance amplifier is responsible for detecting the DC common mode at both ends of the BTL Offset, feed back to the current control terminal to fine-tune the current, thereby reducing the offset, and fine-tune the gain during output clipping to make the gains at both ends of the output exactly the same, so that the output power reaches the maximum and the efficiency is the highest. The reference current module is responsible for the first transconductance The output of the amplifier provides an accurate output midpoint; the second transconductance amplifier is responsible for providing frequency compensation of the system except for Miller compensation, providing sufficient phase margin for the system.

Description

Highly integrated BTL audio power amplifier

technical field

The invention relates to an audio frequency power amplifying circuit, in particular to offset compensation, gain compensation and phase compensation in a BTL application audio power amplifier.

Background technique

Under the same voltage, in order to obtain higher audio output power, the BTL method will be used. As shown in Figure 1, it is a more popular usage. This usage saves the output coupling capacitor and reduces the consumption of external voltage division. However, This usage has two disadvantages: there is no offset compensation circuit, and compensation capacitors are required outside the output. BTL applications have particularly high requirements for the offset voltage at the output of the circuit. If the offset is not handled properly, it will affect the static power consumption and generate a large current noise. , when the large signal output clips, if there is a difference in the gain at both ends, it will affect the overall output efficiency; in addition, the circuit in BTL application is more unstable than that in monophonic application, and the phase margin becomes smaller. The general solution is to connect an external 100nF capacitor, but the present invention improves the internal structure without any external compensation capacitor.

Contents of the invention

Technical problem : the present invention mainly solves the problem of BTL imbalance and gain inconsistency, and the problem of circuit stability.

Technical solution :

A highly integrated BTL audio power amplifier, including 4 amplifiers AMP, 2 transconductance amplifiers, first to ninth resistors, internal compensation capacitors, and a reference current module, wherein the first amplifier and the second amplifier are the main amplifiers, providing The main channel, the third amplifier, the fourth amplifier, the first transconductance amplifier, and the fifth to eighth resistors together form an output offset compensation module, and the third amplifier and the fourth amplifier are respectively the upper and lower drive units of the audio power amplifier;

The first transconductance amplifier is responsible for detecting the DC common-mode offset at both ends of the BTL, and feeding back to the current control terminal to fine-tune the current, thereby reducing the offset. It is exactly the same, so that the output power reaches the maximum and the efficiency is the highest. The reference current module is responsible for providing an accurate output midpoint for the output terminal of the first transconductance amplifier; the second transconductance amplifier is responsible for providing frequency compensation of the system except for Miller compensation. The equivalent capacitance is 100nF, which provides enough phase margin for the system;

Wherein: the positive end of the first amplifier is connected to the reference voltage signal, the negative end of the first amplifier is respectively connected to one end of the first resistor and one end of the second resistor; the output end of the first amplifier is connected to the positive end of the third amplifier;

The other end of the first resistor is connected to one end of the third resistor and the fourth resistor respectively, the other end of the third resistor is connected to the negative terminal of the second amplifier, the positive terminal of the second amplifier is connected to the reference voltage signal, and the second amplifier The output terminal of the fourth amplifier is connected to the positive terminal;

The output end of the third amplifier is respectively connected to one end of the fifth resistor and the other end of the second resistor; the other end of the fifth resistor is respectively connected to the negative end of the third amplifier and one end of the sixth resistor; The other ends of the six resistors are respectively connected to one end of the eighth resistor and the positive end of the first transconductance amplifier;

The other end of the eighth resistor is respectively connected to the negative end of the fourth amplifier and one end of the seventh resistor; the output end of the fourth amplifier is respectively connected to the other end of the seventh resistor and the other end of the fourth resistor;

The negative terminal of the first transconductance amplifier is connected to the midpoint potential signal, and the output terminal of the first transconductance amplifier is connected to one end of the ninth resistance, the negative terminal of the second transconductance amplifier, and the output terminal of the second transconductance amplifier; The positive end of the second transconductance amplifier is respectively connected to the other end of the ninth resistor and one end of the internal compensation capacitor, and the other end of the internal compensation capacitor is grounded.

Beneficial effects : due to the use of the offset compensation network, the circuit fluctuates with the process, the offset is significantly reduced, and the degree of dispersion is also greatly reduced. Compared with the circuit without this compensation network, the output efficiency is also improved to a certain extent. Due to the integration of BTL necessary 104 capacitors simplify the peripherals, make the application easier and the system more stable.

Description of drawings

Figure 1 is a general BTL functional block diagram. The figure includes: a first operational amplifier A and a second operational amplifier B.

Figure 2 is a block diagram with offset compensation and frequency compensation.

The figure includes: the first operational amplifier A, the second operational amplifier B, the third operational amplifier C, the fourth operational amplifier D, the fifth transconductance amplifier E, and the sixth transconductance amplifier F.

Detailed ways

Utilized six amplifiers in the present invention, wherein the first operational amplifier A, the second operational amplifier B determine the gain of the main path, the closed-loop gain of BTL

GV={1+R2/(R1+R3)}+R4/(R1+R3)}+ΣGV (correction); In order to compensate for the fluctuation of the process, it must be set as R2=R4, R1=R3; When the gain of the op amp is inconsistent, there will be a common mode offset, and the compensation circuit will start when it deviates from the midpoint, adjust the gain compensation current, and properly reduce the op amp that clips first, so that the gains of the two channels are exactly the same.

Common mode offset, the circuit itself sets a reference current i0=(VCC-2Vref)/R2, assuming complete symmetry, under the premise of no compensation current, the reference current is divided equally by the first operational amplifier A and the second operational amplifier B, in The current flowing through the resistor R2 in the first operational amplifier A is equal to the current flowing through the resistor R4 in the second operational amplifier B, so the voltage at the midpoint of the output of the first operational amplifier A is:

i0*R2/2+Vref=Vref+VCC/2-Vref=VCC/2

The purpose of setting Vref is to increase the maximum input level that the input end can withstand.

Similarly, the voltage at the midpoint of the output of the second operational amplifier B is also VCC/2.

In practice, the first operational amplifier A and the second operational amplifier B have a common-mode offset, and the third operational amplifier C and the fourth operational amplifier D are responsible for detecting the offset. When the output gain does not match, the vo voltage deviates from the midpoint, and the offset compensation circuit is started , output compensation current, and then adjust the channel with higher gain, so that the gains of the two outputs are equal; when the process fluctuation causes the output common mode offset, the offset compensation module starts, outputs the correction current, adjusts the output midpoint, and achieves the effect of correcting the static offset effect. The third operational amplifier C and the fourth operational amplifier D are simple closed-loop application operational amplifiers with a gain of 3; because they are set in the output drive part, they also reduce the THD of the output, thereby reducing the overall distortion.

Because of the BTL application, two systems A and B working independently are combined together, and the required frequency compensation is more complicated. The compensation point selected by the present invention, from the perspective of the entire system, assumes that the input terminal is grounded, and the compensation point is For the AC ground of the system, if the system is powered on or other interference causes high-frequency offsets in the output of the first operational amplifier A and the second operational amplifier B, then there is an interference pulse at the compensation point. Since it is a BTL application, this interference is passed through the first operation The feedback from the amplifier A and the second operational amplifier B continues to form an oscillation, and this oscillation will continue to form a self-excitation. This phenomenon is not the general phase compensation theory,

Because there is no problem with the frequency compensation of the first operational amplifier A and the second operational amplifier B, they work very stably alone, but when they are combined into a BTL application, in order to stabilize the system, an additional compensation network needs to be re-set. In the present invention, a capacitor to the ground is set at the compensation point as a high-frequency AC ground, so that the high-frequency waves output by the first operational amplifier A and the second operational amplifier B are directly fed back to the ground through the compensation capacitor, and do not flow through the main path to form a Positive feedback. Prevent high frequency self-excitation. The following deduces how to realize the 100nF capacitor.

Suppose the inflowing current is i, the port voltage is v, the voltage across the resistor is VR; the capacitance to ground is c, the equivalent impedance is rin; the equivalent equivalent capacitance is cin; the transconductance of the gm transconductance amplifier is the first operation Amplifier A; then satisfy the following relationship:

i=VR*A+VR/R(1)

v=VR/jωc*R +VR(2)

From (1) and (2), it can be concluded that rin=ri+cin=1/A*R+1/ jωc*R*A (3), and the equivalent diagram is shown in Figure 3.

Where ri=1/A*R; cin=1/jωc*R*A; ri≈0, the equivalent capacitance cin= (A*R) 1/jωc, so the equivalent capacitance is enlarged by (A*R) times; Assume that the transconductance is A=10, R=10K, and the internal integrated capacitor c=1pF, so cin=(10*10k*1p)F=100nF; a small capacitor is used to realize the function of a large capacitor and eliminate the high-frequency self- Excited.

Claims (1)

1. A highly integrated BTL audio power amplifier is characterized in that: comprising first to fourth amplifiers AMP (A, B, C, D), first to second transconductance amplifiers (E, F) and first To the ninth resistor (R1~R9), the internal compensation capacitor (C1), and the reference current module, in which the first amplifier (A) and the second amplifier (B) are the main amplifiers, providing the main path, and the third amplifier (C ), the fourth amplifier (D), the first transconductance amplifier (E), the fifth to eighth resistors (R5~R8) together constitute the output offset compensation module, while the third amplifier (C), the fourth amplifier (D) They are the upper and lower drive units of the audio power amplifier; The first transconductance amplifier (E) is responsible for detecting the DC common-mode offset at both ends of the BTL, and feeding back to the current control terminal to fine-tune the current, thereby reducing the offset. At the same time, the first transconductance amplifier (E) is also fine-tuning the gain when the output clips , so that the gains at both ends of the output are exactly the same, so that the output power reaches the maximum and the efficiency is the highest. The reference current module is responsible for providing an accurate output midpoint for the output of the first transconductance amplifier (E); the second transconductance amplifier (F) is responsible for providing The frequency compensation of the system except Miller compensation, its equivalent capacitance is 100nF, which provides enough phase margin for the system; Among them: the positive terminal of the first amplifier (A) is connected to the reference voltage signal (Vref), and the negative terminal of the first amplifier (A) is respectively connected to one end of the first resistor (R1) and one end of the second resistor (R2); The output terminal of the first amplifier (A) is connected to the positive terminal of the third amplifier (C); The other end of the first resistor (R1) is respectively connected to one end of the third resistor (R3) and the fourth resistor (R4), and the other end of the third resistor (R3) is connected to the negative end of the second amplifier (B) , the positive terminal of the second amplifier (B) is connected to the reference voltage signal (Vref), and the output terminal of the second amplifier (B) is connected to the positive terminal of the fourth amplifier (D); The output terminal of the third amplifier (C) is respectively connected to one end of the fifth resistor (R5) and the other end of the second resistor (R2); the other end of the fifth resistor (R5) is respectively connected to the third amplifier (C) The negative end of the sixth resistor (R6) is connected to one end; the other end of the sixth resistor (R6) is respectively connected to one end of the eighth resistor (R8) and the positive end of the first transconductance amplifier (E); The other end of the eighth resistor (R8) is respectively connected to the negative end of the fourth amplifier (D) and one end of the seventh resistor (R7); the output end of the fourth amplifier (D) is respectively connected to the seventh resistor ( The other end of R7) and the other end of the fourth resistor (R4); The negative terminal of the first transconductance amplifier (E) is connected to the midpoint potential signal, the output terminal of the first transconductance amplifier (E) is connected to one end of the ninth resistor (R9), the negative terminal of the second transconductance amplifier (F) end, the output end of the second transconductance amplifier (F); the positive end of the second transconductance amplifier (F) is respectively connected to the other end of the ninth resistor (R9) and one end of the internal compensation capacitor (C1), and the internal compensation The other end of the capacitor (C1) is grounded.

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