This article uses National Semiconductor 's audio driver LME49810 as an example. It provides 200V peak-to-peak output voltage swing and can drive different types of output stages for high-end consumer and professional audio applications, including active recording studios. Monitors, subwoofers, audio/video receivers, commercial sound systems, non-original audio, professional mixers, distributed audio and guitar amplifiers. In addition, it is also applicable to various industrial audio systems with high voltage and low distortion requirements. Provides a streamlined design for audio systems, enabling designers to more easily develop high-performance audio systems for greater stability and consistency, dramatically reducing split component matching and adjustment during system development and production.
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Design skills
There are many ways to design a high-quality audio system using National's audio drivers. The following are design recommendations.
Input stage: The input stage design is the most critical part of the amplifier. By subtracting the signal from the feedback, the input stage generates an error signal and then drives the error signal to the output. This error signal is usually small enough to provide sufficient linearity for the amplifier.
The LME49810 is a bipolar input amplifier with matching input impedance that is important. Affected by the bias current from the positive input åŸ and the negative input ,, the input impedance mismatch causes the input offset voltage. This input offset voltage will be amplified in accordance with the dead loop gain. Of course, the input bias current of the LME49810 is very low, and for normal applications, the offset voltage appearing at the output is negligible.
Figure 1: Schematic diagram of the input stage and feedback application.
In general, there are two types of audio input designs that are commonly used: AC or DC-coupled inputs. The advantage of an AC-coupled input is that the amplifier input DC offset from the preamplifier, filter stage, or codec stage is typically zero, and there is no need to add any DC servo circuitry to the amplifier to prevent DC faults. The advantage of the DC-coupled input is that it does not require the use of large size and expensive AC-coupling capacitors ; there is no low-frequency distortion caused by the AC-coupling capacitor; it can reduce the noise of the AC-coupled RC network.
Negative feedback coefficient: The negative feedback setting of the power amplifier brings high stability and linearity to the system. Phase shift occurs when the amplifier is operating at high frequencies, while a large negative feedback coefficient mitigates instability and oscillation at high frequencies. In a split-amplifier system, high feedback coefficients can cause poor transient response or high frequency instability. However, the LME49810 has a higher open-loop gain, so its dead-loop gain error and power supply ripple rejection are smaller, maximizing the negative feedback in the circuit, thus increasing the linearity of the system. In general, a voltage gain of 30dB to 40dB is recommended.
Figure 2: Output bias circuit structure.
Compensation: The amplifier's compensation is used to adjust the open-loop gain and phase performance to stabilize the system when feedback is turned off. In general, the greater the stability, the better the compensation. However, the larger the compensation, the lower the bandwidth and slew rate of the audio chip , and the lower slew rate will result in softer audio characteristics, while the higher slew rate will produce sharper and Real audio features. The Miller compensation of the LME49810 is achieved by inserting a capacitor between the 'Comp' and 'BiasM' pins. The most suitable capacitance range is 10p to 100p. In addition, the equivalent series resistance (ESR) of the compensation capacitor should be low to avoid the potential zero point of the capacitor's equivalent series resistance. In general, the use of ceramic capacitors is better than the use of electrolytic capacitors.
Mute: The MUTE pin is controlled by the amount of current flowing in. The 'PLAY' mode is from 50uA to 100uA, and the 'MUTE' mode is below 50uA. It is recommended not to let the current flowing into the MUTE pin exceed 200uA.
Output Bias: The LME49810 has two dedicated pins (BIASP and BIASM) for setting the bias to provide a certain output bias current. The variable resistor Rpot can be used to adjust the bias current of the output stage. Lowering the resistance of Rpot+ R b1 can increase the bias voltage. The multiplier QMULT is used to compensate for the bias voltage to prevent thermal drift in the bipolar output transistor. The QMULT must be connected to the same heat sink as the output transistor.
Output Transistor: The most common output stage in an audio power amplifier is the emitter follower shown in Figure 3. It is often referred to as a dual emitter follower or a Darlington tube. The first follower acts as a driver for the output stage.
Figure 3: Output emitter follower.
The large signal linearity of the emitter follower depends mainly on the size of the load. As the load increases (ie, the load resistance decreases), the output current also increases. The BJT current gain is reduced by the influence of RE and beta roll-off at high current densities. In this case, it may reduce linearity and increase distortion at the output stage. For higher power applications, multi-level output is recommended to maintain high current and better linearity. The LME49810 audio driver has an output current of approximately 50mA, which can be configured as a Darlington or parallel transistor output as required.
Output Stage Transistor Amplification Matching: A dual emitter-follower or Darlington tube usually has a high current gain factor Ic = βIb. In order to improve the stability of the output stage, the current amplification at the negative and positive terminals must match.
Figure 4: Application of the emitter-level negative feedback resistor.
For parallel transistor configurations, the drive capability of a medium power transistor must be determined. The output current (Ic) of the medium power transistor must be greater than the minimum drive current (Ib) of the high power transistor to avoid overloading at the medium power transistor level.
Output transistor voltage range: The maximum range of VCBO and VCEO voltages must be greater than the rail-to-rail range of the supply voltage. For an amplifier with a +/-100V supply, the voltage rating of the transistor should be higher to ensure that it operates properly within specified limits.
Emitter Resistors RE: In high power audio amplifier applications, the matching, current balance, and protection of the output transistors are important for the linearity of the power amplifier. An emitter negative feedback resistor RE is recommended to improve the matching and current balancing of the output transistors. Therefore, we recommend adding this resistor to practical high power audio amplifier applications. However, connecting RE with the output transistor in series reduces the linearity of the amplifier. The resistor RE is the main source of distortion for crossover distortion. This distortion occurs when one end of the output transistor is turned off and the other end is turned on. Therefore, it is necessary to optimize the RE value and maintain the RE as low as possible, which can reduce the influence on the nonlinearity.
Figure 5: Output configurable structure of an audio amplifier.
The most effective way to improve crossover distortion is to reduce the resistance of the RE. For REs of the same value, a parallel form of output reduces the overall RE resistance value used to improve linearity. At the same time, if the RE of each stage is large, it will provide better matching and current balance for the output transistors. In addition, RE is also related to the power loss of the output. For an identical RE, a larger output current results in greater power loss. The value of RE depends on the number of parallel output transistors and the load on the speaker. A 0.1 to 0.5 Ω resistor of sufficient power is recommended.
Output Network: The most common output network for power amplifiers is 'Zobel'. All output networks have only one target, which is to improve the stability of the system. Another function of the 'Zobel' network is to protect the output of the amplifier from the inductance of the speaker coil. The resistors and capacitors in the 'Zobel' network suggested here are connected in series and connected from the output of the amplifier to ground.
Figure 6: Output matching circuit for professional audio amplifiers.
The role of the resistor in Figure 6 is to limit the current at a higher frequency to reduce the rating requirements for the capacitor. Resistor resistance values ​​range from 4.7 Ω to 10 Ω. In most cases, the value of the capacitor is chosen to be 0.1μF to mitigate the loading effect of the output speaker at high frequencies. As the level of the output increases, the current drawn into the 'Zobel' network will also increase. Therefore, the power rating of the components in the output network must be sufficient under any conditions, recommended from 3W to 5W. Affected by the amplifier's output impedance, cable impedance, and inductance, a true speaker frequency response exhibits an unsteady characteristic, which is more complicated than a simple shunt resistor and capacitor. This phenomenon can lead to high frequency instability.
Connecting a small coil inductor in series with the output of the amplifier increases stability. This approach isolates the amplifier from the associated capacitance and does not cause significant loss at the audio frequency. The inductance value is generally 1 to 7 μH. Selecting the appropriate inductor value avoids high frequency roll-off at a certain load impedance. Hollow inductors are recommended here to avoid magnetic saturation problems. In addition, it is recommended to add a damping resistor across the output coil to reduce the Q factor, overshoot and ringing of the output LC network. Traditionally, several ohm's wound damping resistors have been used to avoid self-inductance. The lower the resistance, the less the effect of overshoot and ringing. Since the magnitude of the output current depends on the output power and load impedance. Therefore, a 1W to 5W rated damping resistor should be used.
Summary of this article
With the development of high-end audio systems, professional-grade amplifiers require greater output power, higher linearity, and higher stability. National Semiconductor's audio drivers provide engineers with more valuable designs that enable them to create more high-performance and highly stable audio amplifier systems.
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