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Closed-loop HDTV audio reduces space, cost and improves performance

The advantages of closed-loop audio architectures in the high-definition television (HDTV) space have been well proven, with the vast majority of analog-input Class-D amplifiers being closed-loop. As the market now transitions to digital-input amplifiers (I2S/PCM Serial I/Fs) and the ever-present pressures of cost, time-to-market and performance intensify, closed-loop architectures are becoming even more compelling. This article provides a high-level overview of closed-loop architectures and addresses three of their primary advantages in the HDTV space: higher damping factor; increased power supply noise immunity; and better electromagnetic compatibility (EMC) performance.

Closed-loop architectures overview
In the audio world, the debate over closed-loop versus open-loop architectures has raged on for many years. Depending on the end-application or customer preference, valid arguments for both architectures can be made. In the HDTV space, closed-loop amplifiers have proven to be the clear winner. But in high-end audio, the war still rages. The main advantages of closed-loop architectures include improved linearity, gain stabilization, increased bandwidth, and reduced output impedance. Some of the disadvantages include the potential for decreased stability, decreased gain, and added complexity.


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Figure 1: Closed-loop block diagram

Conceptually, it helps to think of a closed-loop amplifier in terms of “pre-distortion” (Figure 1 ). The feedback network samples the amplifier's output, which consists of both the amplified signal and any non-linear distortions introduced to the signal by the amplifier or power supply. The output sample is then attenuated and inverted before being recombined with the incoming source signal. The resultant signal out of the summing node (Point A) is an attenuated input signal with inverted “pre-distortion” in the areas where non-linearities from the amplifier and power supply were previously added. The amplifier then gains up the signal, adding in non-linear distortions. Since the source signal was pre-distorted via the feedback network, you get a canceling effect of the pre-distortion + distortion, yielding a very linear signal.

This is the fundamental advantage of negative feedback, whereby you have a mechanism that dynamically adjusts for non-linearities in the system. In an open-loop architecture, this mechanism does not exist. Thus, the performance requirements placed on the amplifier's linearity and supply regulation are much higher, which typically translates into increased cost and/or performance tradeoffs.

Next: Damping factor

Damping factor advantage
The damping factor is the ratio of the speaker's impedance to that of the amplifier's output impedance. It is an indication of how well the amplifier is able to start and stop the speaker cone movement, especially at lower frequencies and during transients. Amplifiers with high damping factors typically reproduce a tighter, more accurate bass response.

Closed-loop amplifiers have a very low output impedance, which corresponds to a high damping factor. In a closed loop system, feedback compensates for the amplifier's output resistance voltage drop by increasing the voltage output. (The greater the voltage drop across the output impedance, the less feedback supplied to the summing node and, therefore, the greater the output voltage.) The effect of increasing the output voltage is equivalent to decreasing the output impedance of the feedback amplifier.

To better understand how a low output impedance provides greater control over the speaker, we need to look at how a speaker works. Assume you apply an 80Hz burst mode signal for three cycles to the terminals of a speaker. When the signal is applied to the terminals, it drives a current through the voice coil, which in turn generates an electromotive force (EMF) moving the speaker cone back and forth. Ideally, once the signal is removed the speaker cone immediately stops at its resting position. Unfortunately, we have added energy to the system that must be dissipated or damped prior to stopping the speaker cone movement. Two kinds of damping can be found in a speaker: 1) mechanical damping via the speaker suspension and air load on the diaphragm; 2) electrical damping via the magnetics of the speaker. The mechanical damping properties are a function of the speaker construction and materials used, while the electrical damping properties are directly influenced by the damping factor of the amplifier.

After the signal is removed and the speaker begins to ring, it generates a “damping” back EMF that wants to stop the speaker cone movement. This EMF generates a current flow from one terminal to the other through the output impedance of the amplifier. The smaller the impedance, the greater the current flow — hence, the stronger the damping EMF. In summary, the low output impedance allows a large back EMF current to flow, which in turn exerts a strong damping force on the ringing.


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Figure 2: Three cycles of 80Hz burst mode

Figure 2 shows a closed-loop (Magenta) and open-loop amplifier (Red) driving a subwoofer for three cycles with an 80Hz burst mode signal. The amplitude is 28V pk-to-pk, and the 80Hz signal is near the resonant frequency of the subwoofer. In Figure 3 , you clearly can see the closed-loop amplifier damping the ringing much quicker than the open-loop amplifier. In addition to greater damping, a closed-loop amplifier also can start the speaker cone movement more quickly than an open-loop amp.


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Figure 3: Zoom in on the damping

Next: Power supply rejection

Power supply rejection advantage
By definition, a closed-loop system uses feedback to make the system response insensitive to external disturbances. Open-loop systems contain no feedback mechanism. The relative performance of an open-loop system relies entirely on minimizing the external disturbances.

In the case of an audio amplifier, one major external disturbance is the power supply. The disturbance can be minimized with capacitance or by using dedicated switching power supplies that use feedback to guarantee a stable output voltage. In a LCD-TV, significant system cost savings can be achieved by eliminating clean switching power supplies and driving the audio amplifier directly from the +12V or +24V backlight supply.

A common measurement of an amplifier's ability to reject power supply disturbances is power supply rejection. Unfortunately, the measurement technique does not highlight the advantages of the closed-loop system versus an open-loop system in a bridge-tied output configuration. The technique grounds the inputs to the amplifier and modulates the power supply by adding a frequency component on top of the DC supply.

Figure 4: Open-loop block diagram

In the open-loop system, the input voltage is mixed with the incoming power supply ripple (Figure 4 ). With zero input, no mixing occurs and the disturbance on each output is simply cancelled across the bridge-tied load. In a real audio system with sinusoidal input frequencies, the input frequency mixes with the power supply ripple and creates tones and distortion in the audio band. The gain of the open-loop amplifier is also modulated with the power supply ripple. This effect can be seen in a total harmonic distortion plus noise (THD+N) sweep as shown in Figure 5 where a closed-loop amplifier is compared to an open-loop amplifier.


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Figure 5: THD+N vs. Power — Open-loop and Closed-loop Amplifier

In Figure 5 , a 100Hz sine-wave was applied to the input of each system and the input voltage was increased to sweep the THD+N vs. Output power as measured into an 8Ohm load. The supply used was an off-the-shelf, 12V, switching regulator. The output ripple measured at the input to each amplifier was measured at 300mVp when driving 5W of output power into the load. The THD+N differences between the open-loop and closed-loop systems increase as the demand on the power supply results in more voltage ripple. This phenomenon is even more apparent at lower frequencies where the regulator has difficulty correcting for the large output swings.

A closed-loop system thus enables an audio circuit designer to achieve advanced audio performance without spending time or money designing tightly controlled system power supplies specifically for the audio circuit.

Next: EMC benefits

EMC benefits
An additional capability of the closed-loop system is the ability to slow down the rising and falling edges of the output transitions without compromising total harmonic distortion or slew rate control. This is where the gate drivers are slowly transitioned from the OFF state to the ON state, resulting in a more damped system response (lower dV/dt) and lower peaks in EMC (electromagnetic compliance) measurements.


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Figure 6: Scope captures of open-loop (top) vs. closed-loop response (bottom)

Dead time is a key contributor to total harmonic distortion in Class-D amplifiers. This is defined as the time when both the MOSFETs in the output H-bridge are in the OFF state. In open-loop systems, it is critical that the dead time be matched between the output MOSFETs to avoid second order effects. To minimize dead time, the rise and fall of the pulse width modulation (PWM) output edges are transitioned at very fast rates. Figure 6 compares the rise time of a typical open-loop amplifier (measured at 2.4ns) (top), and a closed-loop device (measured at 10ns) (bottom). Notice the EMC contributors in the scope captures — fast rising edges with large overshoots.

The feedback in the closed-loop amplifier corrects for the slower edge transitions by integrating the error between the input signal (desired output response) and the actual output response with the slower edge transitions.

Next: Open-loop vs. closed-loop EMC comparison

Open vs. closed-loop EMC
In Figure 7 , the EMC graph compares an open-loop to a closed-loop amplifier. The board layout was closely matched for the experiment as improper layout is a large contributor to EMC performance.


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Figure 7: EMC performance of a closed-loop amplifier vs. open-loop amplifier

Also note that the spectrum for the closed-loop amplifier was measured with only an LC filter on the output. The open-loop amplifier had additional snubber circuits consisting of an R and C on each output to limit the dV/dt. The snubber circuits not only add to the system bill of materials (BOM), but also increase the board area. Board area reduction can be critical on expensive four-layer boards. Time and money is also saved when engineering time is not spent in an EMC chamber debugging boards.

Summary
In conclusion, we illustrated three main advantages of closed-loop amplifiers in the HDTV market: higher damping factor; improved power supply noise immunity (i.e., higher power supply ripple rejection ratio or PSRR); and better EMC performance.

As the market transitions from analog-input Class-D audio to digital-input amplifiers, closed-loop devices such as the TAS5706 Class-D amplifier, and the TAS5601 and TAS5602 PWM power stages are providing equipment manufacturers better performance, lower cost and quicker time-to-market with integrated solutions.

About the authors
Ryan Kehr is a Member of the Technical Staff at Texas Instruments and New Product Definition Manager for TV audio products. He is an eight-year veteran in the audio group starting as an application engineer for high-power, Class-D audio amplifiers.

Michael Firth is the Product Marketing Manager for TV audio products. Mike received his Bachelors degree in Engineering Physics at the University of Oklahoma. They can be reached through .

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