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Audio Power Amplifier Measurements, Part 2

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Audio Power Amplifier Measurements, Part 2 Audio Amplifiers Texas Instruments Incorporated Audio power amplifier measurements, Part 2 By Richard Palmer Application Specialist, Audio Amplifiers Introduction real-world layout constraints. The measurements of a par- This article is a continuation of “Audio power amplifier ticular audio circuit may vary from the typical specifications. measurements,” which first appeared in the July 2001 A large variance is usually indicative of a PCB layout or issue of Analog Applications Journal (see Reference 1) measurement system issue. Reference 2 provides more and which contains guidelines for measuring the following details about the measurements in this article. three parameters: Supply rejection • power supply rejection ratio (PSRR), Two types of supply rejection specifications exist: power- • supply ripple voltage rejection ratio (kSVR), and supply rejection ratio (PSRR) and supply ripple voltage • efficiency. rejection ratio (kSVR). The only difference between them All measurements were made by using TI Plug-n-Play is that PSRR is a dc specification and kSVR is an ac specifi- APA evaluation modules (EVMs) for the TPA2001D1 filter- cation that measures the ability of the APA to reject ac- free class-D and the TPA731 class-AB mono devices. Note ripple voltage on the power supply bus. All power supply that the graphs in the data sheets reflect typical specifi- decoupling capacitors are removed from class-AB circuits, cations and were measured on test boards specifically and class-D circuits have a small 0.1-µF decoupling capacitor, designed to allow accuracy and ease of measurement. Board C, placed close to the APA power pins to provide a reverse space, layout, and components were not constrained by path for recovery switching currents. It is recommended size/cost requirements. The measurements in this article, that the designer use equal decoupling capacitance values however, were taken by using circuits on EVMs that reflect when comparing devices from different manufacturers to Figure 1. PSRR and kSVR measurement circuit Audio Power Amplifier AP Analyzer In VOUT CIN (DMM2)** + IN+ Diff Input OUT+ RC Filter for Filter-Free BTL RL + Channel B CIN – Class-D Output IN– OUT– Measurements – VS GND + Channel A – C* AP Generator Out C ** Inputs SVR + – + Balanced, ac-coupled VS ZIN = 100 kW /185 pF Channel A RSVR** (DMM1)** Set Load Reference = RL – Internal Filter£ 10 Hz – 80 kHz Reading Meter = Crosstalk Outputs Unbalanced–Float Data 1 = Analyzer Crosstalk Ch A ON V+ GND Source 1 = Generator Frequency Ch B Track Ch A Z= 20W OUT Regulated Set Load Reference = RL Power Supply Sweep 20 kHz – 20 Hz * The 0.1-µF capacitor, C, is required for class-D operation. ** The PSRR measurement uses the DMMs only since it is a dc value. kSVR measurements use either the analyzer, oscilloscope, or DMMs since they are ac values. RSVR and CSVR are used for kSVR measurements only. 26 Analog and Mixed-Signal Products www.ti.com/sc/analogapps 1Q 2002 Analog Applications Journal Texas Instruments Incorporated Audio Amplifiers get a valid comparison of the performance, since a higher Figure 2. kSVR filter circuit capacitance equates to a better kSVR. PSRR is the ratio of the change in the output voltage, VOUT(dc), to a change in the power supply voltage, VS, CSVR expressed in dB as shown in Equation 1. RGEN RSVR ∆VOUT() dc PSRR = 20log (1) ∆VS V For example, the output voltage of an audio power amplifier GEN RAPA RS that has a PSRR equal to –70 dB would change by 31.6 µV if the supply voltage changed by 0.1 V. kSVR is the ratio of the output ripple voltage, VOUT(ac), to a change in the supply ripple voltage, VS, expressed in dB as shown in Equation 2. VOUT() ac kSVR = 20log (2) output. Here the analyzer, an Audio Precision System–II, VS is configured for a crosstalk measurement1, 2; it sweeps This parameter is normally listed as a typical value in the the ac voltage at constant amplitude over the audio band, data sheet tables at a specified frequency and temperature measuring and presenting a graph of the data points in dB. of 1 kHz and 25°C, respectively. A graph is provided in the The kSVR filter circuit is shown in Figure 2. The dc power supply output impedance, RS, is very low (milli- data sheet of the typical values of kSVR over the audio bandwidth because it is a frequency-dependent term. ohms); and the impedance of the APA to ground, RAPA, as seen by the power supply or signal generator, is very high The PSRR and kSVR measurement circuit is shown in Figure 1. All inputs are ac-coupled to ground. The PSRR (hundreds of ohms). The value of RSVR is added to the measurement requires only the two DMMs. The power circuit to increase the equivalent impedance of the power supply and is chosen to be approximately equal to the ac supply voltage, VS, is initially set, then read from the meter on the power supply. When the power supply meter signal source generator output impedance, RGEN. A voltage does not have the desired resolution, DMM1 is used to divider is formed between RSVR and RGEN that provides a reasonable amplitude ac signal at the APA power pin. measure VS. DMM2 then measures VOUT across the load. CSVR is added to ac-couple the signal generator to the VS is then stepped up or down by a specific amount, and APA. The filter cutoff frequency, fC, should be set 3 dB the corresponding value of VOUT is measured. The differ- ences of the two measurements are then substituted into below the lowest frequency of the audio band, fMIN, which Equation 1, and the PSRR is calculated for that specific in this case is 20 Hz. Equation 3 provides the value for fC, change in supply voltage. PSRR is specified as a typical which is ~14 Hz. value that is valid for a given supply voltage range at 25°C. fMIN fC = (3) The kSVR measurement requires the signal generator, 2 analyzer, a DMM, and the kSVR filter components RSVR 1, 2 and CSVR. The RC measurement filter is used when the The equivalent resistance is then calculated with Equation 4, analyzer cannot accurately process the square wave output where RAPA is the supply voltage divided by the quiescent of the filter-free class-D APAs. DMM1 is used to measure current of the device (VS/IQ). VS at the APA power pin. The generator injects a small sine wave signal onto the power bus, and the audio analyzer RREQ=+ GEN R APA!() R SVR +≈+ RR S GEN R SVR (4) measures this ac voltage at the APA power pin and at the Continued on next page Figure 3. kSVR of the TPA2001D1 and the TPA731 0 – 20 VS = 3.3 V AV = 12 dB Class-AB W R=L 8 AV = 6 dB Class-D – 40 C=B 1Fm BTL SVR – 60 k(dB) Class D – 80 Class AB –100 20 200 2000 20,000 Frequency (Hz) 27 Analog Applications Journal 1Q 2002 www.ti.com/sc/analogapps Analog and Mixed-Signal Products Audio Amplifiers Texas Instruments Incorporated Continued from previous page Those devices with BYPASS pins will have improved kSVR as the capacitance on the pin is increased. Those The value for CSVR is then calculated with Equation 5. operated SE have lower kSVR, particularly at the extreme low and high ranges of the audio band. This is primarily 1 CSVR = (5) due to the resonance of the output ac coupling capacitor. 2π××fRCEQ The kSVR graphs of the TPA2001D1 and the TPA731 are shown in Figure 3. Both of these devices are differential The capacitor will most likely be electrolytic due to the input and BTL output. The TPA731 was measured with value required. It will have some reactance that will vary the inputs floating. Newer devices are typically measured with frequency as shown by Equation 6. with the inputs ac-grounded. 1 X = (6) Efficiency measurements CSVR 2π××fCCSVR Efficiency is the measure of the amount of power that is delivered to a load for a given input power provided by the At 20 Hz the impedance will be quite high—approximately supply. A class-AB APA acts like a variable resistor net- the value of RGEN and RSVR; and at 20 kHz the value will work between the power supply and the load, with the be in the milliohms. output transistors operating in the linear region. They The actual values for the measurement circuit were dissipate quite a bit of power because of this mode of RGEN = 20 Ω, RS ≈ 0, RAPA = 5 V/6 mA = 833 Ω, CSVR = operation and are therefore inefficient. The output stage 330 µF, RSVR = 20 Ω, and fC = 12 Hz. This yields a capaci- in class-D amplifiers acts as a switch that has a small tive reactance of 24 Ω at 20 Hz, and 24 mΩ at 20 kHz. The resistance when operated in the saturation region, which value of the ac signal must be adjusted at low frequencies provides a much higher efficiency. so that the desired voltage is applied to the APA power A circuit for measuring the efficiency of a class-AB or pin. The value of the dc voltage from the power supply class-D system is shown in Figure 4. The simplest setup must also be adjusted, since IQ will create a small voltage occurs when the power supply voltage and current meters drop across RSVR. are accurate and have the resolution required. When the Figure 4. Efficiency circuit, BTL Oscilloscope or AP Generator Out Audio Power Amplifier V2 Analyzer CIN (DMM2) + IN+ Diff Input OUT+ RC Filter for + + + Filter-Free Channel A BTL R2 V Channel 1 (A) CIN – Class-D OUT Output – – IN– OUT– Measurements – Outputs Balanced ZL* – + VS GND Z=OUT 40W V3 Set Load Reference = RL (DMM3) Set Frequency of Signal + + + – R1** VS Channel 2 (B) V1 – – (DMM1)** Inputs Balanced, dc-coupled V+ GND ZIN = 100 kW /185 pF Set Load Reference = RL Regulated Set dBrB Ref to Ch A Power Supply * Load ZL is a speaker for class-D APAs and is a purely resistive load for class-AB APAs.
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