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An Audio Circuit Collection, Part 1

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An Audio Circuit Collection, Part 1 Texas Instruments Incorporated Amplifiers: Op Amps An audio circuit collection, Part 1 By Bruce Carter Advanced Analog Products, Op Amp Applications Introduction connected to VCC+, and ground is connected to VCC– or This is the first of two articles on audio circuits. New oper- GND. A virtual ground, halfway between the positive sup- ational amplifiers from Texas Instruments have excellent ply voltage and ground, is the “ground” reference for the audio performance and can be used in high-performance input and output voltages. Voltage swings above and below applications. this virtual ground to VOM±. Some newer op amps have There have been many collections of op amp audio circuits different high- and low-voltage rails, which are specified in in the past, but all of them focus on split-supply circuits. data sheets as VOH and VOL, respectively. Often, the designer who has to operate a circuit from a 5 V is a common value for single supplies, but voltage single supply does not know how to perform the conversion. rails are becoming lower, with 3 V and even lower voltages Single-supply operation requires a little more care than becoming common. Because of this, single-supply op amps split-supply circuits. The designer should read and under- are often “rail-to-rail” devices to avoid losing dynamic stand the introductory material. range. “Rail-to-rail” may or may not apply to both the input and output stages. Be aware that even though a Split supply vs. single supply device may be specified as “rail-to-rail,” some specifica- All op amps have two power pins. In most cases they are tions may degrade close to the rails. Be sure to consult the labeled VCC+ and VCC–. Sometimes, however, they are data sheet for complete specifications on both the inputs labeled VCC and GND. This is an attempt on the part of and the outputs. It is the designer’s obligation to make the data sheet author to categorize the part as split-supply sure that the voltage rails of the op amp do not degrade or single-supply, but it does not mean that the op amp has system performance. to be operated with the split or single supply shown by the data sheet. It may or may not be able to operate from dif- Virtual ground ferent voltage rails. Consult the data sheet, especially the Single-supply operation requires the generation of a “virtual absolute maximum ratings and voltage swing specifications, ground” at a voltage equal to VCC/2. The external circuit before operating at anything other than the recommended can be a voltage divider bypassed by a capacitor, a voltage power supply voltage(s). divider buffered by an op amp, or preferably a power-supply Most analog designers know how to use op amps with a splitter such as the Texas Instruments TLE2426. Figure 1 split power supply. In a split-power-supply system, the shows how to generate a half-supply reference if the input and output are referenced to ground. The power designer insists on using an op amp. supply consists of a positive supply and an equal and R1 and R2 are equal values selected with power consump- opposite negative supply. The most common values are tion vs. allowable noise in mind. C1 forms a low-pass filter to eliminate conducted noise on the voltage rail. R3 is a ±15 V, but ±12 V and ±5 V are also used. The input and Ω output voltages are centered on ground and swing both small (47- ) resistor that forms a low-pass filter with C2, eliminating some of the internally generated op amp noise. positive and negative to VOM±, the maximum peak output voltage swing. The value of C2 is limited by the drive capability of the A single-supply circuit connects the op amp power pins op amp. to a positive voltage and ground. The positive voltage is In the circuits in the figures that follow, the virtual ground is labeled “VCC/2.” This voltage comes from either the TLE2426 rail-splitter or the circuit in Figure 1. If the latter is used, the overall number of op amps in the design Figure 1. Half-supply generator is increased by one. Passive components The majority of the circuits given in this series have been designed with standard capacitor values and 5% resistors. +V +VCC CC Capacitors should be of good quality with 5% tolerance wherever possible. Component variations will affect the operation of these circuits, usually causing some degree of R1 ripple or increased roll-off as the balance of Chebyshev – R3 VCC/2 and Butterworth characteristics is disturbed. These should + be slight—almost imperceptible. R2 C1 C2 Continued on next page 39 Analog Applications Journal November 2000 Analog and Mixed-Signal Products Amplifiers: Op Amps Texas Instruments Incorporated Continued from previous page an op amp. Since there is already an op amp anyway, why not consider adding a second and doing the entire function Speech filter with analog components? An additional op amp (plus one Human speech most frequently occupies an audio spectrum for the half-supply reference) can perform the filtering of 300 Hz to 3 kHz. There is a requirement, especially in with no aliasing problems, freeing the DSP for other tasks. phones, to limit the frequency response to this range. Figure 2 shows a single-supply phone speech filter in Frequently, this function is performed with a DSP chip. two op amps, five capacitors, and four resistors. This circuit DSP chips, however, require an anti-aliasing filter to reject is designed to be low-power and compact, and is scalable high-frequency components. The anti-aliasing filter requires for even lower power consumption. Figure 2. Single-supply phone speech filter R2 68 kΩ C3 200 pF +VCC +VCC C1 10 nF C2 10 nF 5 1 R3220 kΩ R4 220 kΩ 5 IN ++++ 4 1 µ 3 + 4 C5 1 F --–– 3 OUT R1 2 TLV2221 – 130 kΩ C4 2 TLV2221 100 pF VCC/2 VCC/2 Figure 3. Second-order circuit R1 68 kΩ R3 27 kΩ +VCC +VCC C1 10 nF C2 10 nF 4 3 4 IN +++ 1 C310 nF C4 10 nF 5 2 + –– 7 -- TLV2464 6 R2 11 – 75 kΩ R4 11 TLV2464 180 kΩ VCC/2 VCC/2 C5 1.2 nF C7 3.3 nF +VCC +VCC R5 43 kΩ R6 33 kΩ 4 10 R739 kΩ R8 27 kΩ 4 + 8 12 µ 9 + 14 C9 1 F – 13 OUT TLV2464 – C6 11 TLV2464 1 nF C8 11 470 pF VCC/2 VCC/2 40 Analog and Mixed-Signal Products November 2000 Analog Applications Journal Texas Instruments Incorporated Amplifiers: Op Amps Figure 3 shows a second-order circuit. Nearby out-of-band signals, such as 60-Hz Figure 4. Response of second- and fourth-order speech filters hum, are not rejected very well. This may be acceptable in cellular telephone head- sets, but may not be for large switchboard 10 consoles. A fourth-order speech filter, although more complex, can be imple- 0 mented in a single quad op amp (with an external VCC/2 reference). The response of the second- and fourth- order speech filters is shown in Figure 4. Output (dB) -20 The 60-Hz rejection of the fourth-order 2nd order 4th order filter is greater than 40 dB, while that of the second-order filter is about 15 dB. Both filters have been designed to have an -40 imperceptible 0.5-dB roll-off at 300 Hz and 10 Hz 100 Hz 1.0 kHz 10 kHz 100 kHz 3 kHz. Frequency Crossover filter Inside any multiple-speaker cabinet is an array of inductors and capacitors that directs different frequency ranges to each speaker. The A number of audiophiles are beginning to talk about the inductors and capacitors, however, have to handle the full virtues of bi-amplification, or even multiple amplification. output power of the power amplifier. Inductors for low fre- In this technique, the crossover network is applied to the quencies in particular tend to be large, heavy, and expensive. audio source before amplification instead of after it. Each Another disadvantage of the inductor and capacitor speaker in the cabinet is then driven by a separate amplifier crossover network is that it is a first-order network. At stage that is optimized for the speaker. high volume, destructive levels of audio can be transferred The primary reason for bi-amplification or multiple to speakers not designed to handle a given frequency amplification is that human hearing is not equally sensitive range. If a speaker is incapable of moving in response to to all frequencies. The human ear is relatively insensitive stimulation, the only way the energy can be dissipated is to low and high frequencies, but audio amplifiers are in heat. The heat can build up and burn out the voice coil. designed to have flat response (constant power) across Continued on next page Figure 5. Crossover network R2 200 kΩ C3 4 nF C2 2 nF +V +VCC CC U1A TLCO74 U1B C1 4.7 µF R1 200 kΩ 4 TLCO74 2 R3200 kΩ R4 200 kΩ 4 IN – 1 5 µ 3 + 7 C5 4.7 F –+ 6 BASS 11 – C4 11 1 nF V /2 CC V /2 R6 200 kΩ CC +VCC R7 200 kΩ +VCC U1C TLCO74 U1D C6 2 nF R5 200 kΩ 4 TLCO74 9 C7 1 nF 4 – 8 C8 1 nF 12 µ 10 + 14 C9 4.7 F + 13 TREBLE 11 – VCC/2 R8 11 800 kΩ VCC/2 41 Analog Applications Journal November 2000 Analog and Mixed-Signal Products Amplifiers: Op Amps Texas Instruments Incorporated Continued from previous page • Because there is not much high-frequency spectral content but a constant level of white noise throughout the audio band.
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