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The speaker audio power amplifiers in handheld media playback devices are mostly class AB amplifiers, which are divided into traditional and fully differential architectures. The fully differential architecture exhibits superior noise rejection and is therefore popular with more and more users. This article will present a fully differential version of the solution to replace the traditional Class AB amplifier.
Class AB Speaker Amplifier
Class AB speaker audio power amplifiers have been dominant in the low-power audio amplifier market due to their mature technology, excellent audio performance, simple application and low price. The speaker amplifier in the handheld device has a maximum output power of about 1W (8. load). Most of these amplifiers are BTL bridged load outputs, that is, without coupling capacitors, the output can be directly connected to the speaker. Class B audio amplifiers currently available on the market for BTL are available in both traditional and fully differential configurations.
Traditional architecture
The non-inverting output terminal VO+ is led back through a one-stage inverting amplification to obtain an inverting output terminal VO-, which is output as a BTL.
Full differential architecture
The fully symmetrical circuit architecture, the input stage and the output stage are implemented in a true differential circuit.
Figure 1 Traditional Class AB amplifier
Figure 2 Fully Differential Class AB Amplifier
Audio noise in the phone
More and more mobile phones have the function of music player. Compared with other music playing devices, mobile phone applications have stricter requirements on audio amplifiers. This is mainly because the electromagnetic environment in mobile phones is complex and may generate various audio noises, affecting users. use.
On the mobile phone motherboard, the high-speed data lines of the display, camera, memory and other devices, the address lines criss-cross, and various RF signal lines, may cause interference to the audio amplifier, once the audio amplifier picks up such noise, after Magnification is possible to the extent that the user can hear it. On the other hand, when the GSM mobile phone communicates with the base station, it transmits data according to the allocated time slot, and the time slot interval frequency is 217 Hz. Since the RF power amplifier extracts a large current (peak current up to ampere level) from the battery at the moment of power transmission, a large ripple appears on the power supply, and this ripple interference (frequency around 217 Hz) will be The power supply enters the audio amplifier and is reflected on the output (in most applications the audio amplifier is powered directly by the battery). If the power disturbance reaches a certain level, the human ear will notice the "å—¡å—¡" noise, which is called TDD Noise (TDMA's slot noise).
When the mobile phone's transmit power level (PCL) is high, the peak current drawn by the RF power amplifier is larger, and the interference is more obvious. In some specific situations, “嗡嗡†noise may be more easily heard by users. Case 1: The customer chooses the prelude soothing music as the ringtone of the caller, and the mobile phone is at a higher PCL level (such as PCL=5, GSM900), which happens to be called at a certain moment, and the ringtone is played at the maximum volume, because the ringing is low at this time. "嗡嗡" noise will be relatively obvious. Case 2: The prompt tone when receiving a text message is very easy to expose the background “嗡嗡†noise due to the short sounding time. Situation 3: The user makes a hands-free call in a quiet environment, and the “click†sound will be particularly obvious. This kind of sound causes the user's dislike, which seriously affects the normal operation of the call!
Due to the limitations of the phone's external structure, the speakers may be placed close to the antenna of the phone; or the speakers are far from the audio amplifier, connected by long leads or FPC; or the phone PCB layout is too compact to be able to audio The circuit performs better wiring protection and places noise suppression filter components. Common limitations in these actual product designs can lead to severe RF interference and increase the likelihood of "click" noise.
Fully differential architecture provides excellent immunity to interference
The fully differential amplifier, which is fully symmetrical from the input non-inverting and inverting terminals, provides excellent CMRR, PSRR, and Click-POP tone rejection. The PSRR metric reflects the amplifier's ability to reject power supply noise. The traditional architecture of a Class AB amplifier typically has a PSRR of around -62dB, while a fully differential amplifier can easily achieve -80dB or better performance. It can be seen that the fully differential amplifier has significantly better RF noise rejection than the traditional Class AB amplifier.
In the case of a Class AB amplifier with a conventional architecture, once RF noise occurs (the interference band may be GSM900 or DCS1800), only a few external PF-level capacitors are adjusted without changing the layout. Capacitive components are difficult to suppress noise. As a result, more and more users are switching from traditional Class AB amplifiers to fully differential Class AB amplifiers. According to the experience of many users, if you use a fully differential amplifier,
Following the usual audio circuit layout rules, there is usually no need to worry about the audible noise caused by interference, including radio frequency interference. This greatly simplifies system design and reduces risk.
CP2296 Compatible with Traditional Architecture Class AB Amplifier
Chiphomer has introduced a range of fully differential Class AB amplifiers to meet different application needs. The CP2296 is Chispomer's featured fully differential Class AB amplifier with a PSRR of -85dB. The CP2296 is also available in a CSP9 package that is compatible with Class AB amplifiers that replace the traditional legacy architecture on the market. When users are dissatisfied with the performance of traditional Class AB amplifiers, there is a new option – the CP2296, which allows you to enjoy the superior performance of a fully differential Class AB amplifier without having to redesign the PCB.
Figure 3 CP2296 excellent PSRR suppression
The CP2290 is a traditional architecture Class AB amplifier designed by Chiphomer and is fully compatible with mainstream manufacturers in the market. The following is a compatible design of the CP2290 and CP2296, which is also suitable for compatibility with other manufacturers' traditional architecture Class AB amplifier products.
Differential Input Mode Compatible Design When the source provides a differential signal, the CP2290/96 operates in differential input mode and can be designed as shown in Figure 4.
When using the CP2296, the two Rf resistors in the figure are NC, (the CP2296 has a built-in feedback resistor of 40Ω).
When using the CP2290, place a 20k Ω resistor on the two Rf resistors in the figure. When using the CP2290, it is recommended to fix the Rf to 20k Ω and change the gain by adjusting Ri. When you replace the CP2290 with the CP2296, the gain remains the same (without changing Ri) and the software volume settings are compatible. It can be seen that in this mode of operation, the CP2290 can be directly replaced with the CP2296 without redesigning the PCB.
Figure 4 CP2296 differential mode compatible with traditional AB design
Single-ended input mode compatible design
When the source can only provide single-ended signals, the CP2290/96 operates in single-ended input mode and the circuit can be designed as shown in Figure 5. It should be noted that even in single-ended input mode, the fully differential version provides superior performance over traditional Class AB amplifiers. When using the CP2296, the RS and Rf resistors in the figure are NC. When using the CP2290, put a 20k Ω resistor at Rf in the figure, a 0Ω resistor on the RS, and two elements Ri and Ci at the non-inverting terminal IN+ (in the red circle in Figure 5). For the same reason, the fixed Rf is 20k Ω, and the gain is changed by adjusting the Ri at the IN-end.
It can be seen that in the single-ended input mode, only a 0Ω resistor RS for selection can be added to the original CP2290 design to achieve compatible design of the CP2290 and CP2296 with minimal changes.
Figure 5 CP2296 single-ended mode compatible with traditional AB design
As can be seen from the alternatives in the above two modes of operation, the CP2296 can easily replace the traditional Class AB amplifier products, and the compatible alternatives are very simple, with minimal changes (no need to change in differential input mode) However, it is convenient for users to switch from a traditional class AB amplifier to a fully differential class AB amplifier.
summary
I recommend that users use fully differential Class AB audio amplifiers in the design of new projects because they provide superior anti-interference performance, greatly reducing the possibility of various types of RF noise, reducing design difficulty and risk. If you have other considerations, you still need to use a traditional Class AB audio amplifier. The author recommends careful analysis and evaluation of possible interferences in new products, especially RF interference. Where necessary, consideration should be given to using a fully differential version of a compatible design with a traditional Class AB amplifier (such as the CP2296 mentioned here) to avoid risk.
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