1.Sound Amplification

2. Contents Preamplifiers Requirements Signal Voltage & Impedance Levels Preamplifier Stages Voltage Amplifier Design Constant-Current Sources Current Mirrors Performance Standards Power Amplifier Classes Thermal Dissipation Limits Single-Ended Versus PushPull Operation 3. Contents Switching Amplifiers Amplifier Grounding Cross Over Network Audio terminations: line in/out Audio terminations: aux in/out Audio terminations: mic in 4. Preamplifiers Requirements Due to vast number of audio equipment manufacturers, the equipments are differing on the standards of output impedance or signal voltage For this reason, it is conventional practice to use a versatile preamplifier unit between the power amplifier and the external signal sources to perform the input signal switching and signal level adjustment functions This preamplifier either forms an integral part of the main power amplifier unit or, is a free-standing, separately powered unit 5. Signal Voltage and Impedance Levels For tuners and cassette recorders, the output is either that of the German Deutsches Industrie Normal (DIN) standard or the line output standard In (DIN) standard, the unit is designed as a current source, which gives an output voltage of 1 mV for each 1000 ohms of load impedance The line output standard, designed to drive a load of 600 ohms or greater, at a mean signal level of 0.775 V rms 6. Signal Voltage and Impedance Levels Units having DIN type interconnections, of the styles shown in Figure will conform to the DIN signal and impedance level conventions Din Plug 4 Pin 7. Signal Voltage and Impedance Levels The connectors having phono plug/socket outputs, of the form shown in Figure The permissible minimum load impedance will be within the range 600 to 10,000 ohms The mean output signal level will commonly be within the range 0.251 V rms 8. Voltage Amplifier Design The nonlinearity in a bipolar junction transistor characteristics affects the performance of an amplifier circuit The principal nonlinearity in a bipolar device is that due to its input voltage/output current characteristics If the device is driven from a high impedance source, its linearity will be substantially greater, since it is operating under conditions of current drive 9. Voltage Amplifier Design For example, for the circuit shown in figure the Q2 transistor is driven by Q1 which provides a very high impedance 10. Voltage Amplifier Design The input transistor, Q1 , is only required to deliver a very small voltage drive signal to the base of Q2 so that the signal distortion due to Q1 will be low Q2 ,is driven from a relatively high source impedance, composed of the output impedance of Q1 parallel with the base-emitter resistor, R4 To study the effects of feedback, we can connect the collector of Q2 to emitter of Q1 11. Voltage Amplifier Design Due to feedback the THD(Total Harmonic Distortion) is 0.01% at 1 kHz 12. Voltage Amplifier Design An improved version of this simple two-stage amplifier circuit is shown in Figure 13. Voltage Amplifier Design In this, if the two-input transistors are reasonably well matched in current gain and if the value of R3 is chosen to give an equal collector current flow through both Q1 and Q2 , the DC offset between input and output will be negligible, This will allow the circuit to be operated over a frequency range extending from DC to 250 kHz or more 14. Constant-Current Sources and Current Mirrors The Common emitter configuration is often used for constant current source with constant base current at its input R1 and R2 form a potential divider to define the base potential of Q1 This configuration can be employed with transistors of either PNP or NPN types 15. Constant-Current Sources and Current Mirrors An improved, two-transistor, constant current source is shown as below In this, R1 is used to bias Q2 into conduction, and Q1 is employed to sense the voltage developed across R2 16. Constant-Current Sources and Current Mirrors This voltage is proportional to emitter current, and to withdraw the forward bias from Q2 when that current level is reached at which the potential developed across R2 is just sufficient to cause Q1 to conduct The performance of this circuit is greatly superior to that with single transistor, in that the output impedance is about 10 greater The circuit is insensitive to the potential, Vref. applied to R1 , so long as it is adequate to force both Q2 and Q1 into conduction 17. Performance Standards The performance characteristics of any audio amplifier is dependant on ability of human ear to detect small differences in sound Thus a proper standard is difficult to make as the response of human ear changes from human to human Therefore, the focus is given to achieve more gain with less distortion within the audio amplifier Earlier, the use of ICs was an important criterion for design engineers to determine the performance of an audio amplifier 18. Use of ICs Many engineers were of opinion that the ICs are less preferable than the discrete components This is because of the fabrication method allows multiple PN junctions to be laid side by side This leads to reverse diode leakage currents associated with every component on the chip Additionally, there were quality constraints in respect to the components formed on the chip surface that also impaired the circuit performance 19. Use of ICs In recent IC designs, considerable ingenuity has been shown in the choice of circuit layout to avoid the need to employ unsatisfactory components in areas where their shortcomings would affect the end result Substantial improvements, both in the purity of the base materials and in diffusion technology, have allowed the inherent noise background to be reduced to a level where it is no longer of practical concern 20. Modern Standards The standard of performance that is now obtainable in audio applications is frequently of the same order as that of the best discrete component designs, but with considerable advantages in other respects, such as cost, reliability, and small size The designer of equipment will seek to attain standards substantially in excess of those that he supposes to be necessary This means that the reason for the small residual differences in the sound quality among the hifi systems is the existence of malfunctions of types that are not currently known or measured 21. General Design Considerations Three major design considerations are listed as below Economic considerations Requirements of reliability Nature of IC design The first two of these factors arise because both the manufacturing costs and the probability of failure in a discrete component design are directly proportional to the number of components used Therefore it is better to use less components in a circuit 22. General Design Considerations In an IC, both the reliability and the expense of manufacture are affected only minimally by the number of circuit elements employed Still the discrete component circuits have the advantage of higher voltage swing where the ICs are limited to small voltage operations It is a difficult matter to translate a design that is satisfactory at a low working voltage design into an equally good higher voltage system 23. General Design Considerations The reasons are as stated below increased applied potentials produce higher thermal dissipations in the components for the same operating currents device performance tends to deteriorate at higher inter-electrode potentials and higher output voltage changes available high voltage transistors tend to be more restricted in variety and less good in performance than lower voltage types 24. Power Amplifier Classes The Class of an amplifier refers to the design of the circuitry within the amp For audio amplifiers, the Class of amp refers to the output stage of the amp In practice there may be several classes of signal level amplifier within a single unit The more common amplifier classes are : Class A, Class B, Class AB, Class C, Class D, Other classes 25. Power Amplifier Classes: Class A Class A amplifiers have very low distortion (lowest distortion occurs when the volume is low) however they are very inefficient and are rarely used for high power designs The distortion is low because the transistors in the amp are biased such that they are "on" when the amp is idling As a result of being on at idle, a lot of power is dissipated in the devices even when the amp has no music playing Class A amps are often used for "signal" level circuits (where power requirements are small) because they maintain low distortion 26. Power Amplifier Classes: Class B Class B amplifiers are used in low cost designs or designs where sound quality is not that important. Class B amplifiers are significantly more efficient than class A amps, however they suffer from bad distortion when the signal level is low Class B is used most often where economy of design is needed Before the advent of IC amplifiers, class B amplifiers were common in pocket transistor radios and other applications where quality of sound is not that critical 27. Power Amplifier Classes: Class AB Class AB is probably the most common amplifier class currently used in home stereo and similar amplifiers Class AB amps combine the good points of class A and B amps. They have the improved efficiency of class B amps and distortion performance that is a lot closer to that of a class A amp. With such amplifiers, distortion is worst when the signal is low, and generally lowest when the signal is just reaching the point of clipping. 28. Power Amplifier Classes: Class AB Class AB amps (like class B) use pairs of transistors, both of them being biased slightly ON so that the crossover distortion (associated with Class B amps) is largely eliminated 29. Power Amplifier Classes: Class C They are commonly used in RF circuits Class C amplifiers operate the output transistor in a state that results in tremendous distortion (it would be totally unsuitable for audio reproduction) However, the RF circuits where Class C amps are used employ filtering so that the final signal is completely acceptable Class C amps are quite efficient Class C amps are not used in audio circuits 30. Other classes There are a number of other classes of amplifiers, such as G, H, S, etc Most of these designs are actually clever variations of the class AB design, however they result in higher efficiency for designs that require very high output levels 31. Thermal Dissipation Limits The BJT suffers the problem of thermal runaway The potential barrier of a P-N junction (that voltage that must be exceeded before current will flow in the forward direction) is temperature dependent and decreases with temperature Because there will be unavoidable non-uniformities in the doping levels across the junction, this will lead to non-uniform current flow through the junction sandwich, with the greatest flow taking place through the hottest region 32. Thermal Dissipation Limits If the ability of the device to conduct heat away from the junction is inadequate to prevent the junction temperature rising above permissible levels, this process can become cumulative This will result in the total current flow through the device being funneled through some very small area of the junction, which may permanently damage the transistor This malfunction is termed secondary breakdown Field effect devices do not suffer from this type of failure 33. Thermal Dissipation Limits The operating limits imposed by the need to avoid this failure mechanism are shown in Figure 34. Single-Ended Versus PushPull Operation A transistor can also act as switch other than amplifier Shown in figure is the arrangement in which a transistor can be operated 35. Single-Ended Versus PushPull Operation If we consider first the single-ended layout of Figure when Q1 is O/C, the current flow into R2 is only through R1 and i2 = V /( R1 + R2 ) If Q1 is short circuited, S/C, then 36. Single-Ended Versus PushPull Operation If all resistors are 10 in value, when Q1 is S/C, Vx will be equal to V , and there will be no current flow in R2 For Q1 in O/C, the current i2 will be ( V /20)A If R1 and R2 are 10 in value and R3 is 5 , then the current flow in R2 , when Q1 is O/C, will still be ( V/20)A Whereas when Q1 is S/C, the current will be (0.25 V /10)A and the change in current will be (3 V /40)A 37. Single-Ended Versus PushPull Operation By comparison, for the pushpull system the change in current through R2 , when this is 10 and both R1 and R3 are 5 in value, on the alteration in the conducting states of Q1 and Q2 , will be (2V/15)A, which is nearly twice as large The increase in available output power from similar output transistors when operated at the same V line voltage in a pushpull rather than in a single-ended layout is the major advantage of this arrangement 38. Switching Amplifiers Conventional (audio-) amplifiers are class A or class AB amplifier These amplifiers operate their output devices in the analogue domain This means the resistance of the devices is controlled directly by the strength of the music signal As a result, the devices are neither fully 'on' nor fully 'off'; effectively they are variable resistors The simultaneous voltage across- and the current through the devices in this mode results in dissipation in the power stage of the amplifier and therefore a low efficiency 39. Switching Amplifiers Switching or class-D amplifiers operate the output devices as switches which are turned either 'on' or 'off', making the resistance either zero or infinite Operated in this way, the devices are almost lossless because either the voltage across- or the current through the device is zero Thus the efficiency is high, typically more than 90% for high- as well as low output power 40. Power Amplifier Classes: Class D In a Class D amplifier, the input signal is compared with a high frequency triangle wave, resulting in the generation of a Pulse Width Modulation (PWM) type signal This signal is then applied to a special filter that removes all the unwanted high frequency by-products of the PWM stage The output of the filter drives the speaker Class D amps are (today) most often found in car audio subwoofer amplifiers 41. Power Amplifier Classes: Class D The major advantage of Class D amplifiers is that they have the potential for very good efficiency (due to the fact that the semiconductor devices are ON or OFF in the power stage, resulting in low power dissipation in the device as compared to linear amplifier classes) One notable disadvantage of Class D amplifiers: they are fairly complicated and special care is required in their design (to make them reliable) 42. Power Amplifier Classes: Class D Following is the class D block diagram 43. Amplifier Grounding The grounding system of an amplifier must fulfill several requirements, among which are: 1) The definition of a star point as the reference for all signal voltages 2) In a stereo amplifier, grounds must be suitably segregated for good cross talk performance - A few inches of wire as a shared ground to the output terminals will probably dominate the cross talk behavior 44. Amplifier Grounding 3) Unwanted AC currents entering the amplifier on the signal ground, due to external ground loops, must be diverted away from the critical signal grounds, that is, the input ground and the ground for the feedback arm - Any voltage difference between these two grounds appears directly in the output 4) Charging currents for the power supply unit (PSU) reservoir capacitors must be kept out of all other grounds 45. Amplifier Grounding Reservoir capacitor is a capacitor that is used to smooth the pulsating DC from an AC rectifier 46. Cross Over Network Audio crossover networks are a class of electronic filter used in audio applications Most loud speaker could work in limited portion of the audio spectrum So most hi-fi speaker systems use a combination of multiple loudspeakers drivers, each catering to a different frequency band Crossovers split the audio signal into separate frequency bands that can be separately routed to loudspeakers optimized for those bands 47. Cross Over Network Active crossovers allow drivers covering different frequency ranges to be powered by separate amplifiers Passive crossover simply route the frequencies to their respective speakers 48. Cross Over Network 49. Cross Over Network The capacitor has lower impedance for high frequencies. It acts to block low frequencies and let high frequencies through The inductor has a lower impedance for low frequencies. It acts to block high frequencies and let low frequencies through A capacitor and inductor in series act to block both very high and very low frequencies 50. Cross Over Network 51. Audio terminations: line in/out Consumer electronic devices concerned with audio often have a connector labeled "line in" and/or "line out Line out provides an audio signal output and line in receives a signal input The signal out or line out remains at a constant level, regardless of the current setting of the volume control The impedance is around 100 , the voltage can reach 2 volts peak-to-peak with levels referenced to -10 dBV (300 mV) at 10 k, 52. Audio terminations: line in/out This impedance level is much higher than the usual 4 - 8 of a speaker or 32 of headphones, such that a speaker connected to line out essentially short circuits the op-amp Line in expects the kind of voltage level and impedance that line out provides The line out connector of one device can be connected with the line in of another A line input has a high impedance of around 10 k, as is often labeled as "Hi-Z" input