1. Audio Amplifiers Basics, Circuits and Parameters
MSc. Karol Kropidłowski

2. Presentation Plan
Transistor – what is it? Polarization and classes Pros and cons of classes Applications Basic Parameters of amplifiers Differential amplifier – what is it? Operational amplifiers

3. Transistor – what is it?
Transistor is a semiconductor device used to amplify and switch electronic signals.

4. How transistor works?

5. Symbol and Diode Model
Diode equivalent circuit is very simplistic and does not explain the operation of a transistor but it gives some idea of ​​the voltages that exist between the electrodes.
Bipolar transistor symbols
Diode model citcuits

6. We got transistor, what’s next?
Transistor to be able to work as amplifier must be properly polarized. We will follow the correct polarity on the NPN-type transistor:  - the potential of the collector must be higher than the potential of emitter (we need to feed power to the transistor) -  diode in the base-emitter junction must be polarized in conducting direction, and the collector-base diode in opposite direction (we apply voltage to the base of transistor higher than the threshold voltage and lower than the supply voltage)

7. Polarization
Polarization example for npn transistor:

8. Operating Point and Load Line
Operating point is a point on transistor output characteristic in which it active region and parameters such as Uce and Ic can be designated.
Load line shows all the possible operating points when different values of base current are applied. It also shows how will voltages and current change when we feed input signal to base of transistor.

9. Setting up Operation Point
Let’s assume transistor current gain β=100 We calculate Uce = Uz-Uce_sat=12v-0,9V=11,1V We calculate maximum collector current Uce/Rc=11,1V/1kΩ=11,1mA Let’s chose operating point in the middle of load line (Uwy=5,5V oraz Ic=5,5mA) We calculate base current Ib=Ic/β =55uA and value of base resistor Rb=(Uz-Ube)/Ib= 205,45kΩ

10. Operating Point of amplifier and the class
Setting up operating point in the middle of load line we get amplifier working in A-class. P point on output characteristics.

11. Operating Point of amplifier and the class
A-class input/output waveforms
Green – Input signal Red – Output signal

12. A-class Pros and Cons Pros: No crossover distortion
Very low distortion
Low construction cost (for small powers)
Cons:
Low efficiency of less than 50% (up to 20%)
At higher powers problem with the heat dissipation from the transistors
The need for precise setting of the operating point
The need to compensate for transistor parameters change according to temperature.
Weight, size and price of the amplifier increases exponentially with the output power

13. A-class applications Oldish mobile players (eg. walkman)
Impedance matching (input circuits)
Preamplifiers (audio and not only audio)
Audiophile audio power amplifiers
Studio listenings

14. Operating Point of amplifier and the class
Setting up operating points in way that collector current is minimal (Iceo) We obtain B-Class amplifier. Point A’ on output characteristics.

15. Operating Point of amplifier and the class
B-class input/output waveforms
Green – Input signal Red – Output signal

16. B-class Pros and Cons Pros: High efficiency (theoretically 78,5%)
Very low quiescent current in idle.
Cons:
Huge signal distortion
Only one half of signal is amplified, other half is cut.

17. Operating Point of amplifier and the class
Adding second transistor working with second half of the signal we get 2B-Class amplifier.
Green – Input signal Red – Output signal

18. 2B-class Pros and Cons Zalety: High efficiency (theoretically 78,5%)
Very low quiescent current in idle.
Wady:
Crossover distortion
“metallic” sound of this kind of amplifier

19. 2B-class applications -Outputs of TTL digital gates
Very rarely used in audio practice, because of nonlinear distortion.
Sometimes encountered in budget constructions.

20. Operating Point of amplifier and the class
Setting up operating points in way that there is no collector current. We obtain C-Class amplifier. Point A on output characteristics.

21. Operating Point of amplifier and the class
C-class input/output waveforms
Green – Input signal Red – Output signal

22. C-class Pros and Cons Pros: High efficiency
No quiescent current, no power draw without signal.
Cons:
Huge signal distortion, bigger than in B-class

23. Operating Point of amplifier and the class
Adding second transistor working with second half of the signal we get 2C-Class amplifier.
Green – Input signal Red – Output signal

24. 2C-class Pros and Cons Pros: High efficiency
No quiescent current, no power draw without signal.
Cons:
Crossover distortion bigger than in 2B-class

25. 2C -class applications Sometimes seen in megaphones
Ultrasonic cleaners
Some car alarms itp.
Used in D-Class amplifiers

26. Operating Point of amplifier and the class
Setting up operating points of 2B-class in way that collector current is just above minimal (Iceo) We obtain 2AB-Class amplifier. Point between A’ and P on output characteristics.

27. Operating Point of amplifier and the class
2AB-class input/output waveforms
Green – Input signal Red – Output signal

28. 2AB-class Pros and Cons Pros: High efficiency (theoretically 78,5%)
Low quiescent current
Low signal distortion
Cons:
It requires precise settings of operating points that the quiescent current of both transistors is identical.

29. 2AB-class applications
End stages of power amplifiers (audio, generators, radio transmitters, TV transmitters)
It occurs in operational amplifiers, headphone amplifiers from 0.1 W thru car-audio to live performance amplifiers 4kW+

30. Other amplifier classes
By setting polarity of the transistors as in the 2C-Class and controlling them by PWM we obtain a D-Class amplifier. Idea of converting the audio signal to the PWM control signal is shown in the following figure:

31. Other amplifier classes
Exemplary schematic of an amplifier operating in D-class

32. D-class Pros and Cons Pros: High efficiency (theoreticaly 100%)
At high frequency switching distortion is minimal.
Cons:
Large number of elements
Expensive
Higher the switching frequency more power losses occur.
Complicated transistor control.

33. D-class applications
End stages of power amplifiers(audio, DC and BLDC motor drivers)
Portable devices(mp3, phones,where efficiency is critical)
It occurs in headphone amplifiers from 0.1W thru car-audio 250W + to live performance amplifiers 1,5kW+

34. Other amplifier classes
Class H
For small signals, the amplifier operates as a normal 2AB amplifier but as we approach the maximum output signal, built-in converter increases voltage for end stage in power amplifier which allows to obtain even 4-fold increase in output power, at the same power supply voltage. It occurs most often in power amplifiers used in cars where the limitation is the low voltage (12V). 
It is worth noting that the supply voltage is increased in the rhythm of the signal only in the channel, and only when necessary.
An example of such amplifier is TDA1560Q

35. H-class Pros and Cons Pros: Such as2AB
The ability to achieve higher power output at the same supply voltage
Cons:
Such as 2AB

36. H-class applications End stages in power amplifiers
It occurs in car-audio and performance amplifiers 1,5kW+

37. Basic power amplifiers parameters
Power gain (Kp) Output power[W] THD- Total Harmonic Distortion[%] Input/output impedance Bandwidth [Hz] Efficiency [%] Output noise voltage [mV] Quiescent Current [mA]

38. Basic parameters
Power gain of amplifier is the power output divided by power input. Output power its power that amplifier can produce on nominal load impedance at a given frequency or frequency range, without exceeding a specific distortion ratio within 10 minutes.

39. Basic parameters
Nonlinear distortions (THD) rely on the formation of harmonic and combined frequencies signal. The signal at the output of the device contains additional components that were not present in the input signal. If harmonic distortions are less than 10% they are practically unnoticeable for the typical listener.

40. Basic parameters
Input impedance of the amplifier is impedance, which the input of the amplifier represents for the rated operating conditions. Output impedance determines the value of the load impedance, which may be attached at a specific efficiency of the amplifier.

41. Basic parameters
Bandwidth is the range of frequencies transmitted by the amplifier. Frequency is determined for the 3dB rolloff to flat part of frequency characteristics. According to PN-74 / T-06251/07 for Hi-Fi amplifiers minimum bandwidth should be 40Hz - 16kHz.

42. Basic parameters
Energy efficiency is defined as the percentage ratio of the output power of the amplifier to a power drawn from the power source. This is an important criterion for assessing the quality of the power amplifier.

43. Basic parameters
Output noise voltage at the output is the maximum amplitude of the noise at the amplifier output with input shorted down. Quiescent current is the current drawn by the amplifier from the power source when there is no signal connected to the input.

44. Input Circuits of Power Amplifiers
Historically the A-class amplifier was used as the input circuit of the power amplifier because it ensured low distortion and impedance matching between input and output. High input and low output impedance. However, the A-class amplifier input circuit was replaced by a differential amplifier.

45. Differential amplifier
Amplifier whose output voltage depends on the voltage difference between the inputs of the amplifier. In its simplest version is composed of two transistors coupled together via an emitter resistor Re. This resistor stabilizes the operating points of the two transistors and forces the Ie emitter current flowing in a common circuit which is equal to the sum of the currents of both transistors. At high resistance Re, current Ie does not depend on the intensity of the currents at the inputs, and is constant.

46. Differential amplifier
Exemplary schematic of differential amplifier :

47. Differential amplifier
In the construction of differential amplifiers we look for high differential gain Kur, high control signal damping factor Hs, high input resistance and small signal unbalance and small drift. Transistors should have parameters as close as possible to each other.

48. Differential amplifier
Improving the performance of the amplifier requires an increase in transistor current gain βo, increase in the resistance Re and increase in resistance Rc. Instead of a resistor R and Rc current sources are used, it is easier to produce a current source and current mirror in a single chip than the resistor.

49. Differential amplifier
Differential amplifiers are used as the input circuit of power amplifiers and operational amplifiers. An example of an operational amplifier internals:

50. Operational amplifier
Puzzle time

51. Bibliografia
„Elektronika dla wszystkich” 11/96 „Praktyczny elektronik” 1/2001