1. Classification of power amplifiers
Part I
Classification of power amplifiers

2. POWER AMPLIFIERS Required
To deliver a large current to a small load resistance e.g. audio speaker; or to deliver a large voltage to a large load resistance e.g. switching power supply;
To be of low output resistance in order to avoid loss of gain and to maintain linearity (to minimize harmonic distortion)
To deliver power to the load efficiently

3. POWER TRANSISTORS - BJT
Transistor limitations
1 maximum rated current,
2 maximum rated voltage,
3 maximum rated power, and
4 maximum allowed temperature.

4. Parameter
Small-signal BJT
(2N2222A)
Power BJT
(2N3055)
(2N6078)
VCE (max) (V) 40 60
250
IC (max) (A)
0.8 15 7
PD (max) (W)
1.2
115 45 
35 – 100
5 – 20
12 – 70
fT (MHz)
300 1

6. Current gain is smaller in power BJT
Current gain is smaller in power BJT. The gain depends on IC and temperature may be related to the following:
maximum current that connecting wires can handle
at which current gain falls below a stated value
current which leads to maximum power dissipation.
maximum voltage limitation associated with avalanche breakdown in reverse-biased collector-base junction.
second breakdown in BJT operating at high voltage and current.

7. Instantaneous power dissipation
The second term is small, hence
The average power over one cycle

8. If collector current and collector-emitter voltage are dc quantities, the maximum rated power, PT
The power handling ability of a BJT is limited by two factors, i.e. junction temperature and second breakdown. SOA must be observed, i.e. do not exceed BJT power dissipation.
The safe operating area (SOA) is bounded by IC(max); VCE(sus) and PT (Figure)

9. SOA of BJT (linear scale)

10. SOA of BJT (logarithmic scale)

12. Physical structur of BJT
Cross-section view
Top view

13. POWER TRANSISTORS - MOSFETs
MOSFET is superior over BJT:
Faster switching times
There is no second breakdown.
Stable gain and response over wide temperature range.
May be driven directly by TTL.
Immune to thermal runaway

14. MOSFET’s must also be operated within SOA
For MOSFET 2N6757, the maximum drain current, ID, is 8A and breakdown voltage, VDS (max) is 150V.
Parameter
Power MOSFET
2N6757
2N6792
VDS (max) (V)
150
400
ID (max) (A) (at T = 25C) 8 2
PD (max) (W) 75 20
MOSFET’s must also be operated within SOA

15. The SOA for 2N6757 is shown in the following figure

16. Structure of MOSFET Cross section of VMOS device
Cross section of DMOS device
HEXFET structure

17. Classes of Amplifiers
They are grouped together based on their Q-points on the DC load line.

18. In class-A; the transistor conducts during the whole cycle of sinusoidal input signal

19. In class-B; the transistor conducts during one-half cycle of input signal

20. In class-AB; the transistor conducts for slightly more than half a cycle of input signal

21. In class-C; the transistor conducts for less than half a cycle of input signal

22. Cass–A operation
For maximum swing ( +ve and –ve), transistor is biased such that the Q point is at centre of the load line.
The transistor conducts for a full cycle of the input signal

23. Instantaneous power dissipation in transistor is;
For sinusoidal input signal;
And;
For maximum possible swing;

24. Therefore;
(See graphical representation)

25. When the input signal = 0, the transistor must be capable of handling a continuous power of;
Efficiency;
PL = average ac power to the load
PS = average power supplied by the source (VCC)

26. Maximum theoretical efficiency of class A amplifier is therefore 25%
For maximum possible swing;
Power supplied by the source;
Maximum theoretical efficiency of class A amplifier is therefore 25%
The efficiency;

27. Cass–B operation
Consists of complementary pair electronic devices
One conducts for one half cycle of the input signal and the other conducts for another half of the input signal
Both devices are off when the input is zero
(See Figure)

29. Complementary push-pull circuit
Assuming ideal transistor;
when vI = 0;
both Qn & Qp are off;
when vI > 0;
Qn conducts & Qp is off;
when vI < 0;
Qp conducts & Qn is off
An approximate class-B circuit comprising complementary BJT pair working in push-pull configuration.

30. Assuming cut-in voltage of transistor is 0. 6 V, vO = 0 for a range 0
Assuming cut-in voltage of transistor is 0.6 V, vO = 0 for a range 0.6 V < vI < 0.6 V.
The transfer characteristic becomes non-linear (See Figure)
The range where both transistors are simultaneously off known as the dead band
The output will be distorted – crossover distortion (See Figure)
Crossover distortion can be eliminated by biasing the transistor with small quiescent current – class-AB

31. Dead band

32. Theoretical maximum efficiency of class-B amplifiers

34. Maximum possible value of Vp is VCC.
and

35. The instantaneous power in Qn is;
for 0 < t < 
and
for  < t < 2
The average power in Qn is;

36. (symmetry)
Differentiating for maximum PQn with respect to Vp gives us;
Since each power source supplies half sinewave of current, the average value is;

37. The total power supplied by the two sources is;
The power delivered to the load is;
The efficiency is;

38. Maximum theoretical efficiency of class B amplifier is therefore 78.5%
Maximum efficiency occures when
Under this condition;
Maximum theoretical efficiency of class B amplifier is therefore 78.5%

39. Small ICQ flows through each transistor in the absent of input signal
Cass–AB operation
Small quiescent bias on each output transistor to eliminate crossover distortion
Small ICQ flows through each transistor in the absent of input signal

41. Cass–C operation
B – E junction is reverse-biased to obtain Q-point beyond cut-off.
Transistor conducts for less than half a cycle of input signal
Tuned circuit is required.
Used for RF amplifier.
Efficiency > 78.5%