1. POWER AMPLIFIER Concept of Power Amplifier Power BJTs Power MOSFETs
Classes of Power Amplifier

2. CONCEPT OF POWER AMPLIFIER
To deliver a large current to a small load resistance e.g. audio speaker;
To deliver a large voltage to a large load resistance e.g. switching power supply;
To provide a 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. TRANSISTOR LIMITATIONS
maximum rated current,
maximum rated voltage,
maximum rated power, and
maximum allowed temperature.

4. POWER BJTS Parameter Small-signal BJT (2N2222A) Power BJT (2N3055)
Comparison of the characteristics and maximum ratings of a small-signal and power BJT
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

5. Typical dc beta characteristics (hFE versus lc) for 2N3055

6. POWER BJTS 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. VCE(sus) =115 volt at which these curve merge and the minimum voltage necessary to sustain the transistor in breakdown.
The breakdown voltage, VCE0 ~130 volt when the base terminal is open circuited, IB=0

8. Instantaneous power dissipation
The average power over one cycle
The maximum rated power,

9. The Safe Operating Area (SOA) is bounded by IC(max); VCE(sus) and PT
The SOA of a bipolar transistor plotted on: (a) linear scales and (b) logarithmic scales

10. Example
The parameters of the common emitter circuit are RL=8Ω, VCC=24 volt. Determine the required current, voltage and power ratings of the a power BJT.

11. Power MOSFETs Characteristic of two power MOSFETs Parameter
2N6757
2N6792
VDS (max) (V)
150
400
ID (max) (A) (at T = 25C) 8 2
PD (max) (W) 75 20

12. PERFORMANCE CHARACTERISTIC OF POWER MOSFETs
Faster switching times
no second breakdown.
Stable gain and response over wide temperature range.

13. Classes of Power Amplifiers
Class A - The transistor conducts during the whole cycle of sinusoidal input signal
Class B - The transistor conducts during one-half cycle of input signal
Class AB - The transistor conducts for slightly more than half a cycle of input signal
Class C - The transistor conducts for less than half a cycle of input signal

14. Class A Operation Configuration : No inductor @ transformer are used
Common-emitter amplifier,
dc load line (the Q point is at centre of the load line)
instantaneous power dissipation versus time in the transistor

15. Instantaneous power dissipation in transistor is
For sinusoidal input signal, the collector current;
And the collector emitter voltage;
For maximum possible swing;
Therefore, the instantaneous power dissipation in transistor

16. The power conversion efficiency
The average ac power delivered to the load
The average power supply by the VCC source
The maximum attainable conversion efficiency

17. Example
The common source circuit parameters are VDD=10V, RD=5kΩ and the transistor parameters are Kn=1mA/V2, VTN=1V and =0.
Assume the output voltage swing is limited to the range between the transition point and vDS=9V to minimize nonlinear distortion.
Calculate the actual efficiency of a class A output stage.

18. Exercise
The Q-point of common source circuit is VDSQ=4V
Find IDQ
Determine the max peak to peak amplitude of a symmetrical sinusoidal output voltage if the min value of instantaneous drain current must be no less than 0.1IDQ and the min value of instantaneous drain source voltage must be no less than vDS=1.5V.
Calculate the power conversion efficiency where the signal power is the power delivered to RL.
Ans:
60mA,
5V,
31.25mW, 5.2%