ABOUT CLASS AB AMPLIFIER

1. CLASS AB
AMPLIFIERS
KR RAJSHEKHAR
15ECE18

2. CONTENT
• AMPLIFIERS and its Classification
• Diff. b/w Voltage and power
• POWER AMP.
• Introduction class AB amplifier
• CIRCUIT
• Biasing and OPERATION
• Ac load line/output
• Comparison of all amplifiers

3. AMPLIFIERS AND ITS CLASSIFICATION
• The purpose of any amplifier is to produce an output which follows the characteristics of the
input signal but is sufficiently large enough to supply the needs of the load connected to it.
• According to frequency capabilities.(audio amplifiers , radio frequency amplifiers)
• According to coupling methods.(R-C coupled amplifiers ,Transformer coupled amplifiers
Direct Coupled)
• According to use. (Voltage amplifiers and power amplifier)

4. DIFF. B/W VOLTAGE AND POWER

5. POWER AMP.

6. INTRODUCTION CLASS AB AMPLIFIER
• Class AB amplifiers combine Class A and Class B to achieve an amplifier with more efficiency
than Class A but with lower distortion thanclass B. This is achieved by biasing both transistors
so they conduct when the signal is close to zero (the point where class B amplifiers introduce
non-linearities).
• So, for small signals both transistors are active, acting like a class A amplifier. For large signal
excursions, only one transistor is active for each half of the waveform, acting like a class B
amplifier.

7. CIRCUIT

8. BIASING AND OPERATION
 To overcome crossover distortion, the biasing is adjusted to just overcome
the VBE of the transistors
 In class AB operation, the push-pull stages are biased into slight conduction,
even when no input signal is present.
 This can be done with a voltage-divider and diode arrangement, as shown
 Using equal values of R1 and R2 the positive and
negative supply voltages forces the voltage at
point A to equal 0 V and eliminates the need for
an input coupling capacitor.
 When the diode characteristics of D1 and D2 are
closely matched to the characteristics of the
transistor BE junctions, the current in the diodes
and the current in the transistors are the same;
((current mirror.))
The diode current will be the same as ICQ

9. OPERATION…
 AC Operation
 The Q-point is slightly above cutoff. (In a true class B amplifier,
the Q-point is at cutoff.)
 The ac cutoff voltage is at VCC
 The ac saturation current is:
 In class A , the Q-point is near the middle and there is significant current in the transistors even
with no signal.
 In class B , when there is no signal, the transistors have only a very small current and therefore
dissipate very little power.
 Thus, the efficiency of a class B amplifier can be much higher than a class A amplifier.

10. OUTPUT/AC LOAD LINE

11. COMPARISON OF ALL AMPLIFIERS

12. COMPARISON OF ALL AMPLIFIERS

13. BIBLIOGRAPHY
• http://www.electronics-tutorials.ws/amplifier/amplifier-classes.html
• http://www.learnabout-electronics.org/Amplifiers/amplifiers55.php
• https://en.wikipedia.org/wiki/Amplifier
• EEDP_Lect 06_Amplifiers_3_Classes/PPT/SLIDESHARE.