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Class A Amplifier


Class A Amplifier
A transistor is generally assumed to be linear if it operates in the region of operation
between cutoff and saturation (triode in FETs). Linear operation will result if an
amplifier is biased at the exact midpoint of the linear region, providing the RF drive
signal never exceeds the boundary values (cutoff and saturation voltage). In principle,
a perfectly sinusoidal input signal will result in a perfectly sinusoidal output current
without any harmonics. Such an amplfier is said to be a class A amplfier.
Ideally a class A amplifier can be considered to be linear for a limited range of input
drive. In reality, the linear region contains weak non-linearities which become more
evident as the signal drive level increases. Finite variation of ? with collector current
(Ic) in bipolars will result in non-linearities; in FETs there is a fundamental nonlinearity
due to the square law relationship between input voltage and drain current
(Id). These non-linearities generate significant harmonic content in case of Class A
operation. However, a reactive matching network (typically used to transform the
50 ? antenna load to the optimum load resistance which the transistor wants to see)
can act as a filter attenuating the generated harmonics thus giving a cleaner output
signal .
The operating point of a Class A amplifier is the center of the active region (the signal
level should not push the transistor into cutoff or saturation). Figure 3.6 shows
the operating point (Q-point) of a class A amplifier on a current/voltage transfer
characteristics. Since the transistor is always in active region, the device is always
conducting (even when there is no RF signal). Thus the transistor is constantly drawing
current which represents continous loss of power. In other words, the conduction
angle of the Class A amplfier is 2? . As a result, Class A amplifiers
have the lowest efficiency of all classes of operation. On the other hand, the transistor
is biased at the center of the active region and hence these PAs have very good
linearity. As a result, it is advisable to use Class A amplifiers in cases where linearity
is a stringent requirement but power consumption (efficiency) is less of an issue. It
can be shown that the theoretical maximum efficiency of a Class A PA is always ?
50% . In an inductorless system with resistive loading, the output voltage cannot
rise beyond the supply voltage and hence the efficiency is limited to a theoretical
maximum of 25% . Class A RF Power amplifiers are typically used as low-level
driver amplifiers  where the power consumption is a very small portion of the total
power consumption.




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