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发表于 2007-9-17 16:27:13
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PA设计大全
非常经典的一篇文章
RF transmitting transistor and
power amplifier fundamentals
Power amplifier design
3 POWER AMPLIFIER DESIGN
3.1 Classes of operation and biasing
3.1.1 Class-A
Class-A operation is characterized by a constant
DC collector (or drain) voltage and current. This class of
operation is required for linear amplifiers with severe
linearity requirements including:
– Drivers in SSB transmitters where a 2-tone 3rd-order
intermodulation of at least -40 dB is required
– Drivers in TV transmitters where the contribution to the
gain compression must be very low, i.e. not more than a
few tenths of a dB
– All stages of TV transposers. These are tested with a
3-tone signal and the 3rd-order intermodulation
products must be below -55 to -60 dB. The driver
stages should only deliver a small contribution to the
overall intermodulation, so they have to operate at even
lower efficiency than the final stage (as this is the only
way to reduce distortion in class-A).
Though the theoretical maximum efficiency of a class-A
amplifier is 50%, because of linearity requirements, the
efficiency in the first two applications listed above will be
no more than about 25%. And in TV transposers, the
efficiency is only about 15% for the final stage and even
less for the driver stages.
The transistor power gain in a class-A amplifier is about
3 to 4 dB higher than that of the same transistor operating
in class-B. This is because the conduction period of the
drain current in class-A is 360° and in class-B only 180°
(electrical degrees). Therefore, the effective transconductance
in class-B is only half that in class-A.
3.1.1.1 DISTORTION
SSB modulation is mainly used in the HF range: 1.5 to
30 MHz. When testing transistors for this application,
Philips uses a standard test frequency of 28 MHz. Owing
to its variable amplitude, an SSB signal is sensitive to
distortion.
3.1.1.1.1 2-TONE INTERMODULATION DISTORTION
TEST
3rd and 5th-order products
This is the most common distortion test. In this test, two
equal-amplitude tones 1 kHz apart are applied to the input
of the amplifier under test. Practical amplifiers will never be
completely linear, and the most important distortion
products they produce are the 3rd and 5th order ones,
because these are in or very near to the pass-band.
NOTE TO SECTION 3
For clarity in equations, identifiers such as
R1, +jB2,- jX3 in drawings are written as
R1, +jB2,- jX3 in the body text.
If the frequencies of the two input tones are denoted by
p and q, the 3rd-order products are at frequencies of 2p-q
and 2q-p, see Fig.3-1. The 5th-order products which
usually have smaller amplitudes are at 3p-2q and 3q-2p.
Note, the two intermodulation products of the same order
don’t necessarily have equal amplitudes. This can be due
to non-ideal decoupling of the supply voltages, i.e.
decoupling that is insufficiently effective at all the
frequencies involved. Philips publishes the largest value in
data sheets.
Power relationships
If the tones at p and q are each of 10 W, then the
combination has an average (calorific) power of 20 W. The
two tones can however combine in phase or out of phase,
producing an RF signal of variable amplitude. When the
two tones are in phase, the voltage amplitude is twice that
of one tone, so the power is four times that of one tone (in
this example: 40 W). This maximum power is called the
peak envelope power (PEP) and is commonly published in
data sheets. When the two tones are in anti-phase, their
combined amplitude is zero. In the ideal case, i.e. with no
distortion, the envelope of the combined signal consists of
half sine waves, see Fig.3-2.
Fig.3-1 Position of the main intermodulation
distortion products in the 2-tone test
where p and q are the input tones.
handbook, halfpage MGM031
3p - 2q
2p - q
3q - 2p
2q - p
p q
freq. |
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