文档介绍：Philips Semiconductors
RF transmitting transistor and
Power amplifier design
power amplifier fundamentals
3 POWER AMPLIFIER DESIGN
NOTE TO SECTION 3
Classes of operation and biasing
For clarity in equations, identifiers such as
Class-A R1, +jB2, −jX3 in drawings are written as
R , +jB , −jX in the body text.
Class-A operation is characterized by a constant 1 2 3
DC collector (or drain) voltage and current. This class of
operation is required for linear amplifiers with severe
If the frequencies of the two input tones are denoted by
linearity requirements including:
p and q, the 3rd-order products are at frequencies of 2p−q
– Drivers in SSB transmitters where a 2-tone 3rd-order and 2q−p, see -1. The 5th-order products which
−
intermodulation of at least 40 dB is required usually have smaller amplitudes are at 3p−2q and 3q−2p.
– Drivers in TV transmitters where the contribution to the Note, the two intermodulation products of the same order
pression must be very low, . not more than a don’t necessarily have equal amplitudes. This can be due
few tenths of a dB to non-ideal decoupling of the supply voltages, .
– All stages of TV transposers. These are tested with a decoupling that is insufficiently effective at all the
3-tone signal and the 3rd-order intermodulation frequencies involved. Philips publishes the largest value in
products must be below −55 to −60 dB. The driver data sheets.
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). pq
handbook, halfpage MGM031
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 2p − q 2q − p
efficiency is only about 15% for the final sta