RF Power Amplifiers
In this comprehensive reference, the author presents a full account of RF amplifiers and shows that understanding large-signal RF circuits is simply a matter of understanding the basic principles and their applications.
TABLE OF CONTENTS RF Power Amplifiers
2.4 Bias Circuits
RF PA bias circuits [4, 5, 7, 27, 28] are especially useful in Class A and AB amplifiers intended for linear operation. Requirements imposed on the bias circuits include reproducibility and/or possibilities for individual adjustment and temperature stability. Because a theoretical estimation is complicated, the amount of quiescent current required for minimum IMD in Class AB amplifiers is usually determined experimentally. Typically, minimum IMD is obtained with a quiescent collector current of 1 to 10 percent of the peak collector current. Note that MOSFETs are much more sensitive than BJTs to the quiescent current value.
A common feature of the bias circuits for BJTs and MOSFETs is the absence of emitter resistors. They are almost always used in small-signal amplifiers and assure a stable quiescent point. However, their use is usually avoided in RF PAs because it is extremely difficult to provide a satisfactory RF decoupling of the emitter (or source) to ground. The impedance of the common terminal (emitter or source) must be kept as low as possible to maximize power gain.
Bias Circuits for BJTs
A simple bias source for BJTs is shown in Figure 2-20(a). It uses a clamping diode to provide a low impedance voltage source. A large value capacitor is often connected across the diode to further reduce the AC impedance. The quiescent current can be adjusted by varying R. Usually, the diode is mounted on the heatsink of the transistor, Q, to perform a temperature compensation function. Obviously, it...
Copyright Noble Publishing Corporation 2001 under license agreement with Books24x7
