WMA: RF Power Amplifiers, Classes A through S: How the Circuits Operate, How to Design Them, and When to Use Each

Abstract

This full-day course is for "beginner" through "advanced" design engineers and their supervisors, concerned with designing any type of RF power-amplifier product, e.g., radio transmitter, or RF-power source for induction heating, dielectric heating, plasma generation, or illumination. Learn how all types of linear and nonlinear RF power amplifier circuits operate, how to design them, and when to use each, and be able to design manufacturable RF power amplifiers that work satisfactorily under all foreseen normal and abnormal operating conditions.

With at least ten lettered classes of RF power amplifiers, and several combinations of those classes, many engineers are confused about RF power amplifiers. The complexity of the subject is compounded by the fact that the RF power transistor acts either as a high-resistance current source or as a low-resistance switch, or -- in some amplifiers -- as a high-resistance current source during part of the "on" interval and as a low-resistance switch during another part of the "on" interval ("mixed-mode" operation). The circuit topology does not define unambiguously the transistor operating mode or the amplifier class of operation; examples are shown. This tutorial reviews

  • Saturated (switching-mode) and unsaturated ("linear") families, amplifier classes A through S
  • Transistor Utilization Factor = (Pout per transistor)/(Ipeak * Vpeak), for each class of operation
  • BJTs, MOSFETs, and MESFETs operating in current-source ("linear") mode, in switching mode, and in "mixed-mode" operation
  • Interactions between power transistor and external circuit, determining transistor and circuit operating conditions
  • Controlling RF output amplitude vs. modulating information onto amplitude envelope
  • Benefits and shortcomings of each circuit
  • Overview of reasons for oscillation and how to ensure stability
  • Suitable application areas for each circuit
  • Linear-amplifier systems using RF power-amplifier circuits as building-blocks
  • CAD of switching-mode RF power amplifiers

Organizer

Nathan O. Sokal
Design Automation, Inc.
4 Tyler Road
Lexington, MA 02420-2404; U.S.A.
NathanSokal@compuserve.com
tel: 781-862-8998
fax: 781-862-3769

Sponsor

MTT-17, MTT Education Committee

Technical level

Short Course

Day/ time

Monday, 12 June 2000, 8:00 AM to 5:00 PM

Room

Westin Hotel Americas Ballroom (moved from Hynes Convention Center Room 202)
(Breakfast will be served in the Hynes Ballrooms B and C. Lunch for this workshop is in the Westin Essex ballroom)

Speaker

Nathan O. Sokal, Design Automation, Inc.

Presentation Outline

RF Power Amplifiers -- Overview

1. Physical realizations: monolithic vs. assembly of discrete components

2. Trade-offs; all realizations same trade-off factors

3. Transistor operating mode: Crucial difference: voltage saturation vs. current saturation

4. Amplifier classes vs. transistor operating modes (Caution: Three entirely different circuits have been called "Class F"; here we refer to F1, F2, and F3 to avoid ambiguity.)

5. Applications

6. Efficiency

6.1 Definitions and equations

6.2 Transistor limits

7 . Transistor Utilization Factor

 

Description of Amplifier Classes

1. "Linear" Amplifiers
1.1 Classes A, AB, B
- Circuit operation definitions

- Single-ended and push-pull operation

- Input/output transfer functions

- Bias points; biasing push-pull Class AB for minimum distortion

1.2 Class C amplifier

- Circuit operation

- Efficiency vs. output power, Vcc, and deviation from center frequency

1.3 Class F1 amplifier

1.4 Stability of all "linear" amplifiers

2. Switching-Mode Power Amplifiers

2.1 Benefits vs. "linear" amplifiers

2.2 Efficiency

2.3 Design difficulties

2.4 Advantages, with examples

2.5 Disadvantages

2.6 Class D family vs. Class E family: trade-offs and choice

3. Discussion topics

3.1 Switching (Class E as example) vs. "linear" (Class C or B as example)

3.2 Experimental results, Class E

3.3 Class E applications by modulation type

3.4 Relative advantages and disadvantages, Class D vs. Class E

3.5 Class F2 and F3 as higher-order versions of Class E

3.6 Class S amplifier: description and application

4. Mixed-mode amplifiers

 

System-Level Topics

1 Linear power-amplifier systems
1.1 Predistortion of input signal

1.2 Feed-forward correction of nonlinearity

1.3 Envelope Elimination and Restoration (EER) power amplifier

2. Amplitude control vs. modulation

3. Choice of power-amplifier type

 

Power Amplifier Circuit Design Methods (analytical, numerical, CAD)

1. Complications of real circuit (omitted in analytical; included in CAD)

2. Circuit Computer-Aided Design (CAD)

2.1 Transistor evaluation

2.2 Automatic design of nominal circuit

2.3 Extremely fast circuit simulation (100-1000X speed of SPICE)

2.4 Automatic optimization

3. CAD examples: Classes D and E

3.1 Complications of real circuit

3.2 Comparisons of computed transistor evaluation, circuit simulation, and actual measurement

4. CAD Demonstration (transistor evaluation, automatic circuit design, simulation, and automatic optimization)

5. Impedance-Matching Networks (transmission lines and lumped elements); Effects of Load-Impedance Variation with Frequency

 

Further Discussion of Any Topics of Interest to the Audience