Introduction
Because the typical RF amplifier costs a considerable amount of money, it is important to gain at least a rudimentary understanding of amplifier operational classes and other important specifications before selecting one for a specific application. Not performing some type of “due diligence” could cost dearly. As such, the following provides rudimentary knowledge and additional references should one decide to dig deeper into this very important subject.
Pro Tip: No matter what – always carefully read the datasheet/specifications before deciding to purchase any amplifier!
Amplifier Operational Class Types
Some of us with education or backgrounds in electrical/electronics may recall studying transistor bias modes or the percentage of the time during which the amplifier is “amplifying” or conducting power and different operational classes of amplifiers. The idea is the same here:
- Class A: Conducts over the entire (360°) of the input power cycle.
- Class B: Conducts (with large nonlinearities) over half (180°) of the input power cycle. Not suitable for RF applications.
- Class C: Conducts over less than half (< 180°) of the input power cycle. Primarily used for pulse applications and not addressed in this article.
- Class A/B: Compromise between Class A and Class B where the conduction angle is intermediate; each of the two active elements conducts more than half the time.
From the above list of amplifier operational class types, the two most widely used in RF applications are Class A and Class A/B.
Class A and Class A/B Amplifier Types, Pros and Cons
| Specification | Class A | Class A/B |
| Linearity | Excellent | Poor |
| Harmonics | Low | High |
| Cross-over distortion | None | Present |
| VSWR Capabilities (High reflected power conditions can damage the amplifier) | Excellent (Implemented with hardware) | Poor (Software controlled VSWR foldback protection required) |
| AC Power Requirements | High | Low |
| Efficiency | Low | High |
| Junction temperatures | High | Low |
| Size | Medium to Large | Small |
Class A amplifiers provide the most accurate reproduction of the input signal, have lower harmonics, have no cross-over distortion, and are robust to any impedance mismatches between their outputs and the load (VSWR). However, Class A amplifiers are less efficient, requiring greater power requirements and producing more heat than their Class A/B counterparts.
On the other hand, Class A/B amplifiers are more efficient, produce lower junction temperatures, and are physically smaller than their Class A counterparts. However, Class A/B amplifiers do have some drawbacks. These drawbacks include less-than-ideal linear performance, susceptibility to damage from mismatches between their outputs and the load (VSWR), and can suffer from cross-over distortion.
Other Important Amplifier Specifications
Other important specifications to consider include gain (dB), gain flatness (+/- dB), harmonics (dBc), saturated power (dBm), linear power (dBm), and load Voltage Standing Wave Ratio (VSWR).
Linear Power (P1dB)
Although it is very important to pay attention to all of these specifications in relation to your application, one of the most important to understand is linear power, also known as P1dB. This specification is described as the output power at which the gain has varied by +/- 1dB from its small signal level. If the gain varies by more than +/- 1dB, then the amplifier is not able to reproduce the input signal faithfully, and the signal integrity of the output waveform is suspect and cannot be relied upon. In some instances, this may be okay, but it is not in other areas, such as fully compliant EMC testing to RF immunity standards like IEC 61000-4-3.
Voltage Standing Wave Ratio (VSWR)
Another important specification when researching RF amplifiers is Voltage Standing Wave Ratio or VSWR. When connecting an amplifier’s output to a load, the ideal condition is when the impedance of the output matches the input. When both impedances match, the load absorbs all the power generated by the amplifier, and none is reflected into the amplifier. The problem is that this ideal condition does not exist in real life. The load to the amplifier is typically an antenna, and the input impedance changes depending on frequency. If the VSWR is severe enough (load is completely open or short), then the amount of power reflected into the amplifier is extreme and damages it, rendering it inoperable until repaired. Even when used with extreme care, preventing connection to high VSWR loads is nearly impossible. It is, therefore, important that amplifier manufacturers design their amplifiers to handle (continue to operate without damage) situations where VSWR is severe.
Summary
In summary, this article has covered why amplifier class is important depending on the application and reviewed the pros and cons of Class A versus Class A/B amplifiers. It further described the meaning and usefulness of P1dB and Voltage Standing Wave Ratio capabilities when choosing an RF amplifier.
References
- Jones, N., Amplifier Selection | What you need to know, retrieved from: https://ametek-3.wistia.com/medias/fqxmmg92ck
- Jones, N., Amplifier Selection | What you need to know, retrieved from https://www.ametek-cts.com/-/media/ametekcts/documents/applicationnotes/tn0111_amplifier-selection_what-you-need-to-know.pdf?dmc=1&la=en&revision=b87393d1-275d-4ee4-9a6c-2907423b0e63&hash=9B000A61BB9C18F35E835F56FE7E2445
- Amplifier Research, Orange Book of Knowledge, 9th Download from: https://arworld.us/orange-book-request/
