What Class are you in? The argument for the use of class A Amplifiers in Radiated Immunity testing

By Simon Young, EMV Limited

RF Power Amplifiers are generally specified by their Gain, Frequency Response and Output Power. Other characteristics such as Noise Figure, Gain Stability and Distortion are also considered but for applications involving Radiated Immunity testing one of the major factors to consider is the LOAD TOLERANCE.

The capability to provide power into loads that vary from the ideal 50W is essential. The load impedance's experienced in an EMC test chamber vary widely. For example antenna characteristics, room reflections and resonance's, cable/connector losses and reflections from the Equipment Under Test (EUT) all contribute to the imperfect effective load. Typical antenna VSWR of 2.5:1 and factoring in room and signal path effects can lead to a VSWR in excess of 5.0:1.

In short, the RF power amplifier must be capable of providing full rated power to loads that vary considerably from the ideal 50W.

The Choice of Amplifier for Radiated Immunity Testing

The most important requirement is the test level (V/m) which will determine the output power level of the amplifier. Other factors such as the test distance (ideally 3m), type of room or chamber, type of antenna and harmonic content of the amplifier are also considered. The chamber can be a cell, e.g. G-TEM or TEM or an anechoic chamber, with absorber/ferrite lining. For example, the Loss Prevention Council (LPC) Laboratories have both a G-TEM and a ferrite tile lined anechoic chamber. The two types of chamber are both recognized for Radiated Immunity testing but they present different requirements to the user when determining the power level required for the RF Power Amplifier. The required test levels of in excess of 50V/m mean that LPC have to be very sure of their facts when choosing an amplifier.

If the wrong amplifier is chosen the user can expect interruptions to his testing. Continual interruptions can be caused by the amplifier cutting out due to VSWR problems. More seriously, potential damage to the amplifier due to being used into widely varying loads could lead to long term downtimes during testing. In this situation it could be costly in both time and replacement parts. In either situation the user will be affected by the reduction of revenue as no testing will be possible.

So when choosing the RF power amplifier the user is faced with two major types for Radiated Immunity testing, Class A and AB. The potential user must understand the basic differences between the classes to enable him to make the right choice.

Class A Operation

In Class A operation, the active devices are biased to ensure that collector or plate current flows for 360° of input signal. When operated below the 1dB compression point, the RF signal input and RF output wave-forms vary uniformly about the DC quiescent point and lie within the linear region of the characteristic curves of the active device. While this biasing method provides excellent linearity and low distortion, an additional characteristic is that the Class A amplifier dissipates maximum power in its quiescent state. Therefore the amplifier must be built to handle a great deal of power dissipation.

The Class A design requires the use of larger active devices and, quite often a larger number of devices to share the heat dissipation. Furthermore, additional attention is paid to heat sinking and cooling considerations. When an input signal is applied and RF power is delivered to a load, the RF device runs cooler. As the devices are running below their normal operating temperature, power reflections resulting from operating into high levels of VSWR are not a problem.

The design of the Class A amplifier is superior with regard to its ability to dissipate power. However the Class A amplifier will undoubtedly be larger, heavier and be less efficient with respect to the use of primary power.

Class AB Operation

The Class AB amplifier is biased to produce output current for somewhere less than 360ø and more than 180° of the input signal. It consumes less power in its quiescent state than when an input signal is applied. Since it consumes less power and is therefore more efficient than a Class A amplifier,
Efficiency = RF Power Out/ DC Power Input
less active devices are required and the devices are smaller in area. In addition, less heat sinking is required and the cooling systems tend to be less elaborate. The result of this is that the ability of the Class AB broadband amplifier to absorb reflected power is practically non-existent. In other words high levels of VSWR will cause problems.

The Class A Performance into a Varying Load

The Class A amplifier has been designed to dissipate at least 1000W, power reflected back into the output stage of the amplifier does not present a problem. Even if the output were shorted or open, the resulting total reflection of 100W would not adversely affect the amplifier. It would continue to supply a forward power of 100W regardless of the load, since the additional 100W of reflected power does not increase the device dissipation above its design value of 1000W steady state.

The Class AB Performance into a Varying Load

Here we have a serious problem when dealing with load variations. Its design that assumes nearly ideal loads and the slightest amount of reflected power can cause severe damage to its output stages. To compensate for this problem, Class AB amplifiers use a protection arrangement to limit the amount of reflected power.

The Class AB amplifier must implement a "fold-back" of the available RF output power in an effort to protect its output stages. Typically, the 100W Class AB amplifier would not sustain 100W into a VSWR of 2.0:1 (typical antenna VSWR) and would fold-back to 89W. Therefore with as little as 11% of the output power reflected, the forward power has dropped to 89W. If we take a modest increase in the VSWR to a value of 3.0:1 then 25% of the output power is reflected back but more specifically, the Class AB amplifier has cut back its forward power to 50W.

Figure 1 shows the Minimum Available Power for a Class A and AB 1000W RF power amplifier. It shows the Class A with a load tolerance of 50% even at infinite VSWR but more importantly the Class A amplifier can still deliver full rated power into VSWR's of up to 5.0:1. The Class AB curve is generally applicable to all amplifiers regardless of power level and shows that its performance suffers into a poor VSWR.

An Example to Illustrate the Drawbacks of the Class AB Amplifier

Consider a Class A 100W amplifier that requires 1000W of primary power to provide 100W of RF output power. With no input signal, this amplifier must be capable of dissipating 1000W. When signal is applied the amplifier dissipates 900W while delivering 100W to the load.

A typical broadband Class AB 100W amplifier dissipates considerably less than 100W with no input. When a signal is applied the internal dissipation may rise in excess of 500W.

The above examples assume a perfect 50W load. How does the Class AB amplifier perform with real life loads encountered in typical Radiated Immunity testing situations or applications where impedance's vary? As the load varies from an ideal 50W, output power is reflected back into the output stage.

The Only Class is A

Each amplifier has its niche application. The Class AB amplifier is characterized by its relatively small size and weight as well as its efficient use of primary power. It is effective in applications where size and weight are key and one can guarantee a solid 50W load. Its major weakness is its inability to operate into variable impedance's, typically encountered in Radiated Immunity testing. Another potential drawback is that the Class AB has higher levels of harmonic distortion than a linear Class A amplifier.

The Class A amplifier clearly excels in its ability to deliver power into varying loads. Its major drawbacks of increased size, weight and low efficiency are generally not an issue in most test environments.

Harbinder Bharj, EMC Project Manager at the Loss Prevention Council Laboratories recently had a requirement for a 1000W amplifier from 10kHz to 220MHz and a 500W amplifier from 80MHz to 1GHz. At these power levels there is only one choice of amplifier and that is Class A.

LPC's requirement of a 1000W and 500W amplifier was to add further testing capabilities to their new chamber where test levels of 30 to 60V/m are required for Automotive and Fire Alarm testing.

Class AB RF Power Amplifiers are used in Radiated Immunity testing but much more attention has to be taken when determining the power level. Class A linear RF power amplifiers have superior characteristics over Class AB amplifiers that lend themselves to clear and more accurate Radiated Immunity testing.

Simon Young can be contacted on Tel: 01908 566556, Fax: 01908 560062, Email: info@emv.co.uk or web site: http://www.emv.co.uk

Archive Index

EMC Journal Home


© Nutwood UK Ltd 2001
  
Eddystone Court - De Lank Lane
St Breward - BODMIN - PL30 4NQ
Tel: +44 (0)1208 851530 - Fax: +44 (0)1208 850871
nutwooduk@nutwood.eu.com