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Technical review

BJT transistor, Ic vs Vce

Amplification classes (III)

When communicating with another ham the main objective is to emit a signal displaying an excellent modulation that please to your correspondent's ears. Single-sideband transmitters generate the carrier and modulation at a low frequency and translate it up with a mixer. Linear amplifiers are used to increase the power level up to say 100W. If we have to amplify this signal to higher values because the station if far away, we want that our amplified signal be strictly identical in shape to the original, only its intensity must have increased. We call this characteristics, the linearity.

Linearity

Several factors determine the linearity of an HF so-called "linear" amplifier :

- Changes in device power gain (ratio of output to drive power) over the range from zero to full output

- The collector/emitter (BJT transistor) or drain/source (MOSFET transistor) peak RF voltage excursion

- The regulation (stiffness) of the collector (+Vcc) or drain (+Vdd) supply voltage

- The standing (idle) current and the onset of saturation in the RF transformers. 

The limiting factor is the constancy of power gain over the entire power-output excursion. To visualise this, one can inspect the curve of output vs. drive power in the transistor data sheet. Generally, MOSFETs will exhibit superior linearity as compared to bipolar junction transistors (BJTs). MOSFETs such as the Motorola MRF150 began to displace BJTs (e.g. MRF448, 2SC2652) in the late 1990s. 

The higher the voltage excursion, the longer the linear portion of the output/drive power curve. For this reason, amplifiers powered between 40-50V DC exhibit considerably better linearity, and thus lower IMD, than 13.8V units. Typical 3rd-order IMD (IMD3) values relative to PEP are 32 dB for 50V and 24 dB for 13.8V. Additionally, it is more difficult to design a power supply  for a 13.8 V amplifier due to high current requirements (e.g. typically 80A peak for a 500W PEP amplifier like Ameritron ALS-500MX). So usually a tube amplifier is more linear than any solid-state model (typically IMD3 < -40 dB relative to PEP).

The collector-to-collector (or drain-to-drain) load impedance must be maintained as close to resistive as possible. This in turn requires optimising the wideband output transformer(s), combiner (if used), and low-pass filter passband VSWRs to as low values as possible. The load presented to the low-pass filter output must also be as close as possible to 50 ohms resistive.

The amplification class defines the functioning mode of the electronic circuit. It has been divided in several classes : A, B, AB, AB1, AB2, C, D, E, G, H. Class A is the most appreciated by fans of high fidelity audio and amateur radio because this is the mode in which the signal yield the best performances, in both musicality (for audio applications) and output. However this class has some drawbacks and therefore hams prefer using power devices in Class AB, so many classes that we are going to review.

Class A

To work in Class A, typically the signal to amplify has a working point (quiescent point) that normally lies in the middle of the active region of the load line, as displayed at left. At rest, without input signal, the cathode current flows contineously in the device (tube or transistor), consuming at lot of power, proportional to VceIc. One says that this mode offers a conducting angle of 360.

To avoid distorsions (saturation or blocking) the peak values for the output current and voltage must be always smaller than the working point voltage and current. The maximum signal power of this device is limited by the relationships : 

 

In Class A the output signal is directly proportional to the input; hence the name of linear amplifier. This mode is ideal if you are looking for high quality signals but it is paid by the consumption of much energy. Its efficiency is at best 50%. That means that more than a half of the power is lost in heat, and indeed these amplifiers heat much and request an oversized radiator to dissipate the lost power... 

Class A is the widely used amplification mode in low frequencies and in the first HF stages of radio equipments. And as you probably know if you have a Hi-Fi system, in audio it is the privileged amplification class for the purists.

Class B

Class A or Class B amplifiers are used to increase the power level of weak AM or SSB signals. But Class A amplification yield a low output due to much power dissipation. The best should be to suppress the polarization current in quiescent mode. This is achieved in Class B amplifiers that are biased at cutoff (blocking point) so that the collector current flows for 180 of the input cycle. In other words, as soon as there is an input signal, the device unlocks during half a cycle and conducts.

If we suspect that this tube or transistor will yield a better output as it does no more consume power at rest (quiescent mode), the output signal does only return one alternation of two and we will guess that this mode is not satisfying. Then, as the device will only work in the presence of an input signal, for weak inputs it will not work in the linear region of its characteristics and thus it will not be linear.

At left when a Class B amplifier works close to cut off, its transistors or tubes can only amplify one half of the signal. Therefore class B amplifiers will always consist of two transistors or tubes (push-pull circuit), where each component will supply one half wave of the output signal. At right the control characteristics of two transistors of a Class B amplifier. The non-linearity of the characteristic near zero-current produces cross-over distortions. One correct this effect with Class AB.

To amplify the same way both alternations, Class B amplifiers are usually connected in a push-pull circuit where one transistor amplifies each half of the input signal; there are thus 2 transitors or tubes, each of them amplifying either the positive or the negative alternation. To create the opposite wave for each component, a phase splitter or phase inverter task is inserted in the circuit. But here also there is a problem in the cross-over region that creates a distortion at the time of alternation is changing. One correct this effect in increasing the conducting angle so that the phase is greater than 180. This lead to the Class AB amplification.

In summary, Class B yield a good output with an efficiency of 60%. Drawbacks are the cross-over distortion, unsuited for amateur emissions and the necessity to install transistors or tubes controlled in anti-phase to get a symmetrical set.

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Classes AB, C, ...

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