ASSINGMENT 7
Amplifier Classes of Operation and Biasing Networks
INTRODUCTION .
The main characteristics of an amplifier are Linearity, efficiency, output power, and
Signal gain. In general, there is a trade off between these characteristics. For example,
Improving amplifier’s linearity will degrade its efficiency. Therefore knowing the importance
degree of each one of these characteristics is an essential step in designing an Amplifier. This
Can be jugged based on the application. As an example high output power Amplifier is used
in the transmitter side of a transceiver, whereas high linear amplifier used in the receiver side.
An amplifier is said to be linear if it preserves the details of the signal waveform, that
is to say,
INTRODUCTION .
The main characteristics of an amplifier are Linearity, efficiency, output power, and
Signal gain. In general, there is a trade off between these characteristics. For example,
Improving amplifier’s linearity will degrade its efficiency. Therefore knowing the importance
degree of each one of these characteristics is an essential step in designing an Amplifier. This
Can be jugged based on the application. As an example high output power Amplifier is used
in the transmitter side of a transceiver, whereas high linear amplifier used in the receiver side.
An amplifier is said to be linear if it preserves the details of the signal waveform, that
is to say,
where,
Vi and Vo are the input and output signals respectively.
and A is a constant gain representing the amplifier gain. But if the relationship between Vi and Vo contains the higher power of Vi, then the amplifier produces nonlinear distortion.
The amplifier’s efficiency is a measure of its ability to convert the dc power of the
Supply into the signal power delivered to the load. The definition of the efficiency can be
represented in an equation form as
For an ideal amplifier, the efficiency is one. Thus, the power delivered to the load is equal to
the power taken from the DC supply. In this case, no power would be consumed in the
amplifier. In reality, this is not possible, especially in high frequency realm of RF circuits. In
many high frequency systems, the output stage and driver stage of an amplifier consumed
Power in the amplification process.
The gain of the amplifier (G) is equal to the magnitude of the output signal (Xo) over
the magnitude of the input signal (Xi) as shown in the equation.
Amplifier Classification
Amplifiers are classified according to their circuit configurations and methods of
operation into different classes such as A, B, C, and F. These classes range from entirely
linear with low efficiency to entirely non-linear with high efficiency. The analysis presented
in this chapter assumes piecewise-linear operation of the active device.
The active device used in this research is the field effect transistor. The reason for
choosing this type of transistor is its superior performance in the microwave range
The characteristics of the FET can be described by:
Class A Amplifiers
Class A amplifiers operate over a relatively small portion of a tube’s plate-current or a transistor’s collector-current range and have continuous plate- or collector-current flow throughout each RF cycle. Their efficiency in converting DC-source-power to RF-output-power is poor. DC source power that is not converted to radio frequency output power is dissipated as heat. However, in compensation, Class A amplifiers have greater input-to-output waveform linearity (lower output-signal distortion) than any other amplifier class. They are most commonly used in small-signal applications where linearity is more important than power efficiency, but also are sometimes used in large-signal applications where the need for extraordinarily high linearity outweighs cost and heat disadvantages associated with poor power efficiency.
Class B Amplifiers
Class B amplifiers have their tube control-grids or transistor bases biased near plate- or collector-current cutoff, causing plate- or collector-current to flow only during approximately 180 degrees of each RF cycle. That causes the DC-source-power to RF-output-power efficiency to be much higher than with Class A amplifiers, but at the cost of severe output cycle waveform distortion. That waveform distortion is greatly reduced in practical designs by using relatively high-Q resonant output “tank” circuits to reconstruct full RF cycles.
The effect is the same in principle as pushing a child in a swing through half-swing-cycles and letting the natural oscillatory characteristics of the swing move the child through the other half-cycles. However, low sine-wave distortion results in either case only if the Q of the oscillatory circuit (the tank circuit or the swing) is sufficiently high. Unless the Q is infinite, which it never can be, the amplitude of one-half cycle will be larger than the other, which is another way of saying there always will be some amount of harmonic energy. (Coupling an antenna system too tightly to the resonant output tank circuit of an amplifier will lower its Q, increasing the percentage of harmonic content in the output.)
Another effective method commonly used to greatly reduce Class B RF amplifier output waveform distortion (harmonic content) is to employ two amplifiers operating in “push-pull” such that one conducts on half-cycles where the other is in plate- or collector-current cutoff. Oscillatory tank circuits are still used in the outputs of Class B push-pull amplifiers to smooth switching transitions from the conduction of one amplifier to the other, and to correct other nonlinearities, but lower-Q tank circuits can be used for given percentages of harmonic content in the output. (Tank circuits can be loaded more-heavily for given percentages of harmonic output where two amplifiers operate in push-pull.)
The effect is the same in principle as pushing a child in a swing through half-swing-cycles and letting the natural oscillatory characteristics of the swing move the child through the other half-cycles. However, low sine-wave distortion results in either case only if the Q of the oscillatory circuit (the tank circuit or the swing) is sufficiently high. Unless the Q is infinite, which it never can be, the amplitude of one-half cycle will be larger than the other, which is another way of saying there always will be some amount of harmonic energy. (Coupling an antenna system too tightly to the resonant output tank circuit of an amplifier will lower its Q, increasing the percentage of harmonic content in the output.)
Another effective method commonly used to greatly reduce Class B RF amplifier output waveform distortion (harmonic content) is to employ two amplifiers operating in “push-pull” such that one conducts on half-cycles where the other is in plate- or collector-current cutoff. Oscillatory tank circuits are still used in the outputs of Class B push-pull amplifiers to smooth switching transitions from the conduction of one amplifier to the other, and to correct other nonlinearities, but lower-Q tank circuits can be used for given percentages of harmonic content in the output. (Tank circuits can be loaded more-heavily for given percentages of harmonic output where two amplifiers operate in push-pull.)
Class AB Amplifiers
As the designation suggests, Class AB amplifiers are compromises between Class A and Class B operation. They are biased so plate- or collector-current flows less than 360 degrees, but more than 180 degrees, of each RF cycle. Any bias-point between those limits can be used, which provides a continuous selection-range extending from low-distortion, low-efficiency on one end to higher-distortion, higher-efficiency on the other.
Class AB amplifiers are widely used in SSB linear amplifier applications where low-distortion and high power-efficiency tend to both be very important. Push-pull Class AB amplifiers are especially attractive in SSB linear amplifier applications, because the greater linearity resulting from having one amplifier or the other always conducting makes it possible to bias push-pull Class AB amplifiers closer to the Class B end of the AB scale where the power-efficiency is higher. Alternatively, push-pull Class AB amplifiers can be biased far enough toward the highly-linear Class A end of the scale to make broadband operation without resonant tank circuits possible in applications where broadband operation or freedom from tuning is more important than power-efficiency.
Class C Amplifiers
Class AB amplifiers are widely used in SSB linear amplifier applications where low-distortion and high power-efficiency tend to both be very important. Push-pull Class AB amplifiers are especially attractive in SSB linear amplifier applications, because the greater linearity resulting from having one amplifier or the other always conducting makes it possible to bias push-pull Class AB amplifiers closer to the Class B end of the AB scale where the power-efficiency is higher. Alternatively, push-pull Class AB amplifiers can be biased far enough toward the highly-linear Class A end of the scale to make broadband operation without resonant tank circuits possible in applications where broadband operation or freedom from tuning is more important than power-efficiency.
Class C Amplifiers
Class C amplifiers are biased well beyond cutoff, so that plate- or collector-current flows less than 180 degrees of each RF cycle. That provides even higher power-efficiency than Class B operation, but with the penalty of even higher input-to-output nonlinearity, making use of relatively high-Q resonant output tank circuits to restore complete RF sine-wave cycles essential. High amplifying-nonlinearity makes them unsuitable to amplify AM, DSB, or SSB signals.
However, most Class C amplifiers can be amplitude-modulated with acceptably low distortion by varying plate- or collector-voltage, because they generally are operated in the region of plate- or collector-saturation so that the RF output voltage is very closely dependent upon instantaneous DC plate- or collector-voltage. They also are commonly used in CW and frequency-shift-keyed radiotelegraph applications and in phase- and frequency-modulated transmitter applications where signal amplitudes remain constant.
However, most Class C amplifiers can be amplitude-modulated with acceptably low distortion by varying plate- or collector-voltage, because they generally are operated in the region of plate- or collector-saturation so that the RF output voltage is very closely dependent upon instantaneous DC plate- or collector-voltage. They also are commonly used in CW and frequency-shift-keyed radiotelegraph applications and in phase- and frequency-modulated transmitter applications where signal amplitudes remain constant.
Class-A Benefits
Since Class-A amps are inefficient, generate lots of heat, and require a far more complex power supply than conventional Class-AB amplifiers, there have to be some compelling reasons to use this arrangement. The first is circuit simplicity. In the light of the above discussion, the circuit is not simple, but for the audio signal it can be far less complex than for a conventional power amp.
The benefit of this is that the signal is subjected to comparatively little amplification, resulting in an open loop (i.e. without feedback) gain which is generally fairly low - probably less than 250 (48dB), and possibly as low as 50 or so (34dB). This means that very little overall feedback is used, so stability and phase should be excellent over the audio frequencies. A well designed Class-A amplifier should not require any frequency compensation (or very little), so the open loop gain will remain reasonably constant over the audio range. This results in superior transient response, and dramatically reduced "Transient Intermediation Distortion" (or TID, aka Dynamic Intermodulation Distortion), which is thought by many designers to be caused by phase and time delays between the input and feedback signals. It may be possible that this is the cause, although the existence of TID is virtually zero in any competently designed amp.
The simple fact is that the more amplifying devices that are introduced into the chain, the more phase shift must be introduced. No amplifying device is capable of responding instantaneously to a change of input - all have some inherent delay (which usually includes different turn-on and turn-off times). With fewer devices in the audio circuit, there must be less delay between a change in the input causing a change in the output. The simplified topology used for most Class-A amps can also be used with Class-AB - often with very good results indeed.
conclusion
Class-A Output device(s) conduct through 360 degrees of input cycle (never switch off) - A single output device is possible. The device conducts for the entire waveform in Figure 1
Class-B Output devices conduct for 180 degrees (1/2 of input cycle) - for audio, two output devices in "push-pull" must be used (see Class-AB)
Class-AB Halfway (or partway) between the above two examples (181 to 200 degrees typical) - also requires push-pull operation for audio. The conduction for each output device is shown in Figure 1.
Class-C Output device(s) conduct for less than 180 degrees (100 to 150 degrees typical) - Radio Frequencies only - cannot be used for audio! This is the sound heard when one of the output devices goes open circuit in an audio amp! See Figure 1, showing the time the output device conducts (single-ended operation is assumed, and yes this does work for RF).When I first wrote this article, I had completely forgotten about the Quad "Current-Dumping" amp, which uses a low power "good" amplifier, with a push-pull Class-C type amp to supply the high currents needed for high power. Although these enjoyed a brief popularity, they seem to have faded away. I was reminded of their existence by an article by Douglas Self ("Class Distinction", in the March 1999 issue of Electronics World ), in which he quite rightly points out that the current-dumper is (at least in part) Class-C.
Class-D Quasi-digital amplification. Uses pulse-width-modulation of a high frequency (square wave) carrier to reproduce the audio signal - although my original comments were valid when this was written, there have been some very significant advances since then. There are some very good sounding Class-D amplifiers being made now, and they are worthy of an article of their own.
Since Class-A amps are inefficient, generate lots of heat, and require a far more complex power supply than conventional Class-AB amplifiers, there have to be some compelling reasons to use this arrangement. The first is circuit simplicity. In the light of the above discussion, the circuit is not simple, but for the audio signal it can be far less complex than for a conventional power amp.
The benefit of this is that the signal is subjected to comparatively little amplification, resulting in an open loop (i.e. without feedback) gain which is generally fairly low - probably less than 250 (48dB), and possibly as low as 50 or so (34dB). This means that very little overall feedback is used, so stability and phase should be excellent over the audio frequencies. A well designed Class-A amplifier should not require any frequency compensation (or very little), so the open loop gain will remain reasonably constant over the audio range. This results in superior transient response, and dramatically reduced "Transient Intermediation Distortion" (or TID, aka Dynamic Intermodulation Distortion), which is thought by many designers to be caused by phase and time delays between the input and feedback signals. It may be possible that this is the cause, although the existence of TID is virtually zero in any competently designed amp.
The simple fact is that the more amplifying devices that are introduced into the chain, the more phase shift must be introduced. No amplifying device is capable of responding instantaneously to a change of input - all have some inherent delay (which usually includes different turn-on and turn-off times). With fewer devices in the audio circuit, there must be less delay between a change in the input causing a change in the output. The simplified topology used for most Class-A amps can also be used with Class-AB - often with very good results indeed.
conclusion
Class-A Output device(s) conduct through 360 degrees of input cycle (never switch off) - A single output device is possible. The device conducts for the entire waveform in Figure 1
Class-B Output devices conduct for 180 degrees (1/2 of input cycle) - for audio, two output devices in "push-pull" must be used (see Class-AB)
Class-AB Halfway (or partway) between the above two examples (181 to 200 degrees typical) - also requires push-pull operation for audio. The conduction for each output device is shown in Figure 1.
Class-C Output device(s) conduct for less than 180 degrees (100 to 150 degrees typical) - Radio Frequencies only - cannot be used for audio! This is the sound heard when one of the output devices goes open circuit in an audio amp! See Figure 1, showing the time the output device conducts (single-ended operation is assumed, and yes this does work for RF).When I first wrote this article, I had completely forgotten about the Quad "Current-Dumping" amp, which uses a low power "good" amplifier, with a push-pull Class-C type amp to supply the high currents needed for high power. Although these enjoyed a brief popularity, they seem to have faded away. I was reminded of their existence by an article by Douglas Self ("Class Distinction", in the March 1999 issue of Electronics World ), in which he quite rightly points out that the current-dumper is (at least in part) Class-C.
Class-D Quasi-digital amplification. Uses pulse-width-modulation of a high frequency (square wave) carrier to reproduce the audio signal - although my original comments were valid when this was written, there have been some very significant advances since then. There are some very good sounding Class-D amplifiers being made now, and they are worthy of an article of their own.
Reference
http://sound.westhost.com/class-a.htm
http://sound.westhost.com/class-a.htm
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