A SIMPLE AMPLIFIER
The block diagram of a simple amplifier is shown in Figure 1. The active electronics i.e. the section which actually amplifies the signal is clubbed together in the block diagram, as the amplifier module.
INPUT CONTROL
Since some means of varying the output signal level is required, a gain control is a must. The gain control is shown as a variable resistance connected across the input signal.
For lower output levels, the gain control is lower i.e. a smaller signal is fed into the amplifier module. Unfortunately, the setting of the external gain control does not reduce the noise generated inside the amplifier module. Hence small input signals will be adversely affected by the internal noise from the amplifier module. As a result, the Carrier to Noise ratio (C/N) deteriorates rapidly for low gain control settings, if the topology of Figure 1 is used. All practical CATV Amplifiers need some extent of slope equalization. This is shown as a box in Figure 1 and will add a further attenuation at the input, for lower frequencies. As a result, the slope control at the input further degrades the C/N performance at lower frequencies.
OUTPUT CONTROL
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block diagram Where the output signals are varied by the use of an attenuator across the output of the CATV Amplifier. This layout has the
disadvantage that the amplifier module is constantly providing the highest output. Only apart of this output signalis utilised, as required. Readers who have read our earlier articles will recall that distortion increases very rapidly with output levels. A 5 dB increase in output level will increase the output distortion by 10 dB !
The slope control will have to be added across the output, as shown in Figure 2. This will present an increased attenuation or burden on the amplifier at lower frequencies, which will result in increased distortion at lower frequencies. If the ideal amplifier characteristics of zero distortion are to be achieved, the output signal level must be maintained to the lowest level actually required. Clearly, a line extender amplifier if based on Figure 1, will produce an increased level ofnoise (poor noise figure). Alternately, if Figure 2 is implemented, the distortion will suffer.
MULTI STAGE AMPLIFIERS
A much better performance level would there fore be achieved if the amplifier gain was split into 2 or more stages and the signal level attenuated partially over the first and second stages so that the input stage does not see too low a signal level. At the same time the output stage is not forced to operate at excessive output levels. Figure 3 shows a block diagram of a practical trunk amplifier (Forward Path Only). The total amplifier gain of approximately 32 dB is divided over 2 sections.
PRE AMPLIFIER
As the name suggests, the Pre Amplifier is the first amplifying stage. The input signal after passing through the forward / reverse path diplexer passes through an external plug-in attenuator. In some products, this plug-in input attenuator can either be a fixed attenuator or a Thermal Equaliser. The pre amplifier receives relatively low level signals and is therefore designed to operate with low noise, at relatively modest output levels. Since output levels are not large, the pre amplifier section does not generate significant distortion.
THERMAL EQUALISER
The thermal equaliser is basically an attenuator whose attenuation changes with the temperature. Since cable attenuation increases with temperature, the thermal attenuator is designed so that that its impedance decreases with temperature. As a result, the thermal attenuator compensates for cable attenuation with temperature and presents an approximately constant input signal, despite changes in ambient temperatures. If high gain is required, without Thermal Compensation, this stage can be simply bypassed, with a shorting link.
FIXED EQUALISER
The optional attenuator / thermal equaliser is followed by a fixed equaliser. This partially compensates for the cable slope. Hence the signal to the pre amplifier is equalised in two stages viz. an optional, external attenuator or thermal equaliser and subsequently a fixed equaliser. It is important to note that only partial cable equalisation is performed prior to the pre amplifier. This ensures that low frequency signals are not overly attenuated when a large equalisation slope is
required. As a result, the pre amplifier maintains a superior low frequency C/N ratio.
The block diagram of a simple amplifier is shown in Figure 1. The active electronics i.e. the section which actually amplifies the signal is clubbed together in the block diagram, as the amplifier module.
INPUT CONTROL
Since some means of varying the output signal level is required, a gain control is a must. The gain control is shown as a variable resistance connected across the input signal.
For lower output levels, the gain control is lower i.e. a smaller signal is fed into the amplifier module. Unfortunately, the setting of the external gain control does not reduce the noise generated inside the amplifier module. Hence small input signals will be adversely affected by the internal noise from the amplifier module. As a result, the Carrier to Noise ratio (C/N) deteriorates rapidly for low gain control settings, if the topology of Figure 1 is used. All practical CATV Amplifiers need some extent of slope equalization. This is shown as a box in Figure 1 and will add a further attenuation at the input, for lower frequencies. As a result, the slope control at the input further degrades the C/N performance at lower frequencies.
OUTPUT CONTROL
block diagram Where the output signals are varied by the use of an attenuator across the output of the CATV Amplifier. This layout has thedisadvantage that the amplifier module is constantly providing the highest output. Only apart of this output signalis utilised, as required. Readers who have read our earlier articles will recall that distortion increases very rapidly with output levels. A 5 dB increase in output level will increase the output distortion by 10 dB !
The slope control will have to be added across the output, as shown in Figure 2. This will present an increased attenuation or burden on the amplifier at lower frequencies, which will result in increased distortion at lower frequencies. If the ideal amplifier characteristics of zero distortion are to be achieved, the output signal level must be maintained to the lowest level actually required. Clearly, a line extender amplifier if based on Figure 1, will produce an increased level ofnoise (poor noise figure). Alternately, if Figure 2 is implemented, the distortion will suffer.
MULTI STAGE AMPLIFIERS
A much better performance level would there fore be achieved if the amplifier gain was split into 2 or more stages and the signal level attenuated partially over the first and second stages so that the input stage does not see too low a signal level. At the same time the output stage is not forced to operate at excessive output levels. Figure 3 shows a block diagram of a practical trunk amplifier (Forward Path Only). The total amplifier gain of approximately 32 dB is divided over 2 sections.
PRE AMPLIFIER
As the name suggests, the Pre Amplifier is the first amplifying stage. The input signal after passing through the forward / reverse path diplexer passes through an external plug-in attenuator. In some products, this plug-in input attenuator can either be a fixed attenuator or a Thermal Equaliser. The pre amplifier receives relatively low level signals and is therefore designed to operate with low noise, at relatively modest output levels. Since output levels are not large, the pre amplifier section does not generate significant distortion.
THERMAL EQUALISER
The thermal equaliser is basically an attenuator whose attenuation changes with the temperature. Since cable attenuation increases with temperature, the thermal attenuator is designed so that that its impedance decreases with temperature. As a result, the thermal attenuator compensates for cable attenuation with temperature and presents an approximately constant input signal, despite changes in ambient temperatures. If high gain is required, without Thermal Compensation, this stage can be simply bypassed, with a shorting link.
FIXED EQUALISER
The optional attenuator / thermal equaliser is followed by a fixed equaliser. This partially compensates for the cable slope. Hence the signal to the pre amplifier is equalised in two stages viz. an optional, external attenuator or thermal equaliser and subsequently a fixed equaliser. It is important to note that only partial cable equalisation is performed prior to the pre amplifier. This ensures that low frequency signals are not overly attenuated when a large equalisation slope is
required. As a result, the pre amplifier maintains a superior low frequency C/N ratio.
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