To be able to process low-power, received radio signals, for example broadcast radio or television signals, the signals are amplified by a low-noise input amplifier in order to obtain a good signal-to-noise ratio. The low-noise input amplifier (low noise amplifier) is additionally of very wideband design in order to be able to detect all of the signals which are in the useful range. After that, the amplified signal, which is at an RF frequency, is converted to an intermediate frequency so as to be processed further at this frequency. Following the frequency conversion, the actual more narrowband useful signal is selected by filters and in this context is attenuated again. The attenuation brought about through frequency conversion and through filtering is compensated for by a second and possibly a third amplifier.
Amplification devices of this type, which are used primarily in television systems, therefore require both a low noise factor and a large signal strength in order to reduce unwanted intermodulation products. To achieve both inherently conflicting characteristics, it is necessary to use external amplifier circuits, which have a high space requirement and cost.
To reduce the space requirement and the cost, it has been proposed that digitally operating control amplifiers (programmable gain amplifier PGA) be used as the input amplifier, this amplifier controlling its gain in adjustable stages and also advantageously having a very low noise factor. As a result, despite a very low input level, a good signal-to-noise ratio is ensured. However, amplifiers of this type have the drawback that when the gain setting changes, as a result of static level fluctuations, an abrupt level change appears on the output signal. This abrupt level change is corrected by the second amplifier stage only slowly on account of the time constant. In the case of digitally modulated signals, this can result in additional bit errors on account of incorrect amplitude values. To prevent the second amplifier from being readjusted, therefore, the output signal from the digital first control amplifier is masked out during the digital first control amplifier's switching operation. This means that continuous transmission is not possible however, or gaps appear in the data stream. For these reasons, amplifier systems comprising digital and analog control amplifiers have not been used to date.