The present invention relates to improvements in variable-gain amplifiers requiring gain control varying in accordance with a well defined law, e.g., linearly, over a wide dynamic range.
Prior attempts at solving this problem have not turned out satisfactory. Thus, a gain control has been proposed including an attenuator consisting of a fixed impedance and a nonlinear element such as a diode, whose dynamic impedance varies as a function of the direct current flowing through it. Another device utilizes field-effect transistors whose gate voltage is varied. Yet another device of this type comprises a variable-resistance element of the diode type, arranged in a negative-feedback path of the variable-gain amplifier to be controlled.
However, these devices have drawbacks. In particular, the gain varies as a function of the temperature and this militates against good reproducibility. Moreover, the law governing the variation of the logarithm of the gain as a function of the control voltage is not linear and the dynamic range is relatively narrow, thereby severely limiting the usefulness of the controlled amplifier stage.