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This paper presents a methodology to design a wideband radio frequency variable gain amplifier (RF-VGA) in a low-cost SiGe BiCMOS 0.35 μm process. The circuit uses two Class A amplifiers based on second-generation controlled current conveyors (CCCII). The main feature of this circuit is the wideband input match along with a reduced NF (5.5–9.6 dB)...
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... Slightly better properties than traditional transconductance OTA amplifiers, such as lower power consumption and higher bandwidth, are offered by current conveyors (CC). An example is [11], which uses two current conveyors, with the resulting gain given by the ratio of their quiescent currents. ...
The following paper is devoted to an amplifier with gain control (Variable Gain Amplifier). The role of amplification and gain control is addressed separately from the very beginning, to achieve the greatest possible independence of the required parameters both for the amplifier itself (gain size, bandwidth, noise figure) and for the gain controller (energy consumption control, distortion). The core of the amplifier consists of an operational amplifier with voltage feedback (VFB) in an inverting circuit, to the feedback of which a gain regulator executed with a diode bridge is connected in parallel. The analysis of the simplified circuit model shows that it is possible to obtain a variable-gain amplifier that maintains its bandwidth. The control of the gain is continuous and only one element, the variable resistance, is sufficient for its control. This resistance can be achieved by a number of components, either by a rheostat, by the resistance of the FET transistor channel (the FET control is related to GND, which is advantageous), or by the resulting dynamic resistance of the diode bridge. Continuous control with one element preserving the bandwidth of an operational amplifier with VFB is unique and the authors are unaware of any other similar solution. If maximum gain is required, the diode bridge is disconnected and does not contribute to the resulting noise of the VGA amplifier. Because the diode bridge is connected between the output of the operational amplifier and its inverting input, and because the inverting input is current controlled, the diode bridge processes only a portion of the output voltage of the amplifier, thus preventing more noticeable distortion. The distortion is further suppressed due to the simultaneous action of four diodes in the bridge, whose characteristics are reciprocal for the signal path. The presented construction is relatively simple and can be a topic not only for other microelectronic structures, but also for applications from discrete components. The achieved parameters of the presented connection are as follows: gain change 0 to 20 dB, constant bandwidth 7 MHz, distance III. harmonics min. 30 dB at an output voltage of 0.5 V, maximum power spectral density at the output 25 ∙10-15 V2Hz-1.
