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variable gain amplifier


VGA with high operating frequency and wide gain control range is one of the crucial components. From now, several VGA circuits using Bipolar, BiCMOS, and CMOS devices were introduced. However, recently a CMOS device is preferably chosen because of its low cost. A 25MHz 20dB variable gain amplifier in 1.2mm CMOS was introduced . This architecture allows the gain to be varied over a wide range –more than 20dB– while bandwidth is almost independent of gain. But for large input signal over several 100mV, a large distortion is expected. The reason is that a large input signal force input transistors to come into the linear or cut-off region. And an input stage composed of PMOS transistors results in slower operation than that of NMOS. Furthermore, many transistors connected to the output nodes make the VGA slow too. A 2mA/3V 71MHz IF Amplifier in 0.4mm CMOS programmable over 80dB range was introduced . In this circuit, in order to enhance operating frequency, an input and a gain control stages are composed of only NMOS transistors, and cascode structures are used to minimize the Miller effect. However, many NMOS transistors at the output node for gain control and PMOS transistors for bias make circuit slow. What is more, for large input signals, it also suffers from distortion. Because large input signals over several 100mV can force the input transistors to come into the cutoff or linear region. Recently, BiCMOS VGA that has high speed/wide dynamic range was introduced . This circuit showed high operating frequency by using bipolar transistors as a gain stage and showed wide dynamic range by using emitter degeneration scheme for large input signal. However, this scheme also has two disadvantages. One is that it is more expensive than CMOS versions, and another is that this input structure has a limitation of operating frequency at low gain state. Because under the low gain and high frequency operating state, some part of emitter currents of an input stage leak to the ground node through parasitic capacitances between the emitter and ground. Therefore, it is very difficult to realize low distortion and large attenuation characteristics at high frequencies with these types of implementation.





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