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Help needed to design 40dB gain control in CMOS LNA

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suling

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Hi, had designed a CMOS LNA operating from 100MHz to 1 GHz using resistive feedback with noise cancelling technique. Required a gain control of 22dB for 1dB/step in this LNA and a 20dB for 1 dB/step attenuator before the mixer (ie, output of LNA). The loading for this LNA after the attenuator block is 50kΩ parallel to 0.2pF (equivalent mixer loading). Can only implement 8dB of gain control in the resistive feedback so as to keep the s11 min at 7dB. The other 14dB of gain control was implemented together with the attenuator block (total 34dB attenuation). Would appreciate if anyone can advise/help to improve the linearity and frequency response.

Thank you in advance.
 

Hi!

Active inductors can be used to extend frequency response of wideband LNA's without using any inductor, but just at the output of the LNA because active inductors are noisy and that increases NF. By other hand active inductors has poor linearity. It depends on your specifications. What is the technology that you are using to design the wideband LNA?

Best regards
 

The technology that I am using is 65nm CMOS. The NF and linearity requirement is very tight. At high gain, the NF must be <2.2dB and IIP3 should be -2dBm (with 2 tone input set to -25dBm) typically.
 

-For wideband LNA, you should absolutely use CasCode configuration at those frequencies
-Gain control can be implemeted by adding switched resistor based attenuator.So you can select the right resistor values by switching CMOS FETs digitally.
-Linearity can be found the right OP of the amplifier.Optimization is necessary.
 

There are many AGC amplifiers covering up to 1 GHz, with ~40 dB down gain control. Or, you can use voltage-controlled attenuators, or digital step attenuators.
Check Mini-Circuits, Hittite, RF MicroDevices, Skylinks and other sources. I have used Mini-Circuits digital attenuators, RFMD voltage-controlled attenuators and AGC amplifiers, without problems.
Or, you can find PIN-diode attenuators used in TV receivers; Philips and others make them as Pi-networks; they can be driven by a control voltage from ~ 4 to 40 dB over VHF/UHF range, possibly over 1 GHz.
 

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