Help needed to design gain controlled LNA

[suling]

Hi, wonder if anybody has come across designing a gain controlled LNA with gain of -10dB to 38dB of <=1dB per step. Would appreciate your recommendation on structure and the breakdown of gain & fine tuning method.

Thank you in advance.

 

[kspalla]

Hi,
The DR is 48 dB, Currently no one is giving this much attenuation range.
Max available range is 31dB with 5 bit.
So hava a LNA with a gain of 44dB if 3dB is the IL for two 5 bity 1dB step attenuators.
the gain will be 44-3-3 i.e 38 and can go the attenuation to 38-31-31 i.e. -27dB.

You can have other combinations like 20dB 1 bit attenuator with cascaded 5bit step attenuator.

Hope this helps you

 

[sekharareddy]

What happens to the Noise figure in this case?wrt to gain variation.

 

[suling]

Yes, the noise figure will be changed. The change will be depending on how u allocate the gain attenuation.

Hi, anybody, can recommend structure to use in designing LNA with accurate gain control of the amount of dB as mentioned above?

 

[kspalla]

As i mentioned above, the structure will be 2 or 3 stage LNA ( depending on gain and operating frequency) with 2 stage 5 bit attenuator.

 

[tanuki]

Here are a few example IC's:


https://www.analog.com/en/amplifiers-and-comparators/variable-gain-amplifiers/products/index.html

 

[kspalla]

This part has beauty to operate in IF and RF frequency range, has mixer if you interest to use but has less gain from the requirement. So have additional LNA

 

[suling]

Hi, kspalla, wonder if u can help to comment on structure of LNA to be used to achieve NF < 2.5dB with the attenuator added on. Currently, had tried to implement a first stage input attenuator in front of an amplifier using resistive feedback structure. The best NF obtainable is about 3dB.

 

[kspalla]

Resistive feedback contribute more noise try reactive like inductive.

 

[suling]

Hi, the LNA is operating in VHF/UHF range. The size will be very big if to use inductive matching on chip.

 

[kspalla]

Have you tried what value of inductance (if required).
I mean to say the value lies hardly in few nH. not even tens.
Just try with inductor and see.

 

[suling]

Able to get NF ≈ 2.5dB at Max gain by reoptimising the LNA input matching. However, it's quite difficult to maintain the input return loss @ <8dB when the attenuation changed. Any comment?