design a variable gain amplifier

[flamingo]

Hi, all:
Need to design a variable gain amplifier, gain range larger than 45dB, used in DTV receiver intermediate frequency amplifier(~40MHz), and the absoluate gain is between 15 - 60dB. Anyone has the experiences on this? what about the configuration, and the key point?
Thank you!

 

[pk3316]

For VGA, normally the structures are switched resistance VGA on load or source resister chain, switched bias current VGA, such as gilbert cell based VGA. Also there are some cross couple load structure VGA which changes the load by adjust the cross coupled transistor pair. The key point may be how to achieve a large gain range or meet the so called dB linear type VGA.

 

[flamingo]

For VGA, normally the structures are switched resistance VGA on load or source resister chain, switched bias current VGA, such as gilbert cell based VGA. Also there are some cross couple load structure VGA which changes the load by adjust the cross coupled transistor pair. The key point may be how to achieve a large gain range or meet the so called dB linear type VGA.

Actually, I have designed a VGA based on CMOS Gilbert cell but the gain range is only about 30~40dB. Due to the complexity of the circuits (some modifications make the Gilbert so huge, about 20 transistors), the noise floor is too high, so I need to alter my design. On the other hand, switching resistors network need digitial control, in my design I need to realize a continous gain control.
Another problem I encountered is how to realize high gain as about 60dB.