How to implement linear-in-dB gain in CMOS?

[jordan76]

Hi

I am wondering how to implement linear-in-dB gain IC in CMOS?
Some claim they do so by a resistor ladder with adjustable transconductance,
could anyone explain more detail about the architecture/schematic to achieve it?

regards,
jordan76

 

[ArchAngel]

I guess what u do is a VGA(variable gain amplifier)
there are lots VGA materials
maybe u should use a few bipolar transistors for their exponential characteristics.

 

[sunking]

a lot of Res in serial and many switch to choose them.
the ratio of the Res is in db

 

[strabush]

You can use a circuit as the one attached, but using differential pair transistors instead of the switches and by controlling the differential voltage of each pair you switch continuously which differential pair passes the signal to the output amplifier.

 

[jordan76]

Thanks for your reply,guys!

True,there are many VGA materials,but most of them are either bipolar circuit or linear instead of logarithmic gain control. In my case, it should be CMOS circuit and THD+N should also very low since it is for audio application.

One of my friends told me that it can be realized using Gilbert Gain Cell, I am wondering how the logarithmic gain control is achieved in CMOS which means diode/bipolar transistor is not available for me. Any hints about that?

Thanks in advance!

regards,
jordan76

 

[strabush]

The Analog Devices AD600 line of db-linear controlled amplifiers are implemented exactly as I specified in my previous post. There is no problem at all to implement it with CMOS.

 

[xwcwc1234]

In terms of IC design , is there any digital process compatible and less chip area PGA available ?