Electronics > Beginners

Programmable gain amplifier amp vs variable gain amplifier

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ricko_uk:
Hi,
what is the difference between programmable gain amplifier and variable gain amplifier? After looking at few results including Wikipedia they both seem to be the same.

Some descriptions seem to imply that the programmable one has SPI or I2C or digital IOs interface to program the gain but others descriptions include also an analog signal to control the gain...?

Thanks

David Hess:
Programmable gain amplifiers usually have discrete selectable gains.  Now people think in terms of digital control but this could also include pin strapping.  Variable gain amplifiers usually use some type of analog control signal.

alanambrose:
... So if you look at the kind of chips that are actually available - if you need the (usually baked-in) discrete gain levels available in one of the off-the-shelf chips then by-all-means use that.

If you need custom gain steps or continuous gain control then use a custom VGA design.

The 'custom gain steps case' can be as simple as an analogue switch switching the gain setting resistors e.g.

http://anagram.net/nuts/Misc/VGA.JPG

Not perfect, but fine for a lot of cases. The 'continuous gain control' case might use a DAC for gain control.

David Hess:
There are a couple of ways to make a gain control using just a DAC but in general, DACs are limited to lower frequency gain control applications.  Where higher performance is required, the DAC will control a separate gain control element like a Gilbert cell, FET, or LDR (light dependent resistor).

alanambrose:
Oh yeah, good point - I was thinking of low frequency and discrete gain control rather than, say, RF modulation.

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