Programmable gain amplifier amp vs variable gain amplifier

[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.

[ricko_uk]

Thank you David,
could you please explain the Gilber cell and FET approach? Or suggest where I could find examples? I googled it but nothing came up that seemed like what you mentioned.
Thank you

[unitedatoms]

No difference. Except if precision resistors are included (in pin selectable chips), then thermal drifts are better matched and datasheet has specs for drifts, and price wise, precision resistors can have significant cost.

[David Hess]

could you please explain the Gilber cell and FET approach? Or suggest where I could find examples? I googled it but nothing came up that seemed like what you mentioned.

There should be lots of examples of the Gilbert cell online.  Tektronix called that configuration the paraphase amplifier:

https://en.wikipedia.org/wiki/Gilbert_cell

Examples using FETs are not as common.  The FET is used as a variable resistor but the trick is using a pair of matched FETs so the resistance of the first FET is measured to control the gate voltage and then the same gate voltage is applied to the second FET.

[ricko_uk]

Thank you all!!

[TimFox]

There are also “multiplying DACs”, such as the ADI AD55xx series, where an external signal input replaces the DC reference in a normal DAC.  This allows a digital word to determine the gain from that input, typically through an op amp current to voltage amplifier.  With an appropriate choice of DAC and supply voltages, this works with an AC analog input signal.