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Design >> Analog Design >> Amplifier analog design - how to effectively do it when gain varies so much?
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Message started by kaylee on Sep 12^th, 2016, 12:32am

Title: Amplifier analog design - how to effectively do it when gain varies so much?
Post by kaylee on Sep 12^th, 2016, 12:32am

For example, I was thinking about building a small current sensing device that has a display to measure small current draws on some of my circuits. The design would be such that a very small resistance (~1m Ohm) is placed in-line and the voltage take across it amplified. However, I have always had trouble with amplifier circuits.

As you know, the gain constant of a transistor varies greatly even within the same batch of devices. I know that certain designs can be made such that their transfer functions depend only slightly on the gain constant, but are all gain applications implemented in this way?

Operation Amplifiers can be a bit simpler because their open-loop gain is so high - but similar to any amplifier circuit the gain is a direct function of the values of resistances (which vary largely!).

So how can amplifier circuits be used effectively in a particular application that requires the exact gain coefficient of the circuit (i.e. the current sensing design I mentioned earlier). Of course you can measure the resistance values before you implement them into your design - but what about commercial designs? The software edited based on the resistance values for every unit manufactured - at least it seems very impractical.

Title: Re: Amplifier analog design - how to effectively do it when gain varies so much?
Post by loose-electron on Sep 15^th, 2016, 7:04pm

Accurate gain control with op-amps is a ratio driven equation (R1/R2) where the absolute value is not important but rather the matching accuracy of resistors. Absolute variance is commonly 20% but device to devices on the same silicon is generally quite good, in the 0.5% to 3% region for larger geometry devices.

For commercial production, there are a large number of trim and adjustment circuits that get used that generally never get written up in academic journals.

Title: Re: Amplifier analog design - how to effectively do it when gain varies so much?
Post by DanielLam on Sep 21^st, 2016, 2:32pm

If you can, make the open loop gain high enough across all corners/variations such that you get the correct accuracy. Otherwise, you need calibration.

Title: Re: Amplifier analog design - how to effectively do it when gain varies so much?
Post by buddypoor on Oct 2^nd, 2016, 3:14am

kaylee wrote on Sep 12^th, 2016, 12:32am:

As you know, the gain constant of a transistor varies greatly even within the same batch of devices.

What do you mean with "gain constant of a transistor"?

In general, the solution to the problems you have mentioned is "negative feedback".
This applies to simple transistor gain stages as well as multi-stage amplifiers (in particular: opamps).
In this case, the gain is primarily determined by RATIOS of the parts values only - and ratios can be realized rather close to the desired values.

Title: Re: Amplifier analog design - how to effectively do it when gain varies so much?
Post by Ancel on Oct 7^th, 2016, 12:07pm

Have a look at this:
http://cds.linear.com/docs/en/application-note/an105fa.pdf