[sdiy] Voltage amplification via op amps

[Magnus Danielson]

From: Peter Grenader <petergrenader at mksound.com>
Subject: [sdiy] Voltage amplification via op amps
Date: Sun, 19 Jan 2003 03:33:45 -0700

> 
> Kids,
> 
> anyone have a quick and dirty circuit for (almost) doubling a voltage
> control input which also allows for some sort of trim pot adjustment?  I
> assume an op amp with 2 to 1, but I was hoping someone has worked this out
> already so I could...ah...'borrow' it?
> 
> Let me be a little clearer:
> 
> I've got a VC input, works great. Now I want to amplify it's range, but will
> need an adjustment (trim pot) to assure the adjusted range is exactly what I
> need (you see, 'doubled' above was a bit of a generalization).
> 
> I can't change the range on the input voltage, that needs to stay where it
> is - I need to increase it after it arrives into the circuit it's
> controlling.  I also undersatand that I'm dealing with the limitation of the
> supply voltages to the op amp itself, but I think the max level of the input
> vc will allow me this headroom without problems.
> 
> i hope.

Well, the traditional op-amp solution for non-inverting gain is this:

Toss the input signal onto the positive input of an op-amp.
Toss a resistor (R1) from the output to the negative input of the same op-amp.
Then toss another resistor (R2) from the negative input down to ground.

The resistors effectively acts like a voltage divider, giving the negative
input the R2/(R1+R2) part of the output voltage. This negative feedback creates
a servo-loop such that the op-amp attempts to drive its output so that the
negative input equals (well, allmost) the positive input. This makes the gain
(R1+R2)/R2. To get the gain of 2 you just toss in the same resistors (say 10k).

You now have two options to make this gain trimable. Either you insert a
trimmer into the divider chain, or you create some excess gain and then have a
trimmer on the input signal.

If you insert a trimmer into the divider, you would remove half the trimmer
resistance from each of the resistors, so that they effectively meet in the
middle of the trimmer. The trimmer tap is then used to feed the op-amp.

Depending on the op-amp and trimmer, the excess inductance of the trimmer could
be an issue, so unstability would resolve (you don't want inductance in the
feedback-loop of an op-amp). This may or may not be an issue. A small cap
between the op-amp negative input and output may however solve the issue if it
does occur.

The other way is then to put the trimmer on the input signal. What I recommend
you to do is to let the trimmer have a fixed resistor to ground, so that of the
full resistance from signal input to ground, the trimmer only reaches the upper
part, say one tenth or something. By selecting the gain of the op-amp to be
slightly above 2 such that the middle setting of the trimmer is nearly exact to
form the total gain of 2 you can allow yourself to trim the gain around 2.

By setting up the op-amp to have a gain of 2.2 (R1 = 100k and R2 = 110k) and
by letting the trimmer mid-point equal 2.0 (meaning that the full resistance of
the trimmer and additional resistor is 0.2+2.0 = 2.2 where 0.4 is in the
trimmer and 1.8 is in the resistor, we find that a 10k trimmer and a 72k
resistor ougth to do the trick.

Did that help you?

Cheers,
Magnus