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Reply-To: "Kevin Aylward"
From: "Kevin Aylward"
References: <3D92F99F.firstname.lastname@example.org> <email@example.com> <70Vk9.19089$wH.1079@sccrnsc01>
Subject: Re: Current Feedback Amp Questions
X-Newsreader: Microsoft Outlook Express 6.00.2800.1106
Date: Fri, 27 Sep 2002 12:28:23 +0100
NNTP-Posting-Date: Fri, 27 Sep 2002 12:28:24 BST
"Linear" wrote in message
> >Thanks for the reply. This looks like it won't work for me becasue
> needs a
> >minimum gain of 10 (I need a gain of 2 max.) ...
> Don't give up on using an amplifier just because it has a minimum
> gain of 10.
> You can easily use that amplifier in a gain of 2 with the addition of
> single resistor. Place the resistor directly across the +input and
> the -input. Lets suppose that your gain of 2 is defined with a
> resistor (Rf) of 1K and a gain resistor (Rg) of 1K. The parallel
> combination is 500 Ohms. Then your resistor across the inputs (Rn)
> be 100 Ohms. This combination of three resistors satisfies the amp
> requirements of gain of 10 while giving an actual gain of 2. The
> use to calculate the value of the resistor across the inputs (Rn) is
> Rn = Rf / Minimum gain.
> Use Rg to define the actual gain in the following way:
> Actual Gain = Rf / Rg.
> This is a simple way to work around the minimum gain requirement.
> somebody can make one of those neat text schematics to illustrate this
> The disadvantage of using this technique is that the amplifier noise
> amplified by the minimum gain requirement, in this case 10, no matter
> the actual gain of the circuit is. But if amplifier noise is not
> for you then go ahead and use it.
It also effectively reduces the GBW of the op-amp as well. This means
that the closed loop bandwidth, gain accuracy, and distortion are all
poorer. Secondly, it is much more usual to use a capacitor in series
with the compensation resistor. This at least keeps the loop gain up at
dc and low frequencies.
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.
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