op-amp values?

Thread Starter

pinkyponky

Joined Nov 28, 2019
195
Hi all,

As we know that formula for gain of non-inverting amplifier is G = 1+ (Rf/R1) and also Vout = Vin*G.

When applied Vin=4V (to non-inverting terminal), then, how to choose the resistor values in order to get the gain=1.25 and to get Vout=5V. To get the gain=1.25, the Rf value should be smaller than the R1. But, In general circuits, I have seen that Rf is higher than the R1. If this is the case, then I can't achieve what I want from the amplifier. I'm believing that Rf value should be smaller than the R1 value to achieve required gain, if I'm right, then, is that the resistor values should be Ohms or KOhms?. So, please can you help me?.
 

Ian0

Joined Aug 7, 2020
3,761
But, In general circuits, I have seen that Rf is higher than the R1.
That's because most of the circuits you have seen have a gain that is >2. You are perfectly correct that Vf should be smaller than R1.
The lower number of kΩ are generally the best for op-amps, so 3k and 12k would do nicely, as would 30k and 120k.
Don't go any higher unless you have a FET input op-amp, otherwise bias current will add errors.
Don't go much lower because the feedback resistors are a load that the op-amp has to drive.
 

Ian0

Joined Aug 7, 2020
3,761
The gain is still greater than unity, so it should remain stable, provided that a unity-gain-stable op-amp is used, and nowadays unity-gain-stable is the rule rather than the exception. They are a lot more common than they used to be.
Having said that, I wouldn’t provoke it with anything that looks even a little bit like a capacitive load.
Attenuating the input to the op-amp should be a technique of last resort, as all it does it increase the noise. Bob Pease’s article on “noise gain” is well worth a read.
 

LvW

Joined Jun 13, 2013
1,337
pinkyponky, remember the classical unity-gain amplifier (buffer) with a feedback resistor of even ZERO !
It is true that - in some rare cases - such small gains may cause stability problems (due to heavy feedback).
However, most opamps are unity-gain stable (check the data book) and you should not expext any problems with Rf<R1.
 
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