very basic about opamp

[garg29]

Hi,

Just a basic question. As in the figure below if i want to change the gain without changing the feedback resistor, R1, can i change the gain by changing value of R2, resistor. Will that be fine. as the gain for this circuit is:-

Gain=1+R1/R2

if R1 is constant = 100K

Gain = 1 + 100/R2

If this way is correct, how can i caluculate the limiting values of R2 resistor?

Thanks in advance for any help

 

[vinodjn]

current requirment is increased as you decreased the R2.

 

[Sudhir.K.A]

Hi garg29,

I think it is alright to change the gain by changing any of the two resistor.
But as u can see the total gain doesnot vary linearly with R2 resistor.

Coming to the limitations of R2:

Making R2 infinite is ok . (It is nothing but buffer with resistance in feed back path)

Making R2 zero. Doesnot moke the output gain of the opamp as infinite. Total gain will be limited by Opamp open loop gain.


regards,
Sudhir

 

[garg29]

actually Mr. sudhir as u replied to my previous question (Programable Gain Amplifier) and i told after applying -5 to Vee cd4051 started working, till yesterday it was working but today i checked it, it isn't working, i tried changing the ic and other components but it isn't working out. Can You please send me your mobile number i'm also from India.

 

[Kral]

garg29,
Sudhir.K.A is correct. There is another practical problems in achieving high closed loop gains by making R1/R2 large: The output error due to the input offset voltage of the op amp is equal to Vos (1+(R1/R2)), Where Vos is the input offset voltage spec.
Regards,
Kral

 

[Sudhir.K.A]

Hi garg29,

You can contact me on my gmail id k.a.sudhir@gmail.com or send me an invite to google talk, so that we can chat when you want


Now verify the following :
All voltage measurements be madewith respect to Vss of Analog Mux.

1. uC ground =0V
2. uC A,B,C control inputs should be +5V
3. Vdd of the of CD4051 = +5V
4. Vee of CD4051 should be -5V
5. as polymath also pointed out, Opamp Vdd be = +5V
6.Opamp ground = -5V
7.Input voltage |Vin| < 5 V
8.There is inhibit pin for CD4051 is connected to ground.

If all these voltages are under limit, i dont see why there should be any problem.

As other members have suggested , you can also consider using programmable gain amplifiers from Maxim Semiconductors.

cheers,
Sudhir.K.A

 

[memarian]

Dear,

about offset voltage error:

if you select R3=R1¦¦ R2 the effect of offset voltage minimized from (vos(1+R1/R2))

to vos only.

another problem with high closed loop gain: when you choose a gain in a closed

loop circuit,your circuit bandwiths is BW=fT/G;that FT is Gain Bandwidth Product

of your opamp and is constant for a opamp and it means higher gain lower bandwidth.


Good Luck.

 

[Kral]

memarian,
Making R3 = R1||R2 will eliminate the output error due to input bias current, but it will not afect the output error due to input bias voltage.
Regards,
Kral