[SOLVED] amplifier gain setting resistors selection

[shahbaz.ele]

dear all
I am using OP177 IC as amplifier.
my signal was in micro-volts and I set gain of 20000 using 100ohm/2Mohm resistors.
It worked well.
but the problem is that when I used the same op-amp for frequency signal greater than 500Hz, its gain reduces.
even for 1KHz signal it reduces to 10000 gain.
can some one suggest me that what is the relation between resitor values and frequency/bandwidth.

 

[crutschow]

It's related to the circuit gain and the gain-bandwidth-product (GBP or frequency response at Av=1) of an op amp. For the OP177 the GBP is only 0.6MHz typical. To get the frequency response you divide the GBP by the circuit gain. This gives a bandwith of 0.6MHz / 20,000 = 30Hz. So to get more bandwidth you either need a higher frequency op amp or you need to add more gain stages so you can reduce the gain of each stage. For example two stages would require only a gain for each stage of 141.4 for a total gain of 20,000 which would increase the bandwidth to about 4KHz.

 

[shahbaz.ele]

Ok its useful information,
I also interested to know that is there any difference of selecting gain resistors, (like you can select 1ohm/100ohm for 100 gain, also 1Kohm and 100Kohm.
what are the requirements for resistors selection.

 

[tsushant]

For lower values of resistors, more current will flow through your circuit, increasing power dissipation in circuit. On the contrary for high values of resistor, more noise will be generated which will ultimately deteriorate performance of your system. So you'll have to optimize values of resistors. Practically values between 1K to 100K acceptable, for values greater than 100k, you'll have add a small capacitor (typically in nF) in parallel to resistor to reduce noise. Using resistors less than 100ohm is not recommended.

 

[crutschow]

The minimum value is related to how low an input impedance the circuit driving the op amp can tolerate and also how much load the op amp can drive. For example most op amps can't drive a 100 ohm feedback resistor. On the other hand, too high a resistor value can lead to offset problems due to the input bias current and higher circuit noise due to thermal noise in the resistor. Typically values of a few kΩ up to a a few hundred kΩ are used. But values outside these ranges can be used if the limitations are addressed.

 

[shahbaz.ele]

thanx
dear @tsushant and crutschow

 

[shahbaz.ele]

can we say TL081 (GBP = 3GHz) is better than OP177 (0.6GHz)
with gain of 20000 that i am using with op177

- - - Updated - - -

sorry these GBP are in MHz
3MHz for TL081 and 0.6MHz for OP177

 

[Audioguru]

A TL081 is an "ordinary" opamp. But a TL071 is a TL081 selected for low noise and is used in many audio circuits. They cost the same.

 

[shahbaz.ele]

can you suggest me some better rather best amp IC for 20K gain. upto 300Hz freq

 

[Audioguru]

Texas Instruments make some excellent opamps. The single OPA134, dual OPA2134 and quad OPA4134 opamps are very good for audio circuits.