I am posting the circuit diagrams of both the amplifiers:-

1.circuit of AD 620 instrumentation amplifier,designed for getting a gain of about 106.

2.circuit of non inverting amplifier using op amp ic 741,designed for getting a gain of about 101.

The gain equations of both the amplifiers are written inside a box near both the circuit diagrams iam attaching with this post.

I tried both the above amplifiers for amplifying the output of a vibration sensor or analyzer.

But when i gave the input from the function generator both the circuits amplified well.

For the non inverting amplifier using 741 ic,when i gave the input from the function generator at first i was getting a gain of about 50 and 68 when the input signal frequency was about 9.7 KHz.When i changed the input signal frequency to about 1 KHz i got the gain of about 100.But the original gain designed for this amplifier circuit was 101.When i gave the sensor output signal to the input of the non inverting amplifier it gave a gain of about 450.


When i used AD620 amplifier circuit,I get a gain value of about 100 when i was giving input signal from the function generator.When i gave the output signal from the vibration sensor as the input to the AD620 amplifier it also gave about a gain of about 450.

Why is the gain becoming very high when i give the sensor output as the input to the amplifier?

Which amplifier should i use for my need(for amplifying the sensor output)?

Should i make any modifications to my circuit.If then please suggest those modifications clearly.

Somebody please help me and reply fast.

 

The LM741 is very old, and has a high noise floor, low Gain Bandwidth Product.

Use any other op-amp with better specifications you can and use it.

 

I think it’s a little thing called Gain Bandwidth Product:

http://en.wikipedia.org/wiki/Gain–bandwidth_product

BW = A*GBW where BW is the bandwidth, A is the gain, and GBW is the Gain Bandwidth Product.

In my example, when the input signal freq was 9.7KHz i got a gain of ~50 for a GBW of 9.7K * 50 = 485KHz, which confirms with the low end of the spec for a 741. So, if the GBW of my 741 is 485KHz then at 1KHz, we can expect a maximum gain of 485KHz/1KHz = 485.


If i insert a low pass RC filter in between the vibration sensor output and the 741 op amp input will the gain become alright?I think some times the vibration sensor output will be at very high frequencies.So a low pass RC filter may remove these high frequency components.

What should be value of R in the RC filter which i should connect to the input of the opamp ic 741?Is it 100 K and what should be capacitance value in this input RC low pass filter?

 

It's not just the Bandwidth.

The LM741 is noisy, slow, needs dual supplies that the output cannot reach, and has a low slew rate.

Try a TL071 or the other op-amp you have, and measure the differences. The price is the same, if not cheaper for the newer rail-to-rail high bandwidth/low noise op amps. The 741 is effectively obsolete, it's useful in schools, since they are a few cents each and if they are fried it is no big deal.

If designing a product, you need to look at a design around parts that are relatively new (NOT Cutting Edge new), so that many products incorporate those parts, their end of life is decades away or "very long term", rather than a decade in the past.

 

Interestingly that is why universities use the 741. My lecturer told me that it is so bad (the 741) you can easily measure the offsets, errors and problems assosiated hence useful for teaching!