# 3-stages of op-amp

#### AboRoman

Joined Dec 9, 2011
5
Hello, I hope you're doing well

I have been asked in my HW to use two or three stages (not more than three stages) of op-amp amplifiers to design an amplifier circuit that will amplify a signal from 150 uV to an output voltage of 5 V with the following constraints:
1. The first stage should be a diffrence amplifier as shown in the figure v1 = 0 V, v2 is the source signal, R2 = 20 kΩ and Rg = 60 kΩ.
2. All the other resistors used in the design should be in the range of 2 kΩ to 100 kΩ.
3. For the all the op-amps VCC = 10 V. the problem here is, all what I studied up to now is " Analysing Circuits " , I have not yet arrived to the level of designing . and to make things worse, the only op-amps I have studied about are the ideal ones .

I really need your help at this one , and how to proceed from this point on

thanks and sorry for the Inconvenience

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#### bountyhunter

Joined Sep 7, 2009
2,512
You need to calcultae the desired TOTAL GAIN (from input to output) based on the input signal and output signal.

The diff amp shown has a specific gain based on the specified 60k and 20k resistors.

HINTS:

In a classic diff amp, R1 and R2 are equal. Also, Rf and Rg are equal.

You will need to add two more gain stages to get enough total gain.

You can either use two additional inverting or non inverting stages, since two inv stages end up with same polarity as two non-inverting stages.

• AboRoman

#### crutschow

Joined Mar 14, 2008
32,899
If you can analyze the differential circuit for it's gain, then you need how much gain you need in the following stages based upon the input and output signal levels. If the required gain for the following stages is not to high, then you may only need one additional sage.

If you have only a single supply voltage, then the following stage(s) must have non-inverting gain to avoid having the signal trying to go below ground.

If you look at the op amp data sheets you can see how far they deviate from the "ideal". Often they are close enough that the deviations can be mostly ignored. One concern for low level DC amplification is the offset voltage.

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• AboRoman

#### bountyhunter

Joined Sep 7, 2009
2,512
If you have only a single supply voltage, then the following stage(s) must have non-inverting gain to avoid having the signal trying to go below ground..
good point, I didn't notice it had only one rail

• AboRoman

#### bountyhunter

Joined Sep 7, 2009
2,512
If you look at the op amp data sheets you can see how far they deviate from the "ideal". Often they are close enough that the deviations can be mostly ignored. One of concern for low level DC amplification is the offset voltage.
+1

Most 'standard grade" op-amps have offset voltage in the 5 mV range, which is about 33X the size of your input signal. You would need a super precision instrumentation op-amp.

• AboRoman