Electrical – Non-inverting amplifier with variable gain from 1 to 15

amplifiercircuit analysiscircuit-designnon-invertingoperational-amplifier

I need to design a non-inverting amplifier with gain from 1 (unity) to 15. From what I read, in theory this can be achieved by using a pot as the feedback resistor, but in practice this will only work depending on the open-loop gain of the op-amp.
Below is the circuit which I intend to use, using the AD8544 as an op-amp on single supply and RV1 used to set the gain from unity to 15. The input signal will have Vcc/2 as the base reference, hence why R1 is connected to Vcc/2. Input frequency can be anywhere from 5 to 600Hz.

The datasheet of the AD8544 shows a graph of the open-loop gain VS frequency (figure 30). Does this graph show that the gain could be up to 40dB for frequencies 5 to 600Hz ?

Is there anything which can be improved in the circuit shown below?

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Best Answer

Yes, it means the open loop gain is 40dB, i.e. ~100 V/V (20 log 100 = 40 dB), up to around 300kHz. It is a typical open loop gain of differential signal.

Since you want to amplify the input signal by a factor of 1 to 15 and the output signal is:

out = in * (1 + RV1/R1)

you have to make sure RV1 will vary from 0 to 14 * R1 = 14 * 3.48 = 48.72 Ohm, so 50 kOhm resistor will work.

Of course you cannot get more than Vcc on the output, the opamp will saturate. Therefore for your maximum amplification of 15, when RV1 = 14R1, your maximum input signal for linear amplification may be Vcc / 15, this will be then amplified to the value of Vcc. All the input voltages above Vcc / 15 will result in output of Vcc. In general, for any amplification factor of G the maximum input signal for linear amplification is Vcc / G, and opamp will saturate for voltages above that.