The Op-Amp used as an Inverting or Non-Inverting Amplifier When Vo is connected

Question

The Op-Amp used as an Inverting or Non-Inverting Amplifier

When Vo is connected through a resistor to VIn-, th at configuration is called negative feedback and Vo tends to move is a direction to mak e (VIn+ - VIn-) smaller. Consider the inverting configuration circuit in the OpAmpConfigs.jpg . Assume that VIn- is virtually same as VIn+, about zero volts. The cu rrents through Ri and Rf are the same since input bias current = 0. The voltage across each res istor is proportional to resistance value. Vo = Vi * Acl = Vi * (-Rf/Ri) = -1V * (-2k/1k) = +2 volt s. The voltage Vi must supply current of magnitude (Vi/Ri) through Ri.

Consider the non-inverting configuration circuit in the OpAmpConfigs.jpg . Assume that VIn- is virtually same as VIn+, about +1 volt. The currents through Ri and Rf are the same since input bias current = 0. The voltage across ea ch resistor is proportional to resistance value. Vo = Vi * Acl = Vi * (1+( Rf/Ri)) = +1V * (1+( 2k/1 k)) = +3 volts. The voltage Vi must supply current of magnitude Ibias through VIn+. Figure 1.a is Inverting configuration with voltage gain = (-Rf/Ri). Figure 1.b is Non-inverting configuration with volt age gain = 1+(Rf/Ri).

Which configuration requires the least current from Vi?

Which configuration has the highest resistance conn ected to Vi?

Which configuration can produce gain of +1?

Write your answers to the 3 above questions.

Explanation / Answer

1) The input impedance of non-inverting op-amp is infinite while that of Inverting op-amp it is equal to the resistance between the input and the inverting terminal, so a better impedance matching can be obtained.

Hence least current will be required by the non-inverting configuration because of the higher value of the input resistance

2) The input impedance of non-inverting op-amp is infinite hence the highest resistance will be connected in the case of non-inverting amplifier

3) Gain(non-inverting) = 1 + Rf/R1 and Gain(non-inverting) = -Rf/R1

we can achieve the gain of 1 from non-inverting configuration by putting the value of Rf << R1, in that case provide will be 1

FOR YOUR INFORMATION ONLY:

one of the advantages is that unlike a non-inverting OP-amplifier which has a minimum possible gain of 1, an inverting Op-amp can be used as an attenuater with a gain less than 1 as well.

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