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Analysing negative feedback for different Opamp types

1. Describe how the returning signal affects the output of the op-amp, in negative feedback.

2. If the input signal is 20 mV, RF = 68 kohm and R1 = 2.7 kohm in an LF351 op-amp configured using the non-inverting input and using negative feedback, what are the values of ideal closed loop voltage gain and exact voltage gain?

3. If the input signal is 20 mV, RF = 68 kohm and R1 = 2.7 kohm in an uA741 op-amp configured using the non-inverting input and using negative feedback, what are the values of ideal closed loop voltage gain and exact voltage gain?

4. If the input signal is 0.5V, RF = 1kohm and R1 = 2.7 kohm in an LM158 op-amp configured using the non-inverting input and using negative feedback, what are the values of output current and load power?

Solution Preview

1. The operational amplifier amplifies the voltage in a linear manner within its quoted operating bandwidth and any increase/decrease in input voltage level is correspondingly reflected in a linear increase/decrease in output level voltage. With negative feedback a proportion of this increase or decrease in output voltage is fed back to the other input and the potential at the other input tends to rise or decrease to match the increase or decrease in the input voltage; another way of saying this is that the differential potential between the inputs of the op-amp in this situation tends to stay constant. Any change in input level at one terminal is reflected by linear change in output voltage and corresponding change in potential at other terminal trying to keep the differential input the same.

We can analyze the ideal op-amp for the non inverting case with negative feedback.

With negative feedback the output voltage V(OUT) is given as the product of the potential difference between the inputs V+ - V- and the open loop voltage gain A(OL) algebraically by (1) below.

V(OUT) = {V+ - V-}*A(OL) (1)

As the input impedance of an op-amp is very ...

Solution Summary

Negative feedback as configured for the LF351, UA741 and LM158 Ompam ICs is analysed and the an explanation on How the returning signals affect the Opamp output is provided. The (ideal) closed loop gain for specified negative feedback networks is deduced for the LF351 and UA741 Opamp types as well as the actual (real) voltage gain are deduced. The LM158 Opamp is analysed to determine the output current and output power for set input conditions and feedback network. All amplifiers considered are of the non-inverting type

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