Design inverting op-amp circuit for voltage gain 10. What care should be taken to operate it linearly?
1 Answer

Figure 1 shows the circuit diagram of inverting op-amp

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$A_V$ is given by, $$|A_v| = \frac{-R_F}{R_1}$$ Since $|A_V|$=10, where $A_V$ is negative but by taking mod we write it in positive,

$$-10 = \frac{-R_F}{R_1}$$

Assume $R_1$ = 1KΩ hence,

$$-10 × 1KΩ = $-R_F$$

$$R_F = 10KΩ$$ Hence, designed circuit diagram,

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Care should be taken to operate it linearly:

There are a number of design considerations and tips to be kept in mind when designing an inverting amplifier circuit using an op amp.

Bandwidth product:

It is worth mentioning at this point that for high levels of gain, the gain bandwidth product of the basic op amp itself may become a problem. With levels of gain of 100, the bandwidth of some operational amplifier ICs may only be around 3 kHz. Check the data sheet for the given chip being used before settling on the level of gain.

Input impedance:

With the value of $R_1$, the input resistor being lower than the feedback resistor RF, care has to be taken when designing he circuit to ensure the input resistance is not too low for any given application. Often values of 10kΩ are used as this provides a reasonable load resistance for many circuits. However the exact requirements need to be determined for each application.

Range of values for $R_1$ and $R_F$:

When designing an inverting amplifier it is necessary to ensure that the values of the resistor used remain within 'sensible' bounds. If the input resistor is made too low, then the circuit may load the previous circuit too much. If high gain levels are required, then this may mean that the feedback resistor, RF must be higher. Even though the input impedance of the integrated circuit itself circuit may be high, using a high value of feedback resistor is not advisable as results may become a little unpredictable. As a very broad rule of thumb values for RF of up to 100kΩ or a little more should be fine. Values above this should be used with a little caution, as the circuit may produce some unexpected effects, although in many instances they may work acceptably well.

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