Negative Feedback Equation
Negative Feedback in Operational Amplifiers
Non-inverting Op-amp Circuit
Example 1. A system has a gain of 80dB without feedback. If the negative feedback fraction is 1/50th. Calculate the closed-loop gain of the system in dB with the addition of negative feedback.
Then we can see that the system has a loop gain of 10,000 and a closed-loop gain of 34dB.
Example 2. If after 5 years the loop gain of the system without negative feedback has fallen to 60dB and the feedback fraction has remained constant at 1/50th. Calculate the new closed-loop gain value of the system.
Then we can see from the two examples that without feedback, after 5 years of use the systems gain has fallen from 80dB down to 60dB, (10,000 to 1,000) a drop in open loop gain of about 25%. However with the addition of negative feedback the systems gain has only fallen from 34dB to 33.5dB, a reduction of less than 1.5%, which proves that negative feedback gives added stability to a systems gain. Therefore we can see that by applying negative feedback to a system greatly reduces its overall gain compared to its gain without feedback. The systems gain without feedback can be very large but not precise as it may change from one system device to the next, then it is possible to design a system with sufficient open-loop gain that, after the negative feedback has been added, the overall gain matches the desired value.Also, if the feedback network is constructed from passive elements having stable characteristics, the overall gain becomes very steady and unaffected by variation in the systems inherent open-loop gain.
Example 3. An operational amplifier with an open-loop voltage gain, AVOL of 320,000 without feedback is to be used as a non-inverting amplifier. Calculate the values of the feedback resistances, R1 and R2 required to stabilise the circuit with a closed loop gain of 20.
The generalised closed-loop feedback equation we derived above is given as:
By rearranging the feedback formula we get a feedback fraction, β of:
Then putting the values of: A = 320,000 and G = 20, into the above equation we get the value of β as:
Because in this case the open-loop gain of the op-amp is very high ( A = 320,000 ), the feedback fraction, β will be roughly equal to the reciprocal of the closed-loop gain 1/G only as the value of 1/A will be incredibly small. Then β (the feedback fraction) is equal to 1/20 = 0.05.
As the resistors, R1 and R2 form a simple series-voltage potential divider network across the non-inverting amplifier, the closed-loop voltage gain of the circuit will be determined by the ratios of these resistances as:
If we assume resistor R2 has a value of 1,000Ω, or 1kΩ, then the value of resistor R1 will be:
Then for the non-inverting amplifier circuit about to have a closed-loop gain of 20, the values of the negative feedback resistors required will be in this case, R1 = 19kΩ and R2 = 1kΩ, giving us a non-inverting amplifier circuit of:Non-inverting Op-amp Circuit
- There are many advantages to using feedback within a systems design, but the main advantages of using Negative Feedback in amplifier circuits is to greatly improve their stability, better tolerance to component variations, stabilisation against DC drift as well as increasing the amplifiers bandwidth.
- Examples of negative feedback in common amplifier circuits include the resistor Rƒ in op-amp circuits as we have seen above, resistor, RS in FET based amplifiers and resistor, RE in bipolar transistor (BJT) amplifiers.
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