Where is known as the open-loop gain of the amplifier. The term open-loop means that the AOL feedback loop is absent from the output to the input. Open loop The value most of the times is usually very large (in the ranges of 100,000 or more) for any AOL IC op-amp. Thus, a small difference in the values of and drives the output of the V+ V− amplifier to near the supply voltage. Fig 2.22 An op-amp without negative feedback (a comparator) Closed loop Figure 2.23 An op-amp with negative feedback (a non-inverting amplifier) Negative feedback is used when a predictable operation is required. It is done by applying a portion of the output voltage back to the inverting input and the closed loop feedback greatly reduces the circuit’s gain. Hence, when using a negative feedback the overall gain and response of the circuit is mostly determined by the feedback network. The op-amp circuit’s response with its input, output and feedback circuits to an input is related mathematically using a transfer function which is very important in many op-amps like in analog computers. High input impedance at the input terminals and the low output impedance at the output terminal are very useful op-amp features. In the non-inverting amplifier given in the figure above, the presence of negative feedback via the voltage divider determines the closed loop-gain = / When is Rf, Rg ACL Vout Vin. Vout enough to pull the inverting input to equal equilibrium will be. The voltage gain of the entire Vin circuit is thus 1 + Taking for example, if = 1 V and Rf = Rg, Vout will be 2 V, exactly Rf/Rg. Vin the amount required to keep at 1 V. Because of the feedback provided by the network, V− Rf, Rg this is circuit is a closed loop one. The closed-loop gain is given by: ACL
Where AOL is known as the open-loop gain of the amplifier. The term open-loop means that the feedback loop is absent from the output to the input. Open loop The AOL value most of the times is usually very large (in the ranges of 100,000 or more) for any IC op-amp. Thus, a small difference in the values of V+ and V− drives the output of the amplifier to near the supply voltage. Fig 2.22 An op-amp without negative feedback (a comparator) Closed loop Figure 2.23 An op-amp with negative feedback (a non-inverting amplifier) Negative feedback is used when a predictable operation is required. It is done by applying a portion of the output voltage back to the inverting input and the closed loop feedback greatly reduces the circuit’s gain. Hence, when using a negative feedback the overall gain and response of the circuit is mostly determined by the feedback network. The op-amp circuit’s response with its input, output and feedback circuits to an input is related mathematically using a transfer function which is very important in many op-amps like in analog computers. High input impedance at the input terminals and the low output impedance at the output terminal are very useful op-amp features. In the non-inverting amplifier given in the figure above, the presence of negative feedback via the voltage divider Rf, Rg determines the closed loop-gain ACL = Vout / Vin. When Vout is enough to pull the inverting input to equal Vin equilibrium will be. The voltage gain of the entire circuit is thus 1 + Rf/Rg. Taking for example, if Vin = 1 V and Rf = Rg, Vout will be 2 V, exactly the amount required to keep V− at 1 V. Because of the feedback provided by the Rf, Rg network, this is circuit is a closed loop one. The closed-loop gain ACL is given by:
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