Operational Amplifier, Op Amp Gain
- the gain of an operational amplifier is very high when operated without feedback. Using negative feedback, the performance of the overall circuit has an accurately defined level of gain enabling it to fulfil many functions.
Op-amp circuit design tutorial includes:
• Operational amplifier basics • Op-amp circuit gain • Op-amp bandwidth • Op-amp slew rate • Op-amp offset null • Op-amp input impedance • Op-amp output impedance
• Op amp circuits
The gain of an operational amplifier or op-amp circuit depends upon a variety of factors including the circuit configuration as well as the components around the operational amplifier chip itself. This page gives a summary or overview of the gain of the different operational amplifier circuits available.
Most op amp chips have very high gain levels, typically of the order of 10 000 to 100 000 at very low frequencies.
Although this gain is not directly used, as it would mean that even very small input signals would send the chip into limiting, by using negative feedback high performance circuits can be generated.
Op amp gain basics
Operational amplifiers are normally used with feedback around the amplifier element itself. This tailors the performance to what is needed. There are two scenarios for which the gain can be considered:
- Open loop gain: This form of gain is measured when no feedback is applied to the op amp. In other words it is running in an open loop format. Gain figures for the op amp in this configuration are normally very high, typically between 10 000 and 100 000. This is the gain of the operational amplifier on its own
- Closed loop gain: This form of gain is measured when the feedback loop is operation, i.e. a closed loop. By applying negative feedback, the overall gain of the circuit is much reduced, and can be accurately tailored to the required level or to produce the required output format as in the case of filters, integrators, etc.. The gain is measured with the loop closed and provided there is a sufficient difference between the open loop and closed loop gain, the circuit will operate according to the feedback placed around it. Although negative feedback is normally used for analogue circuits, there are instances where positive feedback is used. The most common application is for comparators where the output is required at one of two levels. The Schmitt trigger is one example where hysteresis is introduced into the system
General gain scenario
Negative feedback is used to control the gain of the overall op amp circuit. There are many ways in which the feedback can be applied - it may be independent of frequency, or it may be frequency dependent to produce filters for example.
However it is possible to produce a generalised concept for applying negative feedback. From this the more specific scenarios can be developed.
Generic op amp negative feedback configuration
It is possible to calculate a general formula for the op amp gain in the circuit:
The output voltage can then be calculated from a knowledge of the input voltage, gain and feedback:
This can now be used to generate the closed loop op amp gain equation.
Using this generic equation it is possible to develop equations for more specific scenarios. The feedback can be frequency dependent, or flat as required.
The two simplest examples of op am circuits using feedback are the formats for inverting and non-inverting amplifiers.
Inverting op-amp gain
The circuit for the inverting op-amp circuit is shown below. This circuit has the output 180 degrees out of phase with the input and also provides a virtual earth input.
Basic inverting operational amplifier circuit
It is easy to derive the calculation for the op-amp gain. The input to the op-amp itself draws no current and this means that the current flowing in the resistors R1 and R2 is the same. Using ohms law Vout /R2 = -Vin/R1. Hence the voltage gain of the circuit Av can be taken as:
As an example, an amplifier requiring a gain of ten could be built by making R2 47 k ohms and R1 4.7 k ohms.
Non-Inverting op-amp gain
The circuit for the non-inverting op-amp is shown below. It offers a higher input impedance than the inverting op amp circuit.
Basic non-inverting operational amplifier circuit
The gain of the non-inverting circuit for the operational amplifier is easy to determine. The calculation hinges around the fact that the voltage at both inputs is the same. This arises from the fact that the gain of the amplifier is exceedingly high. If the output of the circuit remains within the supply rails of the amplifier, then the output voltage divided by the gain means that there is virtually no difference between the two inputs.
As the input to the op-amp draws no current this means that the current flowing in the resistors R1 and R2 is the same. The voltage at the inverting input is formed from a potential divider consisting of R1 and R2, and as the voltage at both inputs is the same, the voltage at the inverting input must be the same as that at the non-inverting input. This means that Vin = Vout x R1 / (R1 + R2)Hence the voltage gain of the circuit Av can be taken as:
As an example, an amplifier requiring a gain of eleven could be built by making R2 47 k ohms and R1 4.7 k ohms.
Op-amp gain is very easy to determine. The calculations for the different circuits is slightly different, but essentially both circuits are able to offer similar levels of gain, although the resistor values will not be the same for the same levels of op amp gain.
These two circuits provide examples of where the op amp gain is controlled not by the internal gain of the chip itself, but by the external components in the negative feedback loop. There are many other op amp circuits that use feedback to control the gain to provide a number of useful functions.
By Ian Poole
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