Operational Amplifier Circuits Selection

The are very many op amp circuits which can be used and designed, the applications include everything from amplifiers to filters and integrators to astables and multivibrators.

Op-amp Circuits Include:
Introduction     Circuits summary     Circuit design hints & tips     Inverting amplifier     Summing amplifier     Non-inverting amplifier     Inverting vs non-inverting circuits     Variable gain amplifier     High pass active filter     Low pass active filter     Bandpass filter     Notch filter     Comparator     Schmitt trigger     Multivibrator     Bistable     Integrator     Differentiator     Wien bridge oscillator     Phase shift oscillator    

Operational amplifiers are particularly versatile circuit blocks. They find applications in a host of different circuits where their attributes of high gain, high input impedance low output impedance and a differential input enable them to provide a high performance circuit with a minimum of components.

By using negative, and sometimes positive feedback around the op amp chip they can be used in many applications and circuits to provide a variety of different functions from amplifiers and filters to oscillators, integrators and many other functions.

There are many op amp circuits that cover most of the main analogue functions that are needed. As a result of this, operational amplifiers have become the workhorse of the analogue electronics designer.

Op amp inverting amplifier circuit with op amp chips

Op amp inverting amplifier circuit with op amp chips

Op-amp-circuit design tips

There are many different types of circuit that can be developed using operational amplifiers. These are often very easy to design and they give some

Op-amp circuits

Operational amplifiers can be used in a host of different circuits and applications. Being an almost perfect differential amplifier, certainly as far as is needed for most applications, their high input impedance, high gain and differential input makes them an ideal circuit block.

  • Op-amp inverting amplifier :   The op-amp inverting amplifier is possibly the most widely used op amp circuit configuration. Not only does it provide gain, but it can also be used as a virtual earth amplifier.

    This operational amplifier circuit features a resistor from the output to the inverting input to provide feedback, and a resistor from the inverting input to the overall circuit input. The actual circuit input impedance is low, being that of the input resistor.

    The circuit is easy to use, and it can provide a useful low impedance for low impedance matching in applications, possibly where low impedance microphones are used. The low impedance also reduces stray pick-up on the input, another useful attribute for many audio applications.
    Read more about . . . . op-amp inverting amplifier.

  • Op amp summing amplifier:   Based around the inverting amplifier circuit with its virtual earth summing point, this circuit is ideal for summing audio inputs. It is widely used in audio mixer and many other applications where voltages need to be summed.

    When used in radio mixer applications, this operational amplifier circuit is ideal because the virtual earth summing point results in the different inputs not affecting each other regardless of the input voltages and impedance levels.

    As a result the inverting amplifier is used in virtually all analogue audio mixers apart from hte general voltage summing applications for which it finds uses.
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  • Op-amp non-inverting amplifier :   The non-inverting amplifier circuit provides gain and also a very high input impedance. Accordingly it is widely used in many amplifier input stages.

    The non-inverting operational amplifier circuit provides the mainstay for applications where a high input impedance is required - it is even used as a voltage follower by applying the output directly to the inverting input. The very high gain of the circuit means that the input voltage is very accurately followed and a very high input impedance is maintained to ensure there is no loading of the previous stage.
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  • Op-amp comparator:   The comparator circuit is often used to provide a high or low signal dependent upon the relative states of the two inputs. Special comparator chips are normally used for this application, but they follow the same basic concepts as operational amplifiers.

    Although op amps are often used as comparators in many circuits, it is normally best it use a proper comparator because op-amps can lock up under some circumstances. Also comparators are much faster and being designed for the voltage comparison applications, they perform much better in this application.
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  • Op-amp high pass filter:   Op amps are able to provide, easy to design, one, two and three pole filters using a single op amp.

    It is possible to provide a single pole circuit quite easily by adding a capacitor to the circuit, but higher performance levels are achievable by incorporating the high pass network into the feedback and significantly enhancing the performance.

    Using some simple mathematics it is very easy to design an op amp circuit the provides sufficiently high performance for most applications. However if it is necessary to utilise a specific filter type, then this is also possible.
    Read more about . . . . high pass filter.

  • Op-amp low pass filter:   Operational amplifiers can be used to good effect in low pass filter applications. If only a very gentle roll off is needed a capacitor can simply be placed across the feedback resistor and the break point of the two components configured to give the right response.
    Two pole active low pass op amp filter circuit
    Active low pass op amp filter circuit
    It is also possible to develop a two pole filter which will give much better performance and better roll off characteristic.

    The calculations for a simple filter capable of meeting most applications are very straightforward, but for those wanting a specific filter response, this too is possible.
    Read more about . . . . low pass filter.

  • Op-amp bandpass filter:   Although high and low pass filters are very useful, bandpass filters, allowing only a band of frequencies to pass through are also needed and can be easily implemented using op amps.

    Bandpass filters are often needed when a specific band of frequencies is required. Although op amps tend to be used for lower frequencies, there are many instances where an active bandpass filter circuit can be used.
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  • Op-amp notch filter:   Notch filters are used where a single frequency or narrow band of frequencies needs to be removed. These op amp circuits may be used in applications where a single frequency or a small band of frequencies need to be removed. One application might be for removing a line / mains hum from an audio signal.

    These filters can be realised using a single op amp. Further op amp stages can be sued if a deeper notch is required.

    Whilst some circuits offer a fixed notch frequency, which is ideal for removing unwanted signals on a fixed known frequency, other circuits are able to provide a variable frequency notch. Other circuits are able to provide a variable Q notch.
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  • Op-amp Schmitt trigger:   The Schmitt trigger is a form of comparator circuit that has different switching levels dependent upon whether the circuit is switching from high to low or vice versa. This gives the circuit noise immunity to the level of the difference between the two switching levels.
    Typical notch filter response showing the frequency at which there is a minimum response and other frequencies are not attenuated
    Typical notch filter response
    As with a standard comparator circuit, it is wise to use a comparator IC instead of an op amp as the comparator will work much better in this type of application.
    Read more about . . . . op-amp Schmitt trigger.

  • Op-amp multivibrator:   Multivibrators are used in a variety of different applications. Op-amp circuits often provide an effective solution.

    Although not the first idea that might come to mind when thinking of an op amp circuit or applications, the circuit nevertheless exists and can be put to good use on a number of occasions.
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  • Op-amp bistable:   Op amps can be used as a bistable in some applications. Although not best suited to the application, they still work well on most occasions.
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  • Op-amp analogue integrator:   Op amps are ideal for use as integrators. The high input impedance and gain lends itself to this application, although for long integration times very high input impedance chips may be required.

    Op amps were used in this application to create analogue computers - their high impedance input and high gain meant that they were able to provide an excellent basis for an op amp integrator circuit.
    Read more about . . . . analogue integrator circuit.

  • Op-amp analogue differentiator:   The op amp differentiator is another circuit used in analogue computing and finds applications in other areas.

    This circuit is possibly less widely used, but nevertheless a key item in an analogue designers toolbox. One issue can be that the differentiator can be open to picking up noise. By the very function of the differentiation, it means it has a rising characteristic with frequency.
    Read more about . . . . analogue differentiator circuit.

  • Op-amp Wien bridge oscillator:   The op amp Wien bridge oscillator is able to act as a good signal generator circuit.

    Based on a bridge circuit, the Win bridge oscillator is able to provide a good performance, if the gain is increased too far, the level of distortion rises significantly.
    Read more about . . . . Wien bridge oscillator.

  • Op-amp phase shift sine wave oscillator:   The phase shift oscillator circuit is able to provide a good sine wave output.

    The advantage of the op amp phase shift oscillator circuit is that it is able to provide very low levels of distortion, making it a favourite circuit or configuration for sine wave oscillator applications.
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Operational amplifiers are an ideal circuit building block for analogue developers. These integrated circuits combine the sufficiently close both e ideal amplifier for most applications that they can be sued to enable high performance circuits to be designed and implemented with a minimum of components.

As operational amplifiers are widely available in many forms, some as general purpose op amps, but others providing high bandwidth, high input impedance or low offsets, etc, as well as them being available in many packages, often with more than one op amp in a package, these chips are ideal for use in many analogue circuits for many applications.

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