Inverting Op Amp Circuit

- the inverting amplifier using an op-amp is one of the most widely used operational amplifier circuits especially as it can be used as a summing amplifier or virtual earth mixer.

Operational amplifiers can be used in a wide variety of circuit configurations. One of the most widely used is the inverting amplifier configuration.

The inverting amplifier offers many advantages from being very simple to use, requiring just the operational amplifier integrated circuit and a few other components.

The inverting amplifier can also be used as a summing amplifier in applications like audio mixers. The inverting input forms a virtual earth, enabling several signals to be summed together.

Basic inverting amplifier circuit

The basic circuit for the inverting op amp circuit is shown below. It consists of a resistor from the input terminal to the inverting amplifier input of the circuit, and another resistor connected from the output to the inverting input of the op-amp. The non-inverting input is connected to ground.

Circuit diagram of the basic inverting op amp circuit showing the two resistors used to control the gain
Basic inverting operational amplifier circuit

In this inverting amplifier circuit the non-inverting input of the operational amplifier is connected to ground. As the gain of the op amp itself is very high and the output from the amplifier is a matter of only a few volts, this means that the difference between the two input terminals is exceedingly small and can be ignored. As the non-inverting input of the operational amplifier is held at ground potential this means that the inverting input must be virtually at earth potential. As a result, this form of amplifier is often known as a virtual earth amplifier.

As the input to the op-amp itself draws no virtually current 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:

The operational amplifier gain equation for the inverting configuration where the gain is - R2/R1

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.

Inverting amplifier input impedance

One key consideration when designing an inverting amplifier is the input impedance. This governs many of the requirements in terms of coupling between stages, particularly when they are AC coupled.

The input impedance is easy to determine

Circuit diagram of the basic inverting op amp circuit showing the two resistors used to control the gain and the virtual earth point
Inverting amplifier circuit showing virtual earth

Using the diagram it is easy to determine the input impedance. As the inverting input to the amplifier is at earth potential, the input impedance is simply the value of R1.

The input impedance of an inverting operational amplifier is equal to the input resistor R1

Inverting amplifier design considerations

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 R1, the input resistor being lower than the feedback resistor R2, 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 R1 and R2:   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, R2 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 R2 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.

Summing amplifier / virtual earth mixer

The fact that the inverting input to the amplifier is virtually at earth potential provides some significant possibilities for audio mixer applications.

This form of virtual earth mixer, or summing amplifier adds several different signals in a linear or additive fashion. It is not the form of multiplier mixer used in RF circuits where a non-linear action is used.

Circuit diagram of the basic inverting op amp circuit used as a virtual earth mixer / summing amplifier
Op amp summing amplifier / virtual earth mixer

There is almost no limit to the number of inputs that can be added. The gain for each leg being R2 / R1y where R1 y is the input resistor for that particular channel.

Single supply inverting amplifier

Normally op amps use dual supply rails, e.g. ±15V. This enables the chips to operate using a signal related directly to ground without the need to as much coupling, if any.

This system works well in many applications, but it is not always viable to use dual supplies and in these circumstances it is necessary to change the inverting amplifier topology slightly to enable a single supply rail to be used.

This can be achieved by biasing the input of the op amp at half the rail voltage and then having decoupling to ground where necessary and inter-stage coupling to remove any DC voltages.

Circuit diagram of an inverting amplifier / operational amplifier using a single rail supply and the operating voltage set by a potential divider network
Inverting amplifier using a single rail supply

The Single rail inverting amplifier circuit is relatively straightforward. The operating point is set by the two resistors R3 and R4. C4 provides the decoupling to earth or ground and should offer a low impedance at the lowest frequency of operation.

This circuit works well, but when using it a few design issues must be remembered:

  • Resistors R3 & R4:   These two resistors form the bias for the non-inverting amplifier, setting the input voltage and operating point. Typically they are set to provide half the supply voltage and therefore they will be equal in value. Values of around 100kΩ are ofteny chosen because they determine the input impedance.
  • Capacitor C2 value:   The value of this capacitor must be chosen to give a low impedance at low frequencies. It will help to reduce any noise from the power supply that may be present.
  • Operating range:   Dependent upon the requirements for the inverting amplifier circuit, it may be necessary to choose an op amp that is able to operate with its output voltage close to either voltage rail. In this way the maximum output voltage swing can be obtained.

By Ian Poole

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