The Inverting Operational Amplifier

We saw in the last tutorial that the Open Loop Gain, ( Avo ) of an ideal operational amplifier can be very high, as much as 1,000,000 (120dB) or more. However, this very high gain is of no real use to us as it makes the amplifier both unstable and hard to control as the smallest of input signals, just a few micro-volts, (μV) would be enough to cause the output voltage to saturate and swing towards one or the other of the voltage supply rails losing complete control of the output.

As the open loop DC gain of an Operational Amplifiers is extremely high we can therefore afford to lose some of this high gain by connecting a suitable resistor across the amplifier from the output terminal back to the inverting input terminal to both reduce and control the overall gain of the amplifier. This then produces and effect known commonly as Negative Feedback, and thus produces a very stable Operational Amplifier based system.

Negative Feedback is the process of “feeding back” a fraction of the output signal back to the input, but to make the feedback negative, we must feed it back to the negative or “inverting input” terminal of the op-amp using an external Feedback Resistor called . This feedback connection between the output and the inverting input terminal forces the differential input voltage towards zero.

This effect produces a closed loop circuit to the amplifier resulting in the gain of the amplifier now being called its Closed-loop Gain. Then a closed-loop inverting amplifier uses negative feedback to accurately control the overall gain of the amplifier, but at a cost in the reduction of the amplifiers gain.

This negative feedback results in the inverting input terminal having a different signal on it than the actual input voltage as it will be the sum of the input voltage plus the negative feedback voltage giving it the label or term of a Summing Point. We must therefore separate the real input signal from the inverting input by using an Input Resistor, Rin.

As we are not using the positive non-inverting input this is connected to a common ground or zero voltage terminal as shown below, but the effect of this closed loop feedback circuit results in the voltage potential at the inverting input being equal to that at the non-inverting input producing a Virtual Earth summing point because it will be at the same potential as the grounded reference input. In other words, the op-amp becomes a “differential amplifier”.

Inverting Operational Amplifier Configuration

inverting operational amplifier

 

In this Inverting Amplifier circuit the operational amplifier is connected with feedback to produce a closed loop operation. When dealing with operational amplifiers there are two very important rules to remember about ideal inverting amplifiers, these are: “No current flows into the input terminal” and that “V1 always equals V2”. However, in real world op-amp circuits both of these rules are slightly broken.

This is because the junction of the input and feedback signal ( X ) is at the same potential as the positive ( + ) input which is at zero volts or ground then, the junction is a “Virtual Earth”. Because of this virtual earth node the input resistance of the amplifier is equal to the value of the input resistor, Rin and the closed loop gain of the inverting amplifier can be set by the ratio of the two external resistors.

We said above that there are two very important rules to remember about Inverting Amplifiers or any operational amplifier for that matter and these are.

  • 1.  No Current Flows into the Input Terminals
  • 2.  The Differential Input Voltage is Zero as V1 = V2 = 0 (Virtual Earth)

Then by using these two rules we can derive the equation for calculating the closed-loop gain of an inverting amplifier, using first principles.

Current ( i ) flows through the resistor network as shown.

resistor feedback circuit

 

inverting op-amp gain formula

 

Then, the Closed-Loop Voltage Gain of an Inverting Amplifier is given as.

inverting operational amplifier gain equation

and this can be transposed to give Vout as:

inverting operational amplifier gain

op-amp linear output

Linear Output

The negative sign in the equation indicates an inversion of the output signal with respect to the input as it is 180o out of phase. This is due to the feedback being negative in value.

The equation for the output voltage Vout also shows that the circuit is linear in nature for a fixed amplifier gain as Vout = Vin x Gain. This property can be very useful for converting a smaller sensor signal to a much larger voltage.

Another useful application of an inverting amplifier is that of a “transresistance amplifier” circuit. A Transresistance Amplifier also known as a “transimpedance amplifier”, is basically a current-to-voltage converter (Current “in” and Voltage “out”). They can be used in low-power applications to convert a very small current generated by a photo-diode or photo-detecting device etc, into a usable output voltage which is proportional to the input current as shown.

Transresistance Amplifier Circuit

trans-resistance operational amplifier

 

The simple light-activated circuit above, converts a current generated by the photo-diode into a voltage. The feedback resistor sets the operating voltage point at the inverting input and controls the amount of output. The output voltage is given as Vout = Is x Rƒ. Therefore, the output voltage is proportional to the amount of input current generated by the photo-diode.

Inverting Op-amp Example No1

Find the closed loop gain of the following inverting amplifier circuit.

inverting op-amp circuit

 

Using the previously found formula for the gain of the circuit

inverting op-amp gain

 

we can now substitute the values of the resistors in the circuit as follows,

Rin = 10kΩ  and  Rƒ = 100kΩ.

and the gain of the circuit is calculated as    -Rƒ/Rin = 100k/10k = -10.

therefore, the closed loop gain of the inverting amplifier circuit above is given -10 or 20dB (20log(10)).

Inverting Op-amp Example No2

The gain of the original circuit is to be increased to 40 (32dB), find the new values of the resistors required.

Assume that the input resistor is to remain at the same value of 10KΩ, then by re-arranging the closed loop voltage gain formula we can find the new value required for the feedback resistor .

   Gain = -Rƒ/Rin

therefore,   Rƒ = Gain x Rin

  Rƒ = 40 x 10,000

  Rƒ = 400,000 or 400KΩ

The new values of resistors required for the circuit to have a gain of 40 would be,

 Rin = 10KΩ  and  Rƒ = 400KΩ.

The formula could also be rearranged to give a new value of Rin, keeping the same value of .

One final point to note about the Inverting Amplifier configuration for an operational amplifier, if the two resistors are of equal value, Rin = Rƒ  then the gain of the amplifier will be -1 producing a complementary form of the input voltage at its output as Vout = -Vin. This type of inverting amplifier configuration is generally called a Unity Gain Inverter of simply an Inverting Buffer.

In the next tutorial about Operational Amplifiers, we will analyse the complement of the Inverting Amplifier operational amplifier circuit called the Non-inverting Amplifier that produces an output signal which is “in-phase” with the input.


 

« | »

12 Responses to “Inverting Operational Amplifier”

  1. Mal Green

    Please help!
    I’ve built a simple Inverting Op Amp circuit (see Example 1 above) with Rin = Rf = 22k, and the Op Amp is provided by a TI LM324 Quad Op Amp IC.

    The output waveform is inverting, but the negative component (or what should be) is actually showing as a small positive Sine signal!

    I’ve tried Sine input provided by both iPhone app and Signal Generator, and Vcc supplied by either a 9v battery or a variable power supply. I’ve even tried different chips, all with same result.

    Please explain why negative signal is not coming through :((

    Reply
    • Wayne Storr

      Two points here. if Rin = Rf = 22k then the gain is -1 so the output should resemble the input sine wave but inverted. However, you are feeding the op-amp with a 9v single rail supply so the op-amp is saturating on the negative rail when the input sine wave goes positive. Try feeding pin 4 with +9V and pin 11 with -9V to create a dual supply. Pin 3, input sine wave and output signal are referenced from the center point 0V or ground.

      Reply
      • Mal Green

        Wayne … you’re a champion!

        Some of the diagrams we were using did not show the +Vcc and -Vcc supplying the Op Amp, so we mistakenly had Pin 11 connected to ‘earth’ … so your suggestion certainly pointed us in the right direction :) Since our circuit has to be portable and battery powered, I created a voltage divider so Vcc = +9v, -Vcc = 0 and ‘Ground’ tapped in to +4.5v.

        As an aside, the P/S we were using may not have been fully rectified, and may have been “masking” our problem … when we used the P/S in the above circuit, it still clipped the -ve signal, but using a battery everything was ok.

        MANY THANKS!

        Reply
        • Wayne Storr

          Hello Mal. Most Op-amps use a dual +/- supply but the LM324 is design to operate from a dual supply or a single supply up to about 24 volts, I think. To operate from a single 9V supply and stop the output from saturating, try reducing the peak-to-peak input voltage as much as possible, maybe below 0.5 volts until it stops saturating.

          Reply
  2. bindu

    Hi..pls explain me about input and output impedances calculation of practical inverting op amp

    Reply
  3. Jason

    You say that negative feedback reduces the bandwidth of the amplifier. I am now confused, I thought GPB/A = bandwidth. The lower A is, the greater the bandwidth or am I missing something?
    Please help!!!

    Reply

Leave a Reply

Your email address will not be published. Required fields are marked *


six − 3 =

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>