 Fundamentals

# A short primer on this popular Op Amp.

ELMO V. JANSZ

Issue 70, May 2023

The 741 Operational Amplifier is a very widely used integrated circuit. It is easy to set up and use.

In this short primer, we will use it in the following applications:

1. Non-inverting amplifier
2. Inverting amplifier
3. Voltage follower
4. Summing amplifier
5. Difference amplifier

The term Operational Amplifier refers to a high gain DC amplifier with two inputs and a single output. Operational amplifiers were originally used as high performance DC amplifiers in Analog Computers. They are now available in IC packages with high gain and use external circuitry to control their characteristics. When used without any feedback the device is said to be in open loop mode. Its characteristics are generally described in this mode. Op Amps are used today in almost every aspect of electronics – in audio systems, communication systems and consumer electronics to name but a few.

The 741 has five ideal characteristics:

1. The open loop gain is infinity.
2. The output resistance is zero
3. The input resistance is infinity
4. The bandwidth is infinity
5. The output voltage is zero when the input voltage is zero i.e. No offset

The circuit symbol for the 741 Is shown here. There are two inputs marked (-) and (+) called the inverting and non-inverting inputs respectively. The device requires two power supplies typically in the range +/-5V to +/-15V.

We shall now discuss each of the five circuit configurations.

# Non-inverting amplifier

Fig.1 shows the 741 set up as a non-inverting amplifier. Note that the input signal is applied to the (+) input.

The resistor RF is called the feedback resistor as it feeds a part of the output signal back to the input. The output voltage is given by Vo = [ 1 + RF/R1] Vi. The voltage gain Av is then given by:

Av = 1 + RF/R1

Note Av is always greater than unity, and the output is in phase with the input since the input is applied to (+) terminal.

# Inverting amplifier

Fig.2 shows the circuit for an inverting amplifier

The input signal is applied to (-) through the resistor R1 and RF is the feedback resistor. The output voltage is given by:

Vo = - (RF/ R1) Vi

The voltage gain Av, in this case, is given by Av = - RF/ R1

# Voltage follower

The voltage follower configuration is shown in Fig.3 It is basically a unity gain non-inverting amplifier. There is no feedback resistor and the output is an exact copy of the input. The circuit has a high input impedance which makes it very useful as a buffer circuit.

# Summing amplifier

Fig.4 shows a summing amplifier. It uses the inverting configuration with several resistors connected to the input.

The output voltage is given by:

Vo = - [(RF/R1) V1 + (Rf/R2) V2 + (RF/R3) V3] which is very similar to the inverting circuit discussed above except that here we have three inputs.

If R1=R2=R3 = RF the equation becomes:

Vo = - [ V1 + V2 + V3]

If we make R1 = R2 = R3 = 3RF, we have a circuit that gives the average of the inputs. That is:

Vo = - 1/3[ V1 + V2 + V3]

# Difference amplifier

Fig.5 shows a difference amplifier. The input voltages are applied to both the inverting and non-inverting inputs simultaneously.

The output voltage is given by:

Vo = - (RF/R1) V1 + [Rg/(R2+Rg) (R1 + RF)/R1] V2

If R1 = R2 = RF = Rg, then:

Vo = V2 – V1

If RF = AR1 Rg = AR2 and R1 = R2, where A is the amplification also called the differential gain, then:

Vo = A (V2 – V1)

i.e. Vo = RF/R1 (V2 – V1)

The following pages includes some hands-on circuits if you wanted to see the op amp in action for any of the circuits we have described.

# Hands On: Follower Amplifier

Set up the follower amplifier using the schematic and wiring diagram shown. We shall start with the follower circuit as this is the easiest to set up.

Note carefully that the power supply positive is connected to pin 7 on the IC and the negative is grounded. Similarly, the negative of the power supply is connected to pin 4 and the positive is grounded. All grounds must go to a common point to avoid earth loops in the circuit. Set up a Sine Wave generator to 1 KHz 1 volt p-p and apply it to pin 3. The output is at pin 6.

The input and output are observed on a CRO. They should be identical. That is, the voltage gain is unity.

# Hands On: Non-Inverting Amplifier

Set up the non-inverting amplifier using the schematic and wiring diagram shown. The power supply connections are the same as for the Follower Amplifier circuit.

Measure the voltage gain using a CRO. It should be very close to 2. The output should be in phase with the input.

Using the equation Vo/Vi = 1+ RF/ R1 and RF = 10K and R1 = 10K calculate the voltage gain. Any difference between the measured and calculated values is due to component tolerances. Change RF to 27K and 47K and repeat the procedure.

# Hands On: Inverting Amplifier

Set up the circuit using the schematic and wiring diagram shown. Again, the power supply connections are the same as for the previous circuits. Observe the input and output waveforms on a CRO. The output is inverted with respect to the input. In fact, it has undergone a 180-degree phase change with respect to the input.

Measure the voltage gain from the waveforms on the CRO. It should be close to unity.

Using the equation Vo/Vi = -RF/R1 calculate the gain. Compare the measured and calculated values. They should be close. Any difference is due to component tolerances.

Change RF to 27K and 47K and repeat the procedure.

# Hands On: Summing Amplifier

Set up the circuit using the schematic and the wiring diagram shown. The power supply connections are the same as for the previous circuits.

The inputs to the amplifier come from the follower circuit and the oscillator. This gives good isolation between the two parts of the circuit.

Set the input to the follower circuit at 1V P-P 1KHz and measure the output of the summing circuit with a CRO. It should be 2 V and inverted with respect to the input to the follower. This value is in keeping with the equation we had earlier in this article viz.

Vo = - RF (V1/R1 + V2/R2)
and using the values shown in the schematic,
= -10(1/10 + 1/10)
= - 2V

We shall now change the inputs to the summing circuit to be different by replacing the follower by a non-inverting circuit as shown below. Measure the output of the non-inverting circuit with the CRO. It should be approximately 2V. This value is in keeping with the equation we had earlier in this article. Viz.

V2 = (RF/R1 + 1) Vi

Using the values shown in the schematic:

V2 = (10/10 + 1)
= 2V

Next, measure the output of the summing circuit. It should be -3V and is given by:

Vo = - (V1 + V2)
= - (1 + 2)
= - 3 V

# Hands On: Difference amplifier

Set up the circuit using the schematic and the wiring diagram shown. The power supply connections are the same as for the previous circuits.

Apply 1V P-P 1KHz to the input of the non-inverting circuit.

The inputs to the difference amplifier come from the non-inverting circuit and the oscillator. Using the component values shown on the schematic its output is given by:

V2 = (68/33 + 1)
= 3.06V

The output of the difference amplifier is given by:

Vo = RF/R1 (V2 – V1)
= 100/47 (3 – 1)
= 4.2V

Measure these values using a CRO. They should be very close to the calculated values.

About the contributor: Contributor, Elmo Janz, has published several articles in Australian Electronic Magazines and has self-published a book titled ‘Amplitude and Frequency Modulation Basics - A Programmed Instruction Unit’.