Chapter 2     BASIC LINEAR AMPLIFIER CIRCUITS

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AN INTRODUCTION TO THE EXPERIMENTS

 

The following experiments are designed to demonstrate the design and operation of the fundamental linear amplifier circuits whose out­put signal is directly proportional to the input.

 

The experiments that you will perform can be summarized as follows:

 

Experiment No.                                   Purpose

  1. Demonstrates the design and operation of a voltage follower.

  2. Demonstrates the design and operation of a non-inverting amplifier.

  3. Demonstrates the design and operation of an inverting amplifier.

  4. Demonstrates the design and operation of a 2-input summing amplifier.

  5. Demonstrates the design and operation of a difference amplifier.

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EXPERIMENT NO. 1

 

Purpose

The purpose of this experiment is to demonstrate the operation of voltage follower, using a type 741 op-amp.

Pin Configuration of 741 Op-Amp (Fig. 2-19)

Schematic Diagram of Circuit (Fig. 2-20)

Discussion

 

The voltage follower is a special case of the non-inverting amplifier where all of the output voltage is fed back to the inverting input by a straight connection, as shown in figure 2-19. The straight feedback connection produces a voltage gain of approximately 1, so the closed loop gain of the voltage follower is

Closed Loop Gain:

The most important features of the voltage-follower configuration are its very high input resistance and its very low output resistance. These features make it a nearly ideal buffer amplifier for interfacing high-resistance source and low-resistance loads.

 

Design Basics

Voltage gain:

 

Step 1

Set your oscilloscope for the following settings:

  • Channels 1 & 2: 0.5 volt/division

  • Time base: 1 msec/ division

  • AC coupling

Step 2

First check your wired circuit, making sure that it is correct. Don’t forget the +V and —V power supply connections, as they are usually omitted from schematic diagrams! Pin 7 goes to ±V and pin 4 goes to -V. Apply power to the breadboard and observe the input and output traces on the screen of the scope.

NOTE:    Since we will be concerned with, both the input and output signals, we will adopt the convention that the input signal is Channel 1, and the output signal is Channel 2. When view­ing both signals simultaneously on a dual-trace oscilloscope, position the input signal so that it is above the output signal.

 

Step 3

Adjust the output of the generator so that the voltage is 1.5 volts peak-to-peak (3 vertical divisions), and the generator frequency so that there are at least 4 complete cycles on the oscilloscopes screen (at least 400 Hz). What is the difference between the input and output signals?

 

There is no difference between the two signals, as they are in phase. The output voltage is also 1.5 volts peak-to-peak. Conse­quently, the voltage gain of this voltage follower is 1.0, which is al­ways the case.

Step 4

Verify that the voltage gain of a voltage follower is always equal to by randomly varying the input voltage and measuring the corre­sponding output voltage.

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EXPERIMENT NO. 2

Purpose

 

The purpose of this experiment is to demonstrate the operation of a non-inverting amplifier, using a type 741 op-amp.

 

Schematic Diagram of Circuit (Fig. 2-21)

 

Discussion

 

An Op-amp connected in a closed-loop configuration in which the input signal is applied to the non-inverting input (+) is a non-inverting amplifier, as shown in Figure 6-14. A portion of the output is applied back to the inverting input (-) through the feedback circuit.

 

This constitutes negative feedback.

 

Design Basics

Voltage gain =

 

Step 1

Set the oscilloscope for the following settings:

  • Channels 1 & 2: 0.5 volt/division

  • Time base: 1 msec/ division

  • AC coupling

Step 2

Apply power to the breadboard and adjust the generator’s output voltage at 1 volt peak-to-peak and the frequency at 400 Hz (4 com­plete cycles). With the amplifier’s input signal positioned above the output signal on the oscilloscope’s screen, what is the difference be­tween the two signals?

 

The only difference between the two signals is that the output signal is larger than the input signal, as shown in Fig. 2-22. Both signals are said to be in phase, since the output signal goes positive exactly when the input does.

Step 3

What is the peak-to-peak output voltage?   volts

What then is the voltage gain?  

How does this compare with the equation given in the Design Basics section.

 

At this point have the instructor observe a working circuit.

Instructor signature______________________________

By the equation

Voltage gain = 1 + RB/RA

                       = 1 + 10k/10k

                       = 2.0

Step 4

 Keeping the input level constant at 1 volt peak-to-peak change resistor RB , and complete the following table. Do your experimental results agree with the design equation?

RB

Measured V, (peak-to-peak)

Voltage Gain

22Kohm

  Volts 

 

33Kohm

  Volts

 

47Kohm

  Volts

 

82Kohm

  Volts

 

 

 

 

 

 

 

 

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EXPERIMENT NO. 3

 

Purpose

 

The purpose of this experiment is to demonstrate the operation of the inverting amplifier, using the type 741 op-amp.

 

 

Fig. 2-23.

 

Discussion

 

An op-amp connected in a closed-loop configuration in which the input signal is applied through a series resistor to the inverting input (-) is an inverting amplifier, as shown in Fig. 2-23. The output is fed back through Rf  to the inverting input. The non-inverting input is grounded.

 

At this point, the ideal op-amp parameters mentioned earlier are useful in simplifying the analysis of this circuit. In particular, the concept of infinite input resistance is of great value. An infinite input resistance implies that there is no current in or out of the inverting input. If there is no current through the input resistance, then there must be no voltage drop between the inverting and the non-inverting inputs. This means that the voltage at the inverting input (-) is zero because the non-inverting input(+) is grounded. This zero voltage at the inverting input terminal is referred to as virtual ground. This condition is illustrated in Fig. 2-23.

 

Design Basics

 

Voltage gain = VO/Vin = -RB/RA

Step 1

Set the oscilloscope for the following settings:

  • Channels 1 & 2: 0.5 volt/division

  • Time base: 1 msec/ division

  • AC coupling

 

Step 2

 

Apply power to the breadboard and adjust the generator’s output voltage at 1 volt peak-to-peak and the frequency so that there are about 5 complete cycles for the 10 horizontal divisions (500 Hz).

The output signal is of oppositive form, or is inverted, compared with the input signal. The output is said to be inverted, or 180o out-of-phase with the input, since the positive peak of the output signal occurs when the input’s peak is negative.

Step 3

What is the peak-to-peak output voltage?   Volts

  

At this point have the instructor observe a working circuit.

Instructor signature______________________________

The peak-to-peak output voltage should be 1 volt, which is the same as the input. Consequently, the voltage gain is --1.0, where the minus sign indicates that the output is inverted with respect to the input. Also, by the equation:                  

voltage gain = - RB/R= -10k/10k   = -1.0

Step 4

Keeping the input level constant at 1 volt peak-to-peak, change resistor RB, and complete the following table. Do your experimental results agree with the design equation?

RB

Measured V. (peak-to-peak) 

Voltage Gain

22Kohm

  Volts

 

32Kohm

  Volts 

 

47Kohm

  Volts

 

82Kohm

  Volts

 

 

 

 

 

 

 

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EXPERIMENT NO. 4

 

Purpose

 

The purpose of this experiment is to demonstrate the operation of a 2-input summing amplifier, Using a type 741 op-amp:

 

Schematic Diagram of Circuit (Fig. 2-25)

Design Basics

      Output voltage:  

Step 1

Set the oscilloscope for the following settings:

 

     Channel 1: 1 volt/division

     Time base: 1 msec/ division

     AC coupling

 

Step 2

 

Apply power to the breadboard and adjust the peak-to-peak out­put voltage of the function generator (V1) at 1 Volt and adjust the frequency so that there are about 3 full cycles on the scope’s screen (300 Hz).

Step 3

Measure the output voltage at the output of the 1st op-amp (V2). What is it?              volts.

You should have measured a peak-to-peak voltage of 1 volt, since this portion of the circuit is just a voltage follower whose operation was described in Experiment No. 1.

Step 4

Measure the voltage at the output of the 2nd op-amp (V0). What is it?   volts.

At this point have the instructor observe a working circuit.

Instructor signature______________________________

Why?                

 

This 2nd amplifier is the summing amplifier, adding the two input voltages V1 (1 volt) and V2 (also 1 volt). This can be verified by the equation in the Design Basics section so that:


        = - 2.0 volts


The negative sign occurs because we are using the op-amp as an inverting amplifier, so that the output is inverted with respect to the sum of the two inputs which are in phase If we are able to simultaneously observe V1, V2, and V0 on the oscilloscope’s screen.

Step 5

 So far we have only presented the simple case of adding two equal voltages. To demonstrate that the equation in Step 4 and the opera­tion of the summing amplifier still hold for unequal input voltages, disconnect the power from the breadboard and rewire only the 1st op-amp as a non-inverting amplifier, as shown in Fig. 2-27. The 2nd op-amp remains connected as before.

Fig. 2-27

Step 6

Apply power again to the breadboard. What is V2 now (i.e., the output voltage of the new circuit for the 1st op-amp)? Is it what you expected?  

 Step 7

Now measure V0 (the output voltage of the 2nd op-amp). What is it?   volts.

Step 8

Again, disconnect the power supply and rewire the 1st op-amp as a unity-gain inverting amplifier, as shown in Fig. 2-29.

Fig. 2-29

Step 9

Apply power to the breadboard and now measure V0. What do you get?   volts.

You should measure no output voltage! Why?

 

 

You would probably think that the output voltage (V0) would be 2 volts, since V1 and V2 are now each 1 volt. I have played a little trick on you. In Step 8 we were using a unity-gain amplifier, so that the output voltage (V2) was inverted with respect to its input, V1. When these two equal, but out-of-phase voltages were added, they cancelled each other, resulting in a net output of zero. This can be seen by looking at V1, V2, and V0 simultaneously.

 

When V1 goes positive, V2 goes negative by an equal amount. When V1 and V2 are added, the net result is zero. The same analysis applies for when V1 goes negative. In Steps 1 through 7, the two input volt­ages were always in phase.

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EXPERIMENT NO. 5

 

Purpose

 

The purpose of this experiment is to demonstrate the design and operation of an op-amp difference amplifier, using a type 741 op-amp.

 

Schematic Diagram of Circuit (Fig. 2-31)

Design Basics

Vo =

When: R1 = R3, R2 = R4

Step 1

 

Wire the circuit shown in the schematic diagram and then apply power to the breadboard.

Step 2

First connect the non-inverting input resistor (R3) to point 1 and the inverting resistor (R1) to point 2 on the resistor divider string.

 Step 3

 

With your voltmeter, measure the dc input voltages V1 (VB ) and V2 (VA ), recording your results below:

                                                            V1 = VB =   volts

                                                            V2 = VA =   volts

                                                                VB - VA =   volts

Step 4

Now with your voltmeter, measure the output voltage Vo, and re­cord your result below:

                                                             Vo=   volts

Step 5

Now reverse the input connections so that R1 is connected to point 1 and R3 is connected to point 2. Repeat Steps 3 and 4, recording your results below:

 

                                                            V1 = VA  =   volts

                                                            V2 = VB  =   volts

                                                            VB – VA  =   volts

                                                            VO  =            volts