CHARACTERISTICS OF (741) OPERATIONAL AMPLIFIER

Aim: To understand the characteristics of an operational amplifier namely,

1.a. Input Offset Voltage.

1.b Input Bias Current.

2. Intrinsic input impedance.

3. The Slew Rate.

4. Common Mode Rejection Ratio (CMRR).

5. The closed loop response by calculating the gain-bandwidth product.(GBP).

Apparatus:

IC 741, Resistors, CRO, Power supply .

IC 741 Basic Pin Configuration:

1, 5 Offset Null, 2 Inverting Input

3- Non-inverting Input, 4- -Vcc

6- Output, 7 - + Vcc

8- No connection

 

 

I.                   a. Input offset voltage

Step 1. Make the circuit as shown in figure and measure the output voltage. Vos.

Typical values:- R1 = 100 KW, R2 = 1 MW

 

Note:

1.      The supply voltage must be + 15v.

2.      The output voltage will be between 50 and 60 mV [d.c].

Step 2

Calculate the closed loop gain ACL. Evaluate the input offset voltage.

 

 

b. Input bias current

Circuit diagram for input bias current

Typical values:- R1 = 100 kW, R2 = 1 MW , R3 = 100 kW

Bias current IB1 = VA / R1

Bias current IB2 = VB / R3

Supply Voltage = 15 V

 

Step 1.

Connect the circuit as shown in fig.2. Switch on the power supply. Measure the voltage across R1(VA). This will be in millivolts. Measure the voltage across the resistor R3 (VB). This will also be in millivolts.

Step 2:

Calculate IB1 and IB2 and take the average value (IB). This is called the average input bias current. This will be a few nano-amperes.

 

II.                Intrinsic Input impedance

Step 1.

Make the circuit connection as shown in fig. Adjust the input voltage (sine wave) at 1 V peak to peak, using an oscilloscope (use 0.5 V/div) . The frequency of the signal is kept at 1000 Hz.

R = 5 MW

[Hint: Zin = R when Vi2 = Vi]

Step 2.

Under the conditions mentioned in step1 obtain the value of the voltage using the other channel of the oscilloscope. Use again 0.5 V/division. Call this as V1i.

Step 3.

Adjust 5M r potentiometer till V12 falls to that of the input voltage Vi. Without disturbing the setting of the 5Mr potentiometer, disconnect it from the circuit and measure the resistance. This resistance is the input impedance.

 

III.             Slew rate:

 

 

 

R = 10 kW

Step1.

Set the oscilloscope for the following settings.

Ch. 1. 5 V/div.

Ch. 2. 1 V/div.

Step 2.

Connect the circuit as shown in figure. Adjust the square wave input voltage at 5v peak to peak and input frequency at 10 kHz.

Step 3.

Measure the peak to peak output voltage V and record the results.

Step 4.

Measure the time t that it takes for the output voltage to switch from its minimum to its maximum value.

Step 5.

Calculate the slew rate

Slew rate = V volts/μsec

t

IV CMRR.

 

Step 1.

Connect the circuit as shown in figure. Input frequency should be 100 HZ.

Step 2.

Adjust the input voltage to be 2 V RMS.

Step 3.

Measure the output voltage. Calculate the common mode gain.

R1 = 100 W, R2 = kW, R3 = 100 W, R4 = 100 kW

Differential gain Ad =

Common mode gain ACM =

 

CMRR =

 

CMRR in decibels = 20 log 10 (CMRR)

Repeat the experiment for different frequencies like 1kHz, 100 kHz, etc.

 

IV.              Closed loop response by calculating gain bandwidth product (GBP).

R1 = R2 = 10 kW

AV=R2/R1

R1 = R2 = 10 kW

Step 1.

Adjust the input signal so that the output is 0.707 V. (~ 0.7V). This must be done as accurately as possible. Measure the input voltage and calculate the gain.

Step 2.

Slowly vary the frequency of the input voltage so that the output voltage decreases to 0.5 V.

Step 3.

Note down the frequency at which the output voltage falls to 0.5V.

Bandwidth = --- kHz.

Calculate the gain Bandwidth product. Repeat the experiment for the following gain values: 2, 3 and 4. Find the GBW in all these cases. [GBW must remain a constant.]