Characterization of Op-Amp

Objective:

To measure the input bias current, input offset current, input offset voltage and the slew rate, bandwidth and CMRR of Op-Amp.

Theory:

An op-amp is a high gain, direct coupled differential linear amplifier choose response characteristics are externally controlled by negative feedback from the output to input, op-amp has very high input impedance, typically a few mega ohms and low output impedance, less than 100Ω. Op-amps can perform mathematical operations like summation integration, differentiation, logarithm, anti-logarithm, etc., and hence the name operational amplifier op-amps are also used as video and audio amplifiers, oscillators and so on, in communication electronics, in instrumentation and control, in medical electronics, etc. 

Op-Amp terminals:

The circuit schematic of an op-amp is a triangle as shown below in Fig.  op-amp has two input terminals. The minus input, marked (-) is the inverting input. A signal applied to the minus terminal will be shifted in phase 1800 at the output. The plus input, marked (+) is the non-inverting input. A signal applied to the plus terminal will appear in the same phase at the output as at the input. V+/V- denotes the positive and negative power supplies. Most op-amps operate with a widerange of supply voltages. A dual power supply of +15V is quite common in practical op-amp circuits. The use of the positive and negative supply voltages allows the output of the op-amp to swing in both positive and negative directions.

Apparatus /Components required:

  1. Power Supply: Dual variable regulated low voltage DC source
  2. Equipment: CRO, Function Generator, DMM (Digital Multimeter)
  3. Resistors:
  4. Semiconductor: IC741 op-amp
  5. Miscellaneous: Bread board and wires.

Off-Set Null:

Because of mismatches in any of the aforementioned components, current flows into the two branches of the input differential amplifier unequally. Therefore, adding the appropriate R1 and R2 to the null pins balances the voltage at the two input terminals. 

 

Input bias current and input offset current:

 Procedure:

  1. Connect the circuit as given in the figure.
  2. Using a DMM, measure the dc voltage at the (-) terminal & record the values in Table.
  3. By ohm’s law, calculate the input currents; IB+ and IB-. Average these values to find out the input Bias current. Also, find the difference between these two currents to know the input offset current. Record these values in Table.

Observation:

DC voltage at the noninverting terminal V+ DC voltage at the inverting terminal V-  

IB+=V+/220k

 

IB=V/220k

Input bias current

IB=(IB+ + IB)/2

Input offset current

IB=(IB+ – IB)

           

Input offset voltage:

 

 

Procedure:

Connect the circuit of Figure.

Measure the DC output voltage at pin 6 using multi-meter and record the result in Table.

Calculate the input offset voltage using the formula Vi = Vout / 1000 and record the value in table.

 

Observation:

 

Vout V in = Vout/1000

 

Slew rate and bandwidth: 

 

Procedure:

  1. Connect the circuit of Figure.
  2. Provide a 1V peak to peak square wave with a frequency.
  3. With an oscilloscope, observe the output of OPAMP. Adjust the oscilloscope timing the get a couple of cycles.
  4. Measure the voltage change ∆V and time change ∆T of the output waveform. Record the results in Table.
  5. Calculate the slew rate using the formula

SR = ∆V / ∆T 

  1. Calculate bandwidth using the formula

Fmax = SR/(2πVP)

Where, VP →à Peak voltage of output Sine wave

 

Observation: 

VP ∆V ∆T SR = ∆V / ∆T

 

Bandwidth(Fmax)

CMRR: 

The ability of a differential amplifier to reject a common-mode signal is expressed by a ration called Common Mode Rejection Ratio, denoted as CMRR”. 

V1 and V2 are the two input signals and VO is the output. In an ideal op-amp, VO is proportional to the difference between the two signal voltages.

Vod∝ (V1-V2)

Vod = Ad (V1-V2)

=AdVd

 

Where, Ad –>differential gain.

If we apply two input voltages which are equal in all respects to the differential amplifier, ie., if V1=V2, then ideally the output voltage, VO = Ad(V1-V2) must be zero. But the output voltage of the practical differential amplifier not only depends on the difference voltages, but also depends on the average common level of the two inputs. Such an average level of the two input signals is called common mode signal denoted as VC.

VC = (V1+V2)/2

Voc = AC VC 

CMRR is defined as the ratio of the differential voltage gain Ad to common mode voltage gain Ac.

CMRR = Ad/AC

Procedure:

  1. Connect the circuit of Figure.
  2. With an oscilloscope, observe the output Vod and measure Vd of OPAMP and find Ad for two different input voltages V1 and V2.
  3. With an oscilloscope, observe the output Voc and measure VC  of OPAMP and find AC for V1 = V2.
  4. Calculate CMRR using the formula CMRR = Ad/ AC

 Conclusion: Should follow result in conformation with theory.

 

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