what is Op-amp? |Block diagram of op-amp |Ideal Characteristics of Operational Amplifier |Electrical Parameters of op-amp

In this article, we will study the basics of the op-amp, and then we will discuss the block diagram of the op-amp (operational amplifier) and its block in a detailed manner. We will also investigate the characteristics of the operational amplifier, Ideal op-amp characteristics, and Electrical parameters of the op-amp.

What is Op-amp (Operational Amplifier)?

• An operational amplifier is a direct-coupled high gain amplifier usually consisting of one or more differential amplifiers and usually followed by a level translator and an output stage. The output stage is generally a push-pull or push-pull complimentary symmetry pair. An operational amplifier(op-amp) is available as a single integrated circuit package.

• The op-amp(operational amplifier) is a versatile device that can be used to amplify DC as well as ac input signals and was originally designed for performing mathematical operations such as addition, subtraction, multiplication, and integration. Thus the name operational amplifier stems from its original use for these mathematical operations and is abbreviated to ap-amp.

• With the addition of suitable external feedback components, the modern-day op-amp can be used for a variety of applications such as ac and dc signal amplification, active filters, oscillators, comparators, regulators, and others.

Also Read: Classification of Amplifiers

Block Diagram of Op-amp (Operational Amplifier)

• Since an op-amp is a multistage amplifier, it can be represented by a block diagram as shown in the figure below.

• The input stage is the dual input, balanced output differential amplifier. This stage generally provides most of the voltage gain of the amplifier and also establishes the input resistance of the op-amp.

• The intermediate stage is usually another differential amplifier, which is driven by the output of the first stage. In most amplifiers, the intermediate stage is dual input and unbalanced(single-ended) output. Because direct coupling is used, the dc voltage at the output of the intermediate stage as well above ground potential.

• Therefore, generally, the level translator circuit is used after the intermediate stage to shift the dc level at the output of the intermediate stage downward to zero volts with respect to the ground.

• The final stage is usually a push-pull complementary amplifier output stage. The output stage increases the output voltage swing and raises the current-supplying capability of the op-amp.

• A well-designed output stage also provides low output resistance.

Electrical Parameters of op-amp(operational amplifier)

• There are various electrical parameters defined for the op-amp which is listed in the following:

1. Input Offset Voltage:

• The input offset voltage is the voltage that must be applied between the two input terminals of an op-amp to null the output.
• We denote the input offset voltage by V_{io}. This V_{io} could be positive or negative.
• The smaller the value of V_{io}, the better the input terminals are matched.

2. Input offset current:

• The algebraic difference between the currents into the inverting and non-inverting terminals is referred to as an input offset current, I_{io}.

I_{io} = \left | I_{B1} - I_{B2} \right |

• Where I_{B1} is the current into noninverting input and I_{B2} is the current into the inverting input.

• As the matching between two input terminals is improved, the difference between I_{B1} and I_{B2} becomes smaller.

3. Input Bias Current:

• Input bias current, I_B is the average of the currents that flow into the inverting and noninverting input terminals of the op-amp.

I_B = \frac{I_{B1}+I_{B2}}{2}

4. Differential Input Resistance:

• Differential Input Resistance R_i, (often referred to as input resistance) is the equivalent resistance that can be measured at either the inverting or noninverting terminal with the other terminal connected to the ground.

5. Common Mode Rejection ratio:

• The common mode rejection ratio (CMRR) is defined in several essentially equivalent ways by various manufacturers.

• Generally, it can be defined as the ratio of the differential voltage gain A_d to the common-mode voltage gain A_cm; that is

CMRR=\frac{A_d}{A_{cm}}

• The differential voltage gain Ad is the same as the large signal voltage gain A, which is specified on the data sheets; however the common mode voltage gain can be defined as,

A_{cm}=\frac{V_{ocm}}{V_{cm}}

where

• V_{ocm}= output common-mode voltage
• V_{cm} =input common-mode voltage
• A_{cm}= Common-mode voltage gain

• The higher the value of CMRR, the better the matching between two input terminals and the smaller is the output common-mode voltage.

6. Supply Voltage Rejection Ratio:

• The change in an op-amp’s input offset voltage. V_{io} caused by variation in supply voltage is called the supply voltage rejection ratio(SVRR). A variety of terms equivalent to SVRR are used by different manufacturers, such as the power supply rejection ratio(PSRR) and the power supply sensitivity(PSS).

• If we denote the change in supply voltage by \Delta V and the corresponding in input offset voltage by \Delta V_{io}, SVRR can be defined as follows:

SVRR=\frac{\Delta V_{io}}{\Delta V}

7. Slew Rate:

• Slew Rate (SR) is defined as the maximum rate of change of output voltage per unit of time and is expressed in volts per microsecond.

SR=\frac{\mathrm{d} V_o }{\mathrm{d} t} |_{maximum}

• The slew rate indicates how rapidly the output of an op-amp can change in response to a change in the input frequency.

• The slew rate changes with a change in voltage gain and is normally specified at unity.

• The slew rate of an op-amp is fixed; therefore, if the slope requirements of the output signal are greater than the slew rate, then distortion occurs.

• Thus slew rate is one of the important factors in selecting the op-amp for ac applications, particularly at relatively high frequencies.

Ideal Characteristics of Op-amp(Operational Amplifier)

An ideal op-amp would exhibit the following electrical characteristics :

1. Infinite voltage gains A.
2. Infinite input resistance Ri so that almost any signal source can drive it and there is no loading of the preceding stage.
3. Zero output resistance Ro so that the output can drive an infinite number of other devices.
4. Zero output voltage when the input voltage is zero.
5. Infinite bandwidth so that frequency signal from zero to infinite Hz can be amplified without attenuation.
6. Infinite common-mode rejection ratios that the output common-mode noise voltage is zero.
7. Infinite slew rate so that output voltage changes occur simultaneously with input voltage changes.

Equivalent Circuit of An Op-amp

• The below figure shows an equivalent circuit of an op-amp(operational Amplifier). This circuit includes important values i.e. A, R_i, and R_o.

• Note that A v_{id} is an equivalent Thevenin voltage source, and Ro is the Thevenin equivalent resistance looking back into the output terminal of op-amp.

• The equivalent circuit is useful in analyzing the basic operating principles of op-amps and in observing the effects of feedback arrangements.

• The output voltage for the circuit shown is,

v_o=Av_{id}= A(v_1-v_2)

Where,

• A = large signal voltage gain
• v_{id}=difference input voltage
• v_1 =voltage at the non-inverting input terminal with respect to ground
• v_2= voltage at inverting terminal with respect to ground

• The above equation indicates that the output voltage v_o is directly proportional to the algebraic difference between the two input voltages.

• In other words, the op-amp amplifies the difference between the two input voltages; it does not amplify the input voltages themselves.

• For this reason, the polarity of the output voltage depends on the polarity of the difference voltage.

Frequently Asked Questions on Op-Amp