INTRODUCTION:

Operational amplifiers are widely used in our audio amplifiers, inside computers (i.e, DAC and ADC etc), instrumentational amplifier (i.e, used in industries for sensing), comparators, oscillators, filters, log and Antilog amplifier, V-I and I-V converters, integrators, differentiators, inside 555 timers wave generators etc.

Do you ever think what basically is inside an op-amp (IC 741)? , how it is configured for performing various functions as mentioned above and what are its characteristics.  I hope that this article will provide you the basic knowledge about the operational amplifier.

As its name implies OPERATIONAL AMPLIFIER, what will you think about the word OPERATIONAL?. If you are thinking about mathematical operations then you are right. It is a device originally designed for performing a mathematical operation (i.e, addition, subtraction, integration and differentiation) and to deal with analog signals. With the addition of suitable external components, the modern day’s op-amps can be used for a variety of application as mentioned above. It can also be used for amplifying AC as well as DC signals, the word AMPLIFIER  refers to increasing the strength of a signal.

It is very high gain amplifier consisting of differential amplifiers, consist of the level translator and an output stage. OP-amp is basically a configuration of transistors with a suitable value of resistors.

LET’S TAKE A BRIEF REVIEW ABOUT IC 741:

In 1965, Fairchild introduced uA709 which is a first generation op-amp. It has several disadvantages, which requires external components for its protection. The disadvantage is: there is no short circuit protection, latch-up problem, frequency compensation problem which require external components(i.e, 2 capacitors and a resistor).

In 1968, Fairchild introduced uA741 an internal compensated property. Unlike uA709, it has no latch-up problem, protected against short circuit, and frequency stability. uA741 is also known as second generation op-amp.

IC IDENTIFICATION AND PIN CONFIGURATION OF IC 741

Since nowadays many manufacturers are producing 741 op-amps. How can we identify 741 op-amps is manufactured by a particular manufacturer. Many op-amps are identified by using a seven-character ID code. For eg  

Firstly, the prefix identifies a particular manufacturer. Secondly, the designator which tells us two things ( (i) three-digit number identifies the type of op-amp.  (ii) the last letter indicates the operating temperature of a particular op-amp). Thirdly, the suffix indicates the type of packages.  

Pin 7 and pin 4 is used as supply pins, at pin 7 positive supply is to be connected and at pin  4 negative supply is to be connected. It has two inputs (i.e, one is inverting input pin 2 and another is non inverting input pin 3). Pin 1 and pin 5 is offset null pins, a potentiometer (typical value of 10k) is connected between these terminals to set the output to be zero. Pin 8 has no connection with the internal circuitry of the op-amp, it is made for full fill the standard 8-pin packaged IC.  

INTERNAL CIRCUITRY OF IC741 OP-AMP:
     

Basically, 741 bipolar transistor op-amp IC comprises 20 BJT transistors. Now to understand the internal circuitry let’s divide circuit into different blocks.

1. The block outlined in blue comprises of a differential amplifier.
2. The blocks outlined in red comprises current mirrors.
3. The block outlined in magenta, comprise of class A amplifier (i.e, voltage amplifier)
4. The block outlined in green and cyan comprises of a level translator and output amplifier (i.e, class AB amplifier).

Now let’s understand each of the blocks:

The differential amplifier firstly comprises of a matched NPN emitter follower Q1 & Q2 which provides high input impedance and provide gain, secondly, a matched common base Q3 & Q4 PNP transistors which are used to drive the active load Q7, Q5, and Q6. Q5 and Q6 are the matched pair and perform the function of the differential amplifier for the offset null input signal. The current of Q5 & Q6 is controlled by varying a 10k pot connected between the input pins 1&5. The transistors Q1&Q3 are cascaded in series and Q2&Q4 is also cascaded which provides high gain when the input is applied at its input terminals. The differential amplifier also has the capability of rejecting common signals(i.e, noises which are common at both input terminals).

The current mirrors comprise of (Q8-Q9) and (Q12-Q13) are configured as Wilson current mirror. Whereas, transistors Q10-Q11 are configured as wider current mirror these current mirrors maintain constant quiescent current to the circuit for stable operation.

The class A amplifier consists of two NPN transistors Q15&Q19 which is configured as Darlington pair and provides voltage gain, transistor Q22 is used to prevent excess current provided to Q20 (i.e, sink transistor which receives current form the common collector Darlington pair).

Transistor Q16 along with 4.5k and 7.5k resistor (known as voltage level shifter), this circuit uses to prevent the output signal from distortion. Now the amplifier at the output stage in a class AB amplifier (consisting of Q14, Q17&Q20). Q14 & Q20 is a complimentary class AB amplifier which provides output impedance to be (typically 50-75 Ohms)  and provides current gain. Whereas Q17 limits the current at the output.

The current from the current mirror (Q8 and Q9) is divided into the differential amplifier comprises of (Q1-Q3) & (Q2-Q4). Now the current from the common base transistors (Q3&Q4) is summed with the current of the wider current mirror (Q10&Q11), Q7 is used to drive Q5 and Q6. The quiescent current is set in Q16 and Q19 is set due to Wilson current mirror (Q12&Q13). The value of 30pF is used for frequency compensation.  

CONFIGURATION  OF OP-AMP(IC 741):

Two main configurations of op-amp

1) Open-loop configuration: in this configuration 741IC can be used as a very high gain amplifier. In open-loop, the gain is ideally infinite hence the output will saturate either in positive supply voltage or in negative supply voltages. This open-loop system has three basic configurations:

a) DIFFERENTIAL AMPLIFIER:
b) INVERTING AMPLIFIER:
c) NON-INVERTING AMPLIFIER:

2) Close-loop configuration: in this configuration, it is connected as negative feedback arrangement. The feedback network is through a resistor(i.e, a passive component)

a) NON-INVERTING AMPLIFIER:  the input is applied at the non-invert input terminal. The gain can be calculated using the value of resistors. Rf is feedback resistor.

Av= 1 + (Rf / R1)

b) INVERTING AMPLIFIER: the input is applied at inverting terminal.

       Av= – (Rf / R1)

c) DIFFERENTIAL AMPLIFIER: in this input is applied at both the inputs, which will amplify the difference b/w the two inputs.

Av=

d) VOLTAGE FOLLOWER: It is an arrangement using a non-inverting amplifier, in this instead of giving a feedback through a resistor, we will short the feedback Rf and open R1. The gain of feedback circuit will be reduced to (Av= 1), it is used as a buffer. In this, the output is equal to the input. For eg, used in instrumentation amplifier, to pick a very small signal (i.e, in mV) from a transducer.

CHARACTERISTICS OF IC 741(OP-AMP):

1) INPUT BIAS CURRENT:

Since op-amp comprises of BJT transistors, a differential amplifier which requires dc biasing current for stable operation. The value of quiescent dc biasing current drawn by op-amp is called input bias current rating of an amplifier.

uA741 the rating is between 80nA(typical) to 500nA(maximum). The input bias current is the average of two input base current.

I(bias) =  ( I(b1) + I(b2)  ) / 2

2) INPUT OFFSET CURRENT:

The difference between the current into inverting and non inverting input terminals is referred to as input offset current. It tells us how larger one current is from another. 741 has typically 20nA of this current.

Smaller the offset current better the op-amp.

I(b1)= non-inverting  input terminal current

I(b2)= inerting input terminal current

I(io)= |  I(b1) – I(b2)  |

3) INPUT OFFSET VOLTAGE:

This voltage is applied between two input terminal to make an output of an op-amp to be null. 741 has a worst-case input offset voltage of 5mV.

4) SLEW RATE:

Slew rate identifies that how much maximum frequency at the input is applied in order to prevent the output from distortion. 741 has a slew rate of typical 0.5 volts per microsecond (V/us). It is measured by applying a pulse using function generator at the input while connecting the channels of the oscilloscope at the input and output (set oscilloscope at dual mode). This is a very important parameter.

5) Output load:

It requires load at an output greater than 2k Ohm. It has an input impedance of about 2Megaohms, and output impedance in between (50-75) Ohms. Its open loop gain is about 200,000 for lower frequencies.

FREQUENCY RESPONSE:

In open loop, the op-amp 741 has very high gain, this response is not very good. To improve the gain we apply negative feedback network, by applying a negative feedback network the gain starts to fall away quite dramatically, by applying more negative feedback the bandwidth becomes wider. (bandwidth is referred  to the ranges of frequencies the op-amp will support).