LM747 – Dual Operational Amplifier IC

LM747 is a general purpose dual operational amplifier IC and is a member of ‘LM xx’ family. The ‘LM xx’ series are integrated circuits designed by National Semiconductor. The prefix LM stands for linear monolithic, referring to the analog components integrated onto a single piece of silicon. The two amplifiers share a common bias network and power supply leads. Otherwise, their operation is completely independent. The chip is short circuit protected and the internal frequency compensation ensures stability without external components.

 

Pin configuration

LM747 is a 14 pin device as shown in above LM747 pinout and description for each pin is given below.

Pin	Name	Description
4	V-	Common negative supply voltage for both Op-amps
11	NC	No connection
OP-AMP1
12	1OUT	Output pin of the op-amp 1
1	1IN-	Inverting input of op-amp 1
2	1IN+	Non-Inverting input of op-amp 1
3,14	OFFSET  NULL 1	Offset null pin used to eliminate the offset voltage and balance the input voltages for op-amp 1
13	V1+	Positive supply voltage for op-amp1
OP-AMP2
6	2IN+	Non-Inverting  input of op-amp 2
7	2IN-	Inverting  input of op-amp 2
10	2OUT	Output pin of the op-amp 2
5,8	OFFSET  NULL 2	Offset null pin used to eliminate the offset voltage and balance the input voltages for op-amp 2
9	V2+	Positive supply voltage for op-amp 2

The device is available in many packages and can be chosen depending on requirement.

 

LM747 Features and Electrical characteristics

• Low power consumption
• Low noise interference among op-amps
• Short-circuit Protection
• No frequency Compensation Required
• No latch-up
• Large common mode and differential voltage range
• Maximum supply voltage: ±22V
• Differential input voltage: ±30V
• Common-Mode Rejection Ratio: 90dB
• Operating temperature: -55ºC to +125ºC
• Total power dissipation: 800mW

 

LM747 Alternative ICs

LM158, LM258, LM358, LM4558, LM2904

 

LM747 Overview

As mentioned earlier LM747 IC consists of two general purpose operational amplifiers and so the chip can be used to design any op-amp circuits like comparator, differential amplification or mathematical operations. More over the two op-amps can perform two different functions at a time which comes in handy in applications. Also the device has offset pins for making the output more accurate which is a must in some applications. This device is used to be popular a decade ago but at present there are many op-amp chips which are more accurate and more efficient. 

 

How to use LM747 IC

Before going for application of the device let us see the internal connection of two op-amps in the device as shown below.

The two op-amps in the chip can be used to design most of op-amp based circuits like voltage follower, comparator, differential amplification and mathematical operations. And to understand the working of device we will use a single op-amp to construct a simple microphone pre-amplifier circuit as given below.

In the above microphone amplifier circuit the op-amp acts as a Non-Inverting Amplifier (Since input is given to Non-inverting terminal of op-amp.

In circuit:

• Microphone output signal is given as input to op-amp for amplification
• R1 and C1 form a high pass filter for cutting of DC signal from microphone
• Amplified output is listened through the small speaker connected at the output
• R2 and R3 resistors form feedback loop for the non- inverting amplifier of op-amp.
• The equation for the output Vo =Input voltage * Gain = Vi * A = Vi * (1+R2/R3)

For example:

Say R2 = 1MΩ, R3 = 1kΩ and microphone output voltage = 1mV

We will have voltage output Vo= 1m*(1+1000)=1V approximately

 

This voltage appears across the speaker and we can listen to the sound. And with that we have realized the amplifier circuit of op-amp and in a similar way we use the device to design all other application circuits of op-amp.

 

Applications

• Analog circuits
• Mathematical operations
• Amplifiers
• Voltage comparators
• Peak detectors
• Industrial
• Measuring instruments

 

2D-Model

All dimensions are in inches [millimeter measurements in brackets]