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LM741 Opamp amplfier problems

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babboy12345

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i am using 741 to amplifier my Vin in non inverting .But why i dint supply Vin to my circuit,there still got value for Vout???
 

ericgibbs

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i am using 741 to amplifier my Vin in non inverting .But why i dint supply Vin to my circuit,there still got value for Vout???
The 741 is an older design, the outputs will not swing from 0v to +Vsupply.

Can you post a circuit drawing.?
 
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babboy12345

New Member
the circuit just a very simple non inverting amplifier. i still face the problems when Vin=0,but Vout still got value?

From theory,when Vin=0,Vout also =0 rite??
 

ericgibbs

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the circuit just a very simple non inverting amplifier. i still face the problems when Vin=0,but Vout still got value?

From theory,when Vin=0,Vout also =0 rite??
Not for a 741, like I said, if you are using a single supply.:)

If you look at the 741 datasheet the output could say be 1V with the input at 0V.

Modern designs dont use the outdated 741.

To get 0V for 0V in , you need a dual supply.OK.

EDIT: what volts do you measure on the output,?
 
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babboy12345

New Member
R2= 220
R1=100
Vin=0
Vout=8v



i follow the diagram design my circuit.Now i am using LM 324N
 

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audioguru

Well-Known Member
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Your schematic does not show power supply voltages. Maybe it has no power supply.
An ordinary opamp cannot drive a load as low as only 320 ohms. Your feedback resistor values are much too low and become the load. Try 22k and 10k instead of 220 ohms and 100 ohms.
 

Roff

Well-Known Member
i just wan to know,if i dint supply any Vin to my circuit,is that my vout will be zero???
If you have both positive and negative power supplies connected to the +V and -V pins respectively, and if you connect the input pin to ground (not floating), then your output should be very close to zero volts. If you haven't done both of these things, the the output will not be zero volts.
 
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chinsoon

New Member
he hasnt compensate for the offset voltage?

i thought it is a known fact that only ideal op amps have the assumption that Vo=0V when Vi=0V. however, in real life, u often get a small amount of votlage (in mV or uV range) at the output before putting in any input...

correct me if i am wrong...

=.=
 

tresca

Member
Thats right.
There is a dc offset with practical opamps.

As mentioned above, without having a dual supply, and the input pin connected to ground (i.e not left open), the output will be CLOSE but not at 0V due to the dc offset.
 

confounded

New Member
haha at last i can post a useful answer! (i hope)
Funny enough i am reading about the 741 in my textbook at the moment, and am having a related problem with the 741.
There are several things that cause a non 0v output for a 0v input;
Input offset voltage
Input bias current
Input offset current

These effects may sum or partially cancel each other!

Input offset voltage is due to manafacturing variations causing non perfectly balanced input stages. The difference in input voltage you must supply to give 0v output is called input offset voltage.
For the 741 you connect a 10k pot between pins 1 and 5 with wiper connected to the -ve supply rail VEE
The input offset voltage will drift with temperature and time however.

Input bias current is half the sum of the input current with inputs tied together.
Input bias current is simply the base/gate current of the input transistors.
A 741 has bipolar juction transistors on input which draw a relativly high current compared to Jfet input op amps such as the 411
Input bias current causes a voltage drop across the feedback resistors.
To make its effects negligable you must ensure resistance of inputs to op amp match.

Input offset current is due to manafacturing variations resulting in non symmetrical input circuit. The result is even if u match source impedances for input bias current the voltage drop across the resitors will not be the same so the input sees a difference voltage which it amplifies to give non 0v output. The only 'remedy' to this error is to use resistors of moderate value as the larger the resistor the higher the voltage drop.
 

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