Gain of the opamp 741 as per circuit diagram

Status
Not open for further replies.

rajaram04

Advanced Member level 3
Joined
Jun 17, 2012
Messages
871
Helped
6
Reputation
12
Reaction score
5
Trophy points
1,298
Location
earth
Visit site
Activity points
7,687
Hello sir

Here i have shown a circuit diagram of an opamp 741 I.C.

please tell me how much would be the value of Gain if the supply voltage is

a) 6 volts
b) 9 volts
c) 12 volts
d) 15 volts
e) 17 volts

 

Gain is not dependent on supply voltage but the 741 may not function properly with only 6V or 9V supply. Also a 10M feedback resistor may be too high to provide enough feedback current for this very old bipolar op-amp.

Brian.
 

betwixt said:
Gain is not dependent on supply voltage but the 741 may not function properly with only 6V or 9V supply. Also a 10M feedback resistor may be too high to provide enough feedback current for this very old bipolar op-amp.

Brian.
Click to expand...



okk sir but i applied 9 volts 250mA supply to the arrangment & when pointed a simple I.R led with supply towrads it , the output led i connected at pin 6 started glowing & too circuit is responding from a long dist.of 1.5 metres . . so i just want to know how much gain it has so that it is showing such character
 

As it is shown, the gain cannot be calculated. The output at pin 6 depends on what you are feeding into the non-inverting input just as much as it depends on the voltage from the photodiode. Obviously it is amplifying and I suspect if it isn't going unstable the gain is very high. Be aware that the bandwidth of a 741 in this configuration will be very narrow, maybe only a few KHz so if you are using it for IR commuication the output waveform will be extremely distorted.

Brian.
 

The circuit is sketched incorrectly or at least incomplete because the N.I. OP input is unconnected. At present, even the photo diode operation mode can't be clearly determined.

I would expect that the circuit is intended as I/V converter (transimpedance amplifier) with zero photo diode bias. In this case, a 741 would need a bipolar power supply or the photo diode and N.I. input must be connected to a virtual ground.

Typical 741 input bias current is 80 nA, in this case the circuit can possibly work with a 10M resistor.
 




hmm sir it is responding to normal light also

- - - Updated - - -





N.I. means which one ?
 

That's why they are called photo-diodes, they respond to light!

If you want it to respond only to IR you have to use a type with a built in filter or provide one outside the diode to block other wavelengths.

Brian.
 

betwixt said:
That's why they are called photo-diodes, they respond to light!

If you want it to respond only to IR you have to use a type with a built in filter or provide one outside the diode to block other wavelengths.

Brian.
Click to expand...




okk thanks sir , then please tell me what ll i ve to do practically ? what i ve to buy or what changes i ll hv to made ?
 

It depends what you are trying to achieve. If this is for responding to an IR remote control unit you need much wider bandwidth and there are specialized circuits that would be easier to use than a 741. If you are trying to measure the amount of IR light (a light level meter) you CAN use the 741 but better alternatives are available.

To filter out visible light you need a window that only passes IR wavelengths, you can buy IR filters (they look like black plastic) or a cheap source is from older TV/VCR remote control units. Many had a plastic window in them that looks deep red or violet when you shine white light through them. They are not as good as proper IR filters but they will block most of the visible light. You can put more than one of these windows in front of each other to get better filtering.

Brian.
 










okk sir thanks i got it , but here u said "you CAN use the 741 but better alternatives are available."'
so what are they would you please tell me that ?

- - - Updated - - -










okk sir thanks i got it , but here u said "you CAN use the 741 but better alternatives are available."'
so what are they would you please tell me that ?
 

The 741 is a bipolar IC with poor characteristics compared to more modern devices. It was first released around 1970 making it at least 43 years old in design!

If you use a TL071 or TL081 type of amplifier, despite also being quite old, you would see a significant improvement. They have wider bandwidth, lower noise, lower offset voltages and much higher input impedances.
The best amplifer to use depends on what you are using it for though. If you are receiving IR data you would be better to use a dedicated IR receiver IC which has built in filtering and signal conditioning.

Brian.
 

Status
Not open for further replies.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…