Go Down

Topic: Amplification factor of op amps (Read 1 time) previous topic - next topic

john25

We all know that op amps means operational amplifier. But how much amplification can a uA741 can do? Can this op amp amplify upto 1000 times?

retrolefty


We all know that op amps means operational amplifier. But how much amplification can a uA741 can do? Can this op amp amplify upto 1000 times?


Yes, most any opamp can. Open circuit gain (when there is no negative feedback being applies) on most opamps is over 100db (at DC, AC gain is a little more complex to express), that's a gain of 10,000,000,000 if I recall by db conversions correctly. Practical circuit gain using opamps is determined by the ratio of external resistors wired to the opamp to form a classic negative feedback DC amplifier, not by any limitation of the opamp.

More important is that you really shouldn't be using 741 type opamps as it's a second generation opamp dating back forty plus years or so. It has many practical circuit limitiations and very poor specifications for this day and age. Their only remaining purpose in life should be as replacement parts for old existing equipment that used them and are now in need of repair. Much newer opamps with much better specs and as cheap or cheaper then a 741 are easily avalible. Bottom line, ditch the 741s and use real opamps. 

oric_dan

Quote

100db (at DC, AC gain is a little more complex to express), that's a gain of 10,000,000,000


I think you need to divide dB-volts by 20 first, so 100/20 = 5, which is the exponent,
or 10^5 = 100,000 is the open-loop gain.

Yes, 741s are first gen, and not so good. Better choices for inexpensive opAmps
are TL081, LF353, LF442, etc, all available from jameco.com. And not to mention
newer CMOS opAmps like LMC6442.

For high-gain "closed-loop" operation, eg X1000, the most important factor is
the Gain-Bandwidth Product value. I seem to recall 741 GBP = 1Mhz, which
means at X1000, the BW = 1Mhz/1000 = 1000 Hz, which is pretty low.

Some of the other amps mentioned have GBP 5-10X higher. For hi-gain situations,
it's common to use 2 opAmps in series with lower gain in each, then you can get
both hi-gain and also hi-BW.




MarkT

Generally its not sensible to have a single stage gain above 100, problems with instability arise (unintended feedback paths via stray capacitances and inductances, or via the power rails)

The opamp's high gain is not to provide gain as such, its to provide high accuracy (or put another way low input-offset voltage).  In negative feedback circuits you trade gain for linearity and accuracy.
[ I won't respond to messages, use the forum please ]

Grumpy_Mike

Quote
Yes, 741s are first gen,

Well not quite as Lefty said it is a second generation monolithic integrated circuit operational amplifier.
Before that came the uA702 in 1963. That had to be stabilised with and external capacitor to stop it oscillating.
The 741 came out in 1968:-
Quote
The popularity of monolithic op-amps was further improved upon the release of the LM101 in 1967, which solved a variety of issues, and the subsequent release of the ?A741 in 1968. The ?A741 was extremely similar to the LM101 except that Fairchild's facilities allowed them to include a 30 pF compensation capacitor inside the chip instead of requiring external compensation. This simple difference has made the 741 the canonical op-amp and many modern amps base their pinout on the 741s.

Quote from:-
http://en.wikipedia.org/wiki/Operational_amplifier

jwatte


We all know that op amps means operational amplifier. But how much amplification can a uA741 can do? Can this op amp amplify upto 1000 times?


An opamp in theory has infinite gain, and you set the actual gain using the negative feedback resistor ladder.
However, over 5,000 or so, I'd start studying data sheets and maybe measurements in detail.
For a signal that varies (e g, almost every signal in existence :-) you should also look at the "gain product."
If an opamp has a "gain product" of 2.8 MHz, and you want a 1000 factor gain, then your input signal should be less than 2.8 kHz for best results.

Techone

@john25

Just one question : What type of application the op-amp uA741 you are planning to use ? 

Grumpy_Mike

Quote
Can this op amp amplify upto 1000 times?

To answer this question then yes it can but no it can't.

You can get this gain out of this op amp, but it is not a useful gain. This is because amongst other things all op amps have what is known as a DC offset. This acts like a permanent DC input to the amplifier. Therefore if the feedback resistors are such that the DC offset times the gain is greater than operating voltage, what happens is the output goes to the rail (or the maximum it can output) and there is no amplification of your signal.

Go Up