# gain bandwidth product / op amps

Discussion in 'General Electronics Discussion' started by komalbarun, Aug 25, 2013.

1. ### komalbarun

67
0
Nov 25, 2011
Hi all,
1.How do we calculate gain-bandwidth product of an op-amp?

like if :
i have open loop gain Ao = 100,000.
Bandwidth = unknown .
Ao x unknown = ??

2.what does frequency response, badwidth mean? how do these affect the output?

3. What do we mean when we talk about unity gain in op-amps. Does it mean that gain=1?

2. ### BobK

7,682
1,688
Jan 5, 2010
1. You don't, you get it from the datasheet.
2. GBP is the product of the gain and the frequency. Say it is 1 MHz. That means you will have unity gain at 1MHz, gain of 10 at 100KHz, gain of 100 at 10KHz etc.
3. Yes.

Bob

3. ### komalbarun

67
0
Nov 25, 2011
thnx a lot Bobk.

There's still something i want to understant.

cut off frequency = the frequency below which and above which output will not be as expected? that's how it seems to be after a lot of online reading xD.

but, how do we get the -3dB?

10 log 2 = 3 <--ok, i got it.
10
but:
where does the 2 come from Oo?

4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

25,490
2,831
Jan 21, 2010
The 2 comes from a factor of two -- a doubling or halving (in your case you are actually looking for 10 log 0.5).

The the Log (base 10) part comes from the definition of a Bel and the fact that it's logarithmic and specifically log base 10.

The 10 part comes from the fact that you want dB, not B.

5. ### komalbarun

67
0
Nov 25, 2011
Thx to both of you, things are much clearer now ^__^

6. ### eman12

31
0
Aug 24, 2013
Hi komalbarun,

If I was you, I would try to take a look at the datasheet of "LM3886" Op-amp. It talks about GBP and give you an good example in this regard.