| Author |
Message |
14425324
Joined: 17 Aug 2009
Posts: 7
|
 02 Nov 2009 10:44 How is Gain Bandwidth Product defined? |
|
|
|
|
| I am checking the definition of Gain Bandwidth Product in op amp design, but the definitions are different on different materials. Is there anybody can show me the correct definition? Thank you very much!
|
|
| Back to top |
|
 |
LvW
Joined: 07 May 2008
Posts: 1466
Helped: 242
Location: Germany
|
02 Nov 2009 11:11 Re: How is Gain Bandwidth Product defined? |
|
|
|
|
The gain-bandwidth-product (GBW) is the product GBW=Ao*fg .
(Ao=opamp open loop dc gain, fg=3-dB-frequency of the magnitude function).
In most cases the opamp is universal compensated and can be seen as a single pole model. In these cases, the GBW is approximately equal to the transit frequency fT where the gain has dropped to 0 dB.
|
|
| Back to top |
|
|
14425324
Joined: 17 Aug 2009
Posts: 7
|
02 Nov 2009 11:17 Re: How is Gain Bandwidth Product defined? |
|
|
|
|
| LvW wrote: |
The gain-bandwidth-product (GBW) is the product GBW=Ao*fg .
(Ao=opamp open loop dc gain, fg=3-dB-frequency of the magnitude function).
In most cases the opamp is universal compensated and can be seen as a single pole model. In these cases, the GBW is approximately equal to the transit frequency fT where the gain has dropped to 0 dB. |
thank you very much.
but I saw some definition written as GBW = Gcl * fc
where Gcl is closed loop gain, fc is 3-dB-frequency.
Why?
|
|
| Back to top |
|
|
LvW
Joined: 07 May 2008
Posts: 1466
Helped: 242
Location: Germany
|
02 Nov 2009 11:29 Re: How is Gain Bandwidth Product defined? |
|
|
|
|
Yes, thatīs right. But only in case of universal compensation !
The answer is easy: In a single pole response the gain drops with 20 dB/dec. - which means: When the frequency increases by a factor of 10, then the magnitude gets smaller by a factor of 10. Thus, the corresponding product remains constant.
Comment: fc in your formula is the 3-dB-point of the closed loop gain !
|
|
| Back to top |
|
|
14425324
Joined: 17 Aug 2009
Posts: 7
|
02 Nov 2009 12:19 Re: How is Gain Bandwidth Product defined? |
|
|
|
|
| LvW wrote: |
Yes, thatīs right. But only in case of universal compensation !
The answer is easy: In a single pole response the gain drops with 20 dB/dec. - which means: When the frequency increases by a factor of 10, then the magnitude gets smaller by a factor of 10. Thus, the corresponding product remains constant.
Comment: fc in your formula is the 3-dB-point of the closed loop gain ! |
Thanks!
Could you please explan me why in universal compensation we ues Ccl(closed loop gain) but in other condition we need to ues Gol(open loop gain)?
Is universal compensation a compensation approch, or just represent the singal pole op amp?
Thank you very much!
|
|
| Back to top |
|
|
Google
AdSense
|
02 Nov 2009 12:19 Ads |
|
|
|
|
|
|
| Back to top |
|
|
LvW
Joined: 07 May 2008
Posts: 1466
Helped: 242
Location: Germany
|
02 Nov 2009 13:13 Re: How is Gain Bandwidth Product defined? |
|
|
|
|
Could you please explan me why in universal compensation we ues Ccl(closed loop gain) but in other condition we need to ues Gol(open loop gain)?
The GBW is defined as mentioned before (Ao*fg).
In case of a universal compensated opamp this product equals (Acl*fg,cl) as explained before. Thatīs all one can say. That means, for a non-compensated opamp the GBW must be calculated as defined (Ao*fg) - and the other products which can be calculated (Acl*fg,cl) deviate from this product more or less.
Is universal compensation a compensation approch, or just represent the singal pole op amp?
It's the same. The aim of universal compensation is to create a single pole response in the active freqency range (where the gain is > 0 dB).
|
|
| Back to top |
|
|
