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amitsingh New Member Offline Posts: 9 Bangalore, India	Significance of 3-db BW and Unity Gain BW Sep 19^th, 2007, 7:15pm Hi all, � � � � Can anybody enlight me about the role what 3-dB BW and UGB (unity gain BW) plays in an amplifier. say for an example its used in a LDO. How does above mentioned parameters effect its performnce ? Thanks in advance. �:)
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bharat

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Re: Significance of 3-db BW and Unity Gain BW
Reply #1 - Sep 20^th, 2007, 6:43am

Unity gain frequency is the frequency where Gain is unity. The significance of this frequency can be derived
Loop gain A*beta <1 , As A=1 ( unity gain), beta has to be in a way that product A*beta is less than unity. This frequncy is good data point for stability check.
Measure of PM is done at this frequency, which tells me how far I am from -180 degree at unity gain frequency. This measure decides the value value of beta to contain loop gain A*beta <1.

3 dB frequency is also called pole frequency. This is the frequency when the DC gain in dB is reduced by 3 dB or 0.707* DC gain. It is also the frequency, when the DC normalised gain starts seeing the gain roll over at the slope rate of -20dB/decade ( -6dB/octave).We say that pole frequency is signature of the circuit. It tells the presence of capacitance with independent initial condition starts storing the energy from the bias and rolling the gain.

Any comments from others!!

-Bharat

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HdrChopper

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Re: Significance of 3-db BW and Unity Gain BW
Reply #2 - Sep 20^th, 2007, 9:06am

Hi amitsingh,

In respect to an LDO, the GBW of the circuit will determine its ac characteristics, among which the most importants are PSSR and output impedance.
The rejection of the noise coming from the supply line will be better at DC (highest open loop BW), while it will decrease according to the reduction of such loop gain with frequency --> the more GBW the larger the frequency range with better PSRR.

Same thing applies for the output impedance --> it gets larger as you increase the frequency. This will happen at higher frequencies in case your GBW is larger too.

Hope this helps
tosei

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amitsingh New Member Offline Posts: 9 Bangalore, India	Re: Significance of 3-db BW and Unity Gain BW Reply #3 - Sep 20^th, 2007, 6:45pm Thank you tossei. But how about the 3 -db freq role in performance of an Ampr? Thanks in advance :)
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HdrChopper

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Re: Significance of 3-db BW and Unity Gain BW
Reply #4 - Sep 20^th, 2007, 7:49pm

Well,

In essence it it plays the same role as the unity gain BW: assuming the amp is stable, you�ll have a dominant pole which sets the unity gain BW of the system. This pole sets a -20dB/dec gain roll over as Bharat explained before, so depending on the gain you close the loop with you�ll get the -3dB BW from such gain roll over. In other words, typically (for resistive feedback networks) f-3dB=fu / Acl (DC), where fu is the unity gain BW and Acl (DC) is the DC closed loop gain.

From other perspective: the f-3dB sets the BW of the closed loop system, while the UGB does so for the open loop one and both are usually related.

-tosei.

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amitsingh New Member Offline Posts: 9 Bangalore, India	Re: Significance of 3-db BW and Unity Gain BW Reply #5 - Sep 20^th, 2007, 10:41pm Thank you tosei.
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