bandwidth drop with cascaded limiting amplifier

EDAboard.com Forum Index -> Analog Circuit Design -> bandwidth drop with cascaded limiting amplifier

Author

Message

Ipanema

Joined: 27 Oct 2004
Posts: 41

12 Nov 2004 10:39 bandwidth drop with cascaded limiting amplifier

Hi,

Can anyone explain to me why the bandwidth of limiting amplifier(LA) get reduced when cascaded? e.g. single stage LA, BW=10GHz, but with 5 stages of the same LA, BW=2GHz.

Thanks.

Google
AdSense
Google Adsense

12 Nov 2004 10:39 Ads

zigzagfish

Joined: 03 Nov 2004
Posts: 15

17 Nov 2004 10:38 Re: bandwidth drop with cascaded limiting amplifier

of cos when you cascade the 5 the same amps, whose main poles would be the same and aroud 10G as well as other coresponding poles, the overlapping of the poles would decrease the bandwidth significanly, you can see it clearly from the frequency response graphics

Ipanema

Joined: 27 Oct 2004
Posts: 41

18 Nov 2004 6:59 Re: bandwidth drop with cascaded limiting amplifier

Hi,

From my simple understanding, if there exist 5 poles exactly at 10Ghz, I should get 100dB/dec at 10Ghz compared to 1 pole at 10Ghz, which will give only 20dB/dec. The -3dB points will reduce a little bit, but not as much as 2Ghz. I would like to know what make the pole shifted in this case.

Thanks.

bmalp98

Joined: 17 Nov 2004
Posts: 14

18 Nov 2004 10:50 Re: bandwidth drop with cascaded limiting amplifier

Hi,

Bandwith is point where gain drops by 3dB. Cascading 5 stages makes 15dB..

Or, more mathematically, take (A/(wc+jw))^n, bandwidth is where imaginary and real part of the denominator are equal. Develop in Taylor series for the dominant pole, you get
wc^n + n*wc^(n-1) * jw. Equating real and imaginary part:
w = wc/n

Thats what you see.

bmalp98

surianova

Joined: 01 Sep 2004
Posts: 410
Helped: 21
Location: ASIA

19 Nov 2004 5:40 Re: bandwidth drop with cascaded limiting amplifier

my opinion is when a few limiting amplifier cascade together, the gain will increase from first stage to the last stage. As gain increase, the miller capacitance also increase according to formula cgd(1+A). This will form a even lower frequency pole as incresing stages, f=/2pi. RC. The output of the last stage will be
the dominant pole or the first pole.

Suria

bmalp98

Joined: 17 Nov 2004
Posts: 14

19 Nov 2004 12:04 Re: bandwidth drop with cascaded limiting amplifier

Hi,

I don't think so. What you call Miller capacitance has to be connected from the (inverting) output to the input of the amplifier. Physically, this is the base-collector or drain-gate capacitance. Since these capacitances "see" only the local gain of a single stage, there is no increase, right?

Best,
bmalp98