| Author |
Message |
richloo
Joined: 24 Aug 2004
Posts: 103
Helped: 5
Location: Isolated Island
|
 15 Nov 2004 14:40 Need advice on opamp design |
|
|
|
Hello, i am lack of experience in choosing different type of topology in opamp design. Practically, for 2 stage classical & folded cascode opamp, what is the highest gain or gain bandwidth product that they can hit? My application is high frequency filter, is it possible to use opamp-RC rather than gm-C?
Thanks in advance.
|
|
| Back to top |
|
 |
ArchAngel
Joined: 01 Jul 2004
Posts: 49
Helped: 14
|
| 06 Dec 2004 10:44 Need advice on opamp design |
|
|
|
the performance of opamp architechture depends on
process, try sc-filter for ur hf-filter maybe a better solution
|
|
| Back to top |
|
|
yeewong_su
Joined: 12 Oct 2004
Posts: 126
Helped: 3
|
| 07 Dec 2004 3:00 Need advice on opamp design |
|
|
|
| 2 stage opamp can match you need. cascode opamp can give the high gain above 80db
|
|
| Back to top |
|
|
sunking
Joined: 25 May 2004
Posts: 914
Helped: 46
|
| 07 Dec 2004 6:36 Re: Need advice on opamp design |
|
|
|
both them you can use
and Gm_RC is better for high frequency
|
|
| Back to top |
|
|
richloo
Joined: 24 Aug 2004
Posts: 103
Helped: 5
Location: Isolated Island
|
| 07 Dec 2004 16:10 Need advice on opamp design |
|
|
|
I have completed a 2 stage opamp with GBW of 330MHz, DCgain=60dB, GM=20dB and PM=54 degree. I found that i can't go beyond 3rd order unity gain 200MHz SallenKey LowPassFilter since the o/p signal is out of phase.
Can anyone give any comment/suggestion/solution?
|
|
| Back to top |
|
|
richloo
Joined: 24 Aug 2004
Posts: 103
Helped: 5
Location: Isolated Island
|
| 07 Dec 2004 16:13 Need advice on opamp design |
|
|
|
| BTW, the filter i m using is Butterworth... Thanks in advace...
|
|
| Back to top |
|
|
electronrancher
Joined: 24 Mar 2002
Posts: 474
Helped: 34
|
| 07 Dec 2004 22:10 Need advice on opamp design |
|
|
|
richloo - that is a fast amp, what process did you draw in?
i would suggest gm-C instead of op-RC. if your overall amp is 330MHz, the diff pair is probably 500+MHz. Using gm's instead of opamp means you don't have the parasitic poles from an output stage.
|
|
| Back to top |
|
|
djalli
Joined: 10 Nov 2001
Posts: 887
Helped: 15
Location: 1600 Pennsylvania Avenue, Washington DC 20500
|
| 07 Dec 2004 22:57 Re: Need advice on opamp design |
|
|
|
| richloo wrote: |
I have completed a 2 stage opamp with GBW of 330MHz, DCgain=60dB, GM=20dB and PM=54 degree. I found that i can't go beyond 3rd order unity gain 200MHz SallenKey LowPassFilter since the o/p signal is out of phase.
Can anyone give any comment/suggestion/solution? |
I see 300MHz bandwidth. What opamp did you use?
|
|
| Back to top |
|
|
richloo
Joined: 24 Aug 2004
Posts: 103
Helped: 5
Location: Isolated Island
|
| 08 Dec 2004 3:45 Re: Need advice on opamp design |
|
|
|
I used 0.18um CMOS, 2 stage opamp. but the transient response have slow rise time... can anyone tell me how to improve on this? thanks in advance...
Last edited by richloo on 08 Dec 2004 4:39; edited 1 time in total |
|
| Back to top |
|
|
richloo
Joined: 24 Aug 2004
Posts: 103
Helped: 5
Location: Isolated Island
|
| 08 Dec 2004 4:26 Need advice on opamp design |
|
|
|
electronrancher:
thanks for ur advice. i just wish to start gm-c but i can't coz i m running out of time. i am using 0.18um SCMOS. Since u said that it is a fast amp, from ur experience and with the simulated GainMargin & PhaseMargin, do u think possible to go to higher order (>3) filter? how to "tweak" it? thanks in advance.
|
|
| Back to top |
|
|
gevy
Joined: 17 Nov 2004
Posts: 377
Helped: 41
Location: Russia
|
| 08 Dec 2004 16:32 Re: Need advice on opamp design |
|
|
|
You can download the useful E-book
"Inroduction to CMOS OP-AMPS and comparators" by R.Gregorian
http://www.edaboard.com/ftopic99896.html
It contains the answers to many practical questions.
|
|
| Back to top |
|
|
yeechyan
Joined: 14 Jun 2004
Posts: 111
Helped: 1
|
| 02 Jan 2005 4:27 Need advice on opamp design |
|
|
|
| What about you change the input differential pairs from PMOS to NMOS? NMOS is faster than PMOS. Anyway the noise will be higher.
|
|
| Back to top |
|
|
richloo
Joined: 24 Aug 2004
Posts: 103
Helped: 5
Location: Isolated Island
|
| 02 Jan 2005 10:57 Need advice on opamp design |
|
|
|
thanks yeechyan! i have tried NMOS diff input pair n it is ok. but only little improvement... 
i begin to try gm-c, if i wan to drive both resistive and capacitive load, how can i do that w/o introducing additional pole? an inverter?
|
|
| Back to top |
|
|
chency
Joined: 06 Dec 2004
Posts: 44
Helped: 3
|
| 04 Jan 2005 4:08 Re: Need advice on opamp design |
|
|
|
| richloo wrote: |
thanks yeechyan! i have tried NMOS diff input pair n it is ok. but only little improvement...
i begin to try gm-c, if i wan to drive both resistive and capacitive load, how can i do that w/o introducing additional pole? an inverter? |
maybe you replace your first stage with a folded cascode
|
|
| Back to top |
|
|
richloo
Joined: 24 Aug 2004
Posts: 103
Helped: 5
Location: Isolated Island
|
| 04 Jan 2005 10:36 Need advice on opamp design |
|
|
|
chency: do u mean that i have to replace only the diff pair stage?
regard
|
|
| Back to top |
|
|
qutang
Joined: 25 Oct 2004
Posts: 315
Helped: 6
|
| 04 Jan 2005 14:09 Need advice on opamp design |
|
|
|
| the structures of the picture is very useful.
|
|
| Back to top |
|
|
jiangwp
Joined: 27 Jul 2004
Posts: 184
Helped: 9
|
| 05 Jan 2005 13:35 Re: Need advice on opamp design |
|
|
|
The filter performance is function of the parameters of opamp and the value of capacitor and resistor.
On one hand, you should optimize the opamp , also do the value of resistor and capacitor.
The resistor and capacitor is load of the opamp . If their value is large , the opamp is work in the large and small signal region, then you can increase the slew rate of the opamp. If the filter process the small signal , the opamp work in the linear region , the bandwith is dominate .
I think you can improve the bandwith , slew rate and output impedance in the mean time.
|
|
| Back to top |
|
|
