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Design >> Analog Design >> high Q bandpass filter design
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Message started by lhlbluesky_lhl on Oct 21^st, 2012, 1:54am

Title: high Q bandpass filter design
Post by lhlbluesky_lhl on Oct 21^st, 2012, 1:54am

i want to design a active bandpass filter, Q>10, center frequency f0=20KHz, stop band attenuation as large as possible, and use as small as possible external components, and only one off-chip component is allowed (a R or a C), the other components to be on-chip, what type of BPF can i use?
in my opinion, i want to design a two-order BPF, but as i know, f0 is usually decided by at least two R and two C, such as f0=1/(sqrt(C1*C2*R1*R2)*2*pi), so if i use only a R or a C to tune the f0, the linearity is not better.
besides, the opamp non-ideal characteristics can have much influence on the BPF performance. so, how to consider this or how to design the opamp parameters (such as DC gain, GBW, PM, etc).
thanks all.

Title: Re: high Q bandpass filter design
Post by Dan Clement on Oct 21^st, 2012, 6:01am

Look into doing a GmC filter.

Title: Re: high Q bandpass filter design
Post by loose-electron on Oct 21^st, 2012, 3:58pm

Define your requirements better

"Out of band rejection as high as possible"

Needs to be quantized.

After that? Define you S-plane function as needed.

Also, consider an ADC and a FIR filter at that low a frequency.

gmC that low? Nowadays I don't do gmC till I get above 20MHz.

Title: Re: high Q bandpass filter design
Post by lhlbluesky_lhl on Oct 22^nd, 2012, 5:10am

hi, loose-electron. why gmC cannot get that low frequency?

Title: Re: high Q bandpass filter design
Post by Dan Clement on Oct 22^nd, 2012, 5:37am

The power of GmC can be high for lower frequencies.

Title: Re: high Q bandpass filter design
Post by lhlbluesky_lhl on Oct 22^nd, 2012, 5:54am

The power of GmC can be high for lower frequencies.
why? for a given C, the lower f0, the lower gm, and the lower the power. right?

Title: Re: high Q bandpass filter design
Post by Dan Clement on Oct 23^rd, 2012, 4:56am

The minimum power is really set by the technology. So below a min frequency the area and power is not used efficiently.

Title: Re: high Q bandpass filter design
Post by boe on Oct 23^rd, 2012, 7:02am

lhlbluesky_lhl wrote on Oct 22^nd, 2012, 5:54am:

The power of GmC can be high for lower frequencies.
why? for a given C, the lower f0, the lower gm, and the lower the power. right?

And the higher the noise and the larger the mismatch.
- B O E

Title: Re: high Q bandpass filter design
Post by loose-electron on Oct 23^rd, 2012, 4:33pm

lhlbluesky_lhl wrote on Oct 22^nd, 2012, 5:10am:

hi, loose-electron. why gmC cannot get that low frequency?

You can do a gmC filter at any frequency but the transconductor
design often requires a lot of bias current to maintain linearity

Generally I will use a gmC filter for higher frequency applications
(20-500 MHz) and below that look into something based upon op-amps,
switched capacitor design or ADC and FIR filter.

If you are going to hit an ADC anyhow, I would use a minimal
filter for aliasing and then ADC and FIR filter.