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Old 16th March 2006, 12:03 PM   #1
sreten is offline sreten  United Kingdom
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Default high gain 2nd order high pass active filter topology

A question for experts on active filters.

Can a single op-amp be configured for gain ~ 30dB
and also provide a 2nd order high pass function ?

All topologies I've seen have limited gain.

/sreten.
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Old 16th March 2006, 04:32 PM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
just a guess but maybe an MFB might be able to reach up there.

Remember to check your gain bandwidth requirement for the opamp for that extremely high gain.
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Old 16th March 2006, 04:36 PM   #3
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I'm not sure (I am no expert) but I would use an 'undercompensated' opamp like op-37
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Old 17th March 2006, 09:54 AM   #4
sreten is offline sreten  United Kingdom
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Hi,

The real question is related to incorporating bass boost in the
feedback loop of power amplifiers by using a high pass filter.

As gain at high frequencies will not change I can't see any
issues with compensation or lack of gain bandwidth product.

/sreten.
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Old 17th March 2006, 10:34 AM   #5
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I haven't tried it recently, but Analog have a filter design tool that I believe will allow gain.
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Old 17th March 2006, 12:52 PM   #6
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by pinkmouse
I haven't tried it recently, but Analog have a filter design tool that I believe will allow gain.
Hi,

yes there is a field for gain but the tool doesn't seem to work.
(or setting the gain to 30 breaks the tool !)

/sreten.
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Old 17th March 2006, 01:58 PM   #7
ilimzn is offline ilimzn  Croatia
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Yes, in fact it is quite easy to do.

Think of a regular MFB filter with amplifer block with gain = 1. You have a feedback path from output to the input network in form of a capacitor. In order to provide 30dB gain, you need to move the feedback pick-off point from the output of the amp (Av=1) to a point where Av=1 regardless of amp gain, and this in fact will be the junction of the resistor network that sets the 30dB gain, i.e. the inverting input of the 'OPamp'. In order for this to work with reasonable accuracy, the feedback network needs to be low impedance, or at least the impedance as seen from the non-inverting OPamp input should be much lower than that of the filter network at the input of the amp. This is the less of a problem, the higher the gain required, and the better the drive capability of the OPamp.

For instance, assuming a NE5532 OPamp, and assuming twice the minimum load is used, the same load can be presented in the form of the feedback network. Example: if you use 600ohms as the minimum total load for the 5532, and 1.2k load of the next stage, then your feedback network must have 1.2k end-to end. For a gain of 30dB = 31.62, you come up with 1162 ohms from output to inverting input, and 38 ohms from inverting input to ground. The impedance seen at the non-inverting input is roughly equal 38 ohms, so this is perfectly acceptable for all normal values of your filter components. The remaining problem is DC offset, whch can be catered for in a number of ways, but this is outside of the scope of this topic.

I've been using this trick for, amongst other things, making simple two-way active speakers on the cheap, you use a chip power amp instead of the OPamp. Since the chip amp normally requires at least 24dB of gain to insure stability, and by definition drives low impedances, it is quite easy to use in this manner.
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Old 18th March 2006, 09:43 AM   #8
djk is offline djk
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Just pretend the feedback node of the power amp is the output node of a unity gain opamp. If the lower arm feedback resistor is low enough then you won't have any problems.

If the lower arm feedback resistor is a high value, then just add a voltage divider at the output of the amp that has the same ratio as the feedback resistor pair. Sony did this around a 30dB preamp line stage, works fine.

The NAD 3020 integrated amp uses this on the main power amp input too, the lower arm feedback resistor is 27R.
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Old 18th March 2006, 11:38 PM   #9
infinia is offline infinia  United States
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If you think you need 30db of bass boost something is really wrong.
example: Running your mids and HF driver with 10 Watts
about 100 dB SPL. The power amp and woofer would be running 30 db higher or 10,000 Watts to get the low freq extention.
Not too realistic
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Old 18th March 2006, 11:59 PM   #10
AndrewT is offline AndrewT  Scotland
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Hi,
I think he said 30db of gain, not bass boost.
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