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Design >> Analog Design >> building "soft limiting" in an amplifier
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Message started by vivkr on Jul 2^nd, 2008, 2:23am

Title: building "soft limiting" in an amplifier
Post by vivkr on Jul 2^nd, 2008, 2:23am

Hi,

I need to build in some gain compression in one of my amps so that the gain gradually drops off as the signal amplitude increases.

I could use a piecewise-linear gain characteristic but this gives too much distortion around the transition points. I could also reduce the gain of the amp in general by factor 4-5x but this is hardly a good solution for small-signal operation.

Could someone suggest a good method to realize "soft" limiting?

Thanks
Vivek

Title: Re: building "soft limiting" in an amplifier
Post by buddypoor on Jul 2^nd, 2008, 2:39am

Hi vivek,

by reading the phrase "soft limiting" I immediately think of "linear" oscillators which all exhibit such a circuitry: diodes, diode-resistor networks, FET-control, thermistor, OTA as resistor, OTA as controlled amp, ....
And what about an arrangement based on the AGC principle ?

Title: Re: building "soft limiting" in an amplifier
Post by vivkr on Jul 2^nd, 2008, 4:17am

Could you give me some references to AGCs? I was thinking along similar lines (AGC/VGA) but I don't know many good references.

Regards
Vivek

Title: Re: building "soft limiting" in an amplifier
Post by buddypoor on Jul 2^nd, 2008, 6:36am

Hi vivek,

here are two refs
1.) R.E. Prymek: opamp filters control responses in AGC systems, Microwaves & RF =ctober 1991, page 111
2.) S.R. Bullock: Simple technique yields errorless agc systems,
Microwaves & RF, August 1989, page 85

3.) and 4) see pdf attachement

Hope this can help,
regards

Title: Re: building "soft limiting" in an amplifier
Post by buddypoor on Jul 2^nd, 2008, 6:38am

One attachement was lost; here is it.

Attachment deleted as it contained copyrighted material.
John M. Khoury. On the Design of Constant Settling Time AGC Circuits. IEEE Transactions on Circuits and Systems - II: Analog and Digital Signal Processing, vol. 45, no. 3, March 1998.
http://ieeexplore.ieee.org/Xplore/login.jsp?url=/iel4/82/14571/00664234.pdf?arnumber=664234

Title: Re: building "soft limiting" in an amplifier
Post by vivkr on Jul 2^nd, 2008, 10:53pm

Thanks a lot for these references and the attachments. I hope that I will find a solution to my problem here.

Regards
Vivek

Title: Re: building "soft limiting" in an amplifier
Post by thechopper on Jul 3^rd, 2008, 7:07pm

Hi vivek,

Did you consider the possibility of using linear clampers? A comparator and eventually a feedback loop when such comparator is triggered might also help you. When the clamp circuit is on, the effective gain of your amplifier is virtually zero, keepìng the signal at the level driven by the clamper.

Just giving you some other ideas that might work for you.

Rgds
Tosei

Title: Re: building "soft limiting" in an amplifier
Post by vivkr on Jul 3^rd, 2008, 11:23pm

Hi Tosei,

I think this scheme with the comparator would give me more of a hard clamping than soft clamping. What I wanted was something like a VGA/AGC where the gain gracefully degrades until at the end, you have gain =1 or less depending on how you design the amplifier. THD is an important spec for the system, but with a gain compression, it would still be within spec.

Regards
Vivek

Title: Re: building "soft limiting" in an amplifier
Post by buddypoor on Jul 4^th, 2008, 6:25am

Hi, vivek !

A very simple solution to your problem has come just into my mind - sometimes the most simple ideas are the last:

What about a differential stage with a tanh-characteristic, which performs soft limiting above app. 50 mV ?
Regards

Title: Re: building "soft limiting" in an amplifier
Post by thechopper on Jul 5^th, 2008, 10:57am

vivkr wrote on Jul 3^rd, 2008, 11:23pm:

Hi Vivek,

That is not necessarily true since, if have the comparator inside the negative feedback loop: when the comparator kicks in the loop will force it to work as a linear amplifier. You can control the loop gain such that the gain compression is as soft as you want --> the more loop gain, the harder the gain compression. But certainly you have a degree of freedom by controlling the loop gain.

Regards
Tosei.

Title: Re: building "soft limiting" in an amplifier
Post by buddypoor on Jul 6^th, 2008, 12:33am

Hi Tosei,

the principle you have described sounds interesting to me; can you please describe it in more detail ? (opamp with resistors and a comparator in the fedback loop ?). Thank you in advance.

Title: Re: building "soft limiting" in an amplifier
Post by vivkr on Jul 6^th, 2008, 11:27pm

Hi all,

Thanks for the interesting inputs. I will see which of these 2 methods works better for me.

Tosei: I have some experience of the comparator inside a feedback loop and though it worked well on the whole, I had some practical difficulties with it. Might be worth trying to see if those could be removed.

Lutz: The tanh method would be good. I need to see if I can arrange a feedback loop around the whole to get the precise small-signal gain over the range of interest, as the small-signal gain, large-signal gain and the respective signal ranges are quite precisely defined and I need to stick to them.

Thanks again to both of you. If any new ideas come in, then they are also welcome of course.

Vivek

Title: Re: building "soft limiting" in an amplifier
Post by thechopper on Jul 7^th, 2008, 7:12am

Hi Buddypoor,

The approach I proposed is basically a closed loop system intended to control the clamp level
for the output voltage of an amplifier.In principle it could be used for either a closed or open
loop main amplifier (the one whose signal is going to be clamped or "soft limited").

The system would consist of two comparators (one for the positive or one for the negative clamp
thresholds) that would be connected to the output of the main amplifier.
If the amplifier's output signal is within the clamp thresholds, then the comparators are "off".
When the amplifier's output signal reaches either theshold, the corresponding comparator
will try to switch state and will be engaged. However, such comparator will be controlling either a current or voltage
that will be fed back to - for instance - the input of the main amplifier. In this way a linear nega
tive feedback loop will take place which will force the comparator to work in a linear fashion:
the comparator will feed back a signal that will try to compensate for the amplifier's input one in such a
way that the output level stays at the preset threshold.
Therefore such loop will comprise the active (linearly working) comparator and the main amplifier.
The larger the loop gain, the harder the clipping on the output signal. Therefore, by controlling
the gain of such loop (in particular the gain of the comparator) the limiting could be done in a more
soft or hard way.

When analyzing stability for this loop system obviously the comparator has to
be biased at the proper (linear) operating point for performing such analysis.

Also, the tanh method could be combined with my proposed approach, since the in-loop comparator transfer function
characteristic could be done in such way.

Something to particularly consider is the case when the main amplifier is a closed loop one. This is another case
of a multi-loop system: the main amplifier loop and the main amplifier+comparator loop. IF the limiting signal
is fed back to the input of the main amplifier the multi-loop system will have a common break point
and the stability analysis might be simplified.

I hope this better describes what I proposed.

Vivek: I have also some experience with in-loop comparators too and I admit I had a hard time stabilizing those systems,
but this might be relatively simple depending on the type of amplifier and loop characteristics you are dealing with.

Regards
Tosei

Title: Re: building "soft limiting" in an amplifier
Post by buddypoor on Jul 7^th, 2008, 8:39am

Hello Tosei,

thank you very much for the detailed description of the "clamped loop system". I am involved in analog electronics since app. 25 years, but I must confess, up to now I´ve never heard about this principle. Did you invent this method ? At the moment I didn´t get it yet completely - especially as far as stability aspects are concerned.
Nevertheless, I have a question regarding the comparator. Are there any special requirements concerning slope of the linear region (gain) ? I have some doughts to find a proper bias point, but perhaps some simulations can clarify this.
Thank you again
and best regards
buddypoor

Title: Re: building "soft limiting" in an amplifier
Post by thechopper on Jul 7^th, 2008, 8:22pm

Hi Buddypoor,

Unfortunately I´m not the one who invented this although I implemented it a couple of times :). But certainly it existed before I used although I could not provide you with a reference right now (I´m not sure what exactly could have been published about this, but there has to be some paper or publication around I´d have to look for).

As for stability, as I tried to explain before, the main issue is to get the proper comparator bias point such that the analysis provides valid information. One way to do it is to run a transient analysis and stop it at some point in time when the comparator is limiting the amplifier´s output signal. Then you could run an ac analysis using the operating point provided by the transient final state.

I cannot think right now of special requirements as for the slope of the linear region except for possibly the amount limiting you are looking for given a certain overdrive : for example, the more overdrive on the comparator (the larger the signal if it was not clamped) the higher the gain you need in order to keep the clamp level as accurate (and constant) as possible.

Hope this helps.
Regards
Tosei

Title: Re: building "soft limiting" in an amplifier
Post by buddypoor on Jul 7^th, 2008, 11:26pm

Tosei, thank you. I´ll try to do some simulations anywhen in summer (when I have the time for it). Then, perhaps some new question arise.
Regards
budypoor

Title: Re: building "soft limiting" in an amplifier
Post by nano_RF on Aug 24^th, 2008, 10:29am

Hi All,

If amplifier is employed in negative feedback fashion then i wonder if you can achieve the soft-limiting by simply changing the feedback ratio.

Basically have another (resistor plus active device combo in series) in parallel to the main feedback resistor. For the duration where gain compression is not desired the bias level of active device is kept below the threshold, and during the soft limit it is gradually turned on (bias is directly related to the output voltage) and basically allowing the second resistor to be in parallel with the main feedback resistor and the gain is reduced so-forth.

However, this will have some threshold variation.

Regards