GC_ET_AFX_CW_Limiter.html

2021-01-06   09:42:06

     Analog  Active-Log-Limiter 

Definition:
AGC uses feedback to ensure that the output signal always has a certain amplitude,
whereas a LIMITER merely ensures that the output doesn’t exceed a certain amplitude. 

*****  Log-Limiter circuits based on personal designs from 1975, 
******* tutored by
Dr. Joseph Laughter, Bio-Med-Instrumentation, University of TN, Memphis, TN.   

Ref:  http://en.wikipedia.org/wiki/Diode_modeling 

Basic Log Diode Limiter is a single stage ,  Negative-Feedback , Germanium Diode , instant response design.
R(series with diodes) adjusts limiting level in combination with R(feedback gain). 

  *** (!)  Frequently you will encounter the "Anti-Parallel Diode" .      

One Volt Input can be adjusted to One Volt Output at onset of Limiting, or other levels as needed. 

Resistors in series will push the Diode Turn-On Curve up the scale, to the > 1V range, author's choice for SOP. 
Rlmtdiode (777 Ohms) can be varied up towards 3333 Ohms for higher turn-on curves. 
Logging Diodes are Germanium and measure  0.23 V type, matched by comparison ,
using the resistance circuit on the author's  VOM  (Amprobe AM-270).   
 
*******  Graph of the Advantage of  Germanium   over   Silicon diodes.  
*******  Softer Knee produces  much less  InterModulation Distortion during Limiting Action.
*** This is a dynamic affect produced by diode forward current conduction,
*** and varies widely with the particular diode internal impedance (resistance)
*** and varies widely with the particular circuit impedance. 

                                      

(((  Germanium provides Lower Slope on load curve than silicon  ))) 

(((  Germanium produces Less InterModation-Distortion during limiter action. )))

(((  Germanium produces Less Odd-Harmonic Distortion during limiter action. )))

 

 

 

 

***

Below: Waveform display by Robert Keim, using silicon diodes.
Note that Diode Conduction (limiting) occurs at all levels of current flow.
The author has observed (measured) this from 1 micro-Volt upward in low impedance circuits.
( see author's Log-Amp circuits preseented on the " Notes from University of Tennessee page.

In common circuits, this effect is used from the 0.5V forward to 0.7V forward range.
Image Shows graduated (+3dB) effects of driving the Limiter Diodes into Action.

On the top two levels, the limiting action (flat topping) is obvious. 
On the top two levels, the diode turn-on  generates obvious 3rd harmonic energy.

.......   

Here is some background on the author's Active Limiter circuit :

The first stage is a Pre-Amp with a wide-range gain control,
balancing the Rinput and the Rfeedback.  
Since the signal is well above noise level at this point,
we wanted to prevent pop-corn noise from shocking the Roofing-Filter and Active-Limiter stage,
so we used a Log-Negative FeedBack function,
which acts strongly at about  1.7V , directly back into the Primary Vinput. 

In General, the Negative FeedBack have an R(series). 
The R(series) adds to the Diodes Internal-Resistance and Dynamic-Impedance,
...
Stated this way :
VF = VK log (IF/IS(T)) 
where real world "I"
always = V/(R+Dynamic-Impedance) 
and we are adding to the "R" . 

In tests, the resulting combination works well , just as described :  
(1)  R(series)  linear resistance.
(2)  Diode Internal-Resistance , dynamic and Logrithmic.
(3)  Diode-Dynamic-Impedance from the PN junction, dynamic and Logrithmic. 
All function together to lengthen the initial slope
but    will  still produce abrupt   Roll-Over-the-Log  at some upper level.  

1 V signal works well into the Roof-Triad-Filter and also matches the Roll-Over in the Limiter. 
More than 1.7V pushes the Signal into the Limiter enough to begin activating Limiter action.
1 V signal makes run time measurements between stages/modules easy to compare..  

Same idea applies later in the dedicated Limiter Stage 
with the log-Negative-Feed-Back diodes. 

We did Transient plots to compare various combinations,
and did real circuit adjustments with O'scope to check.

Visualize this as Resistor-in-series-with-diodes feedback as : 

(1) Resistive Negative-FeedBack 

(2) with the diodes limiting any signal going above the R(fb) gain setting. 

 

The soft-limiter effect is a human criteria we are chasing  
and it is something that the ear/brain system  can  hear/read.  

The Roof-Triad-Filter produces a very symetrical signal output,
and this reduces the IMD created when the Limiter section is active doing 'hard' limiting.  
"Hard" limiting of an Asymetrical signal will produce more IMD and odd-harmonic distortion.  

The stage for Monitoring the Limiter Action (LED (D3))  
is not traced through SPICE since it does not transfer a signal.
In practice, the LM324 has about 50 Ohms Z(out) 
and the voltage swing is limited  to 4.5 V up or down by the chip itself ( +/- 6 V supply). 
The monitoring LED runs OK as-is because   
the Front to Back voltage ratings on the single LED
are tolerant of this level of push/pull voltage.
 
***

•  

What is the secret of diode clippers?
Can we build these diode circuits in a logical manner
rather than giving them as ready-made circuit solutions?
- ResearchGate. Available from:
https://www.researchgate.net/post/What_is_the_secret_of_diode_clippers_Can_we_build_these_diode_circuits_in_a_logical_manner_
rather_than_giving_them_as_ready-made_circuit_solutions#view=56ff9c6693553b2e143e5750 [accessed Apr 2, 2016].
***

We did an earlier Limiter which was 
Preprocessed with an All-Pass Phase-Rotating stage
to shift Asymetrical signals to be Symetrical, at F( 700 ) Hz, 
in order to reduce odd harmonic distortion created by the instant acting diode limiters.  
InterModulation-Distortion can occur during heavy limiting action,
when signals are close together  and/or  aSymetrical.
Through experiment, it was determined that multiple All-Pass stages were not equal to the Roofing-Triad-Filter.

June 2015 : The All-Pass Filters were discarded because the Roofing-Filter  
 had measurably better results in aligning the phases and producing Symetrical waveforms . 
Rlmtdiode (R Limiter Diode = 4700 Ohms) can be varied up towards 33000 Ohms
for longer linear shaped curves, with good diode curve at top. 
Diode current flow is from aprox. 10 microAmps to 10 milliAmps. 


 

*** Negative-FeedBack Diode-Base-Biasing applied at the grounding of the Shunt Diodes :  
*** an experiment which produced dual levels of Diode Conduction (turn-on) at V(out).
This shows the Negative Feedback signal being used to control the base-bias of the shunt-diode matrix.  
There was good advantage to this more complex design, as it has two distinct turn-on curves. 

Higher level pulse noise would receive tighter (quicker) diode limiting (clamping) .
It was discarded in favor of a simpler Resistor+Diode Negative-FeedBack method. 

 
************************************************************************************************************************

*** Transient analysis for various combinations of Shut vs Negative-FeedBack methods. 
*** These Transient plots are from above Test Circuit with
(1) Shunt-Limiting
(2) combined with Negative-FeedBack Diode Base Bias. 

*** First No Shunt Limit and No Negative-FeedBack
*** Vout max = 12.9V
  

*** Second,  Transient for Shut Limit combined 
*** with Negative-FeedBack base-biasing the Shunt Diodes. 
*** Vout max = 3.6V

 



 *** Test Schematics for various combinations. "DDDG" 

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*** Test Circuit to show variations of the  Linearized-Log  Negative-FeedBack  Method ***
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*** below, the V(out) Limiter varies from  1.7V  up to 2.24V. ***





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*** Below  **********************************************

The Author's   Pre-Amp  includes an Active Log-Limiter, 
whose Limit Level is based on the Gain of the OPA stage.
Gain is calculated by  R(in) / R(feedback). 
As the stage gain is adjusted, in normal usage, 
the V(Limit-out) also changes.  
The Non-Linear Limiter is altered by a Linear relation with Gain. 
As Gain is increased, so the V(limit-log-out) also increases, 
providing more head room for the possibly larger V(out) signal. 
( Any possible pro/con to this is up for discussion. )

The attached Bode demonstrates the results of a full swing of gain from minimum to maximum.
Blue signal is V(in), and rainbow signals are various logs of the V(in) signal.   
*** Given V(in) = 3V,    the V(out)Limiter varies from  1.7V  up to 2.124V. ***

We Published this in the thread "What is the secret of diode clippers?" - ResearchGate.
Available from: https://www.researchgate.net



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Below:  these Bodes show the various types of diodes limiting signals and their natural shape.
Normalized to about 1V, as in actual use, which also aids in visualizing their effects. 
Vsignal is the dark-red trace at 3V,
Vdiodes are rainbow traces taken at V(out) and are in the 1V range.
Vdiodes are actually in the .23V to .60V range
and the circuit is adjusted to a standard 1V V(out) in the selected circuit prototyped.

Author's Spice traces : 
optimized/normalized  to normal 1V standard
so that the curve shape can be visually compared.

Silicon has as a sharper cut-off and Schottky  has a more sloping cut-off. 
The author selected the Germanium soft curve for use in AFX Limiter stage
because the characteristic curve could be measured and matching diodes selected in pairs.  
Another research associate selected silicon (KC9ON) for our commercial production model. 
The dark-red sine wave is the applied test signal.
Note: The sharp knee intermodulation effects are filtered out by he following BandPass Filters.
                                              

*******  Comparison of Germanium vs Silicon diodes from Data Sheets.   
******* Sharper Knee produces more InterModulation Distortion during Limiting Action. 
graphic source unknown 

**** Summary
****** Sharper Knee produces
****** more InterModulation Distortion during Limiting Action. 
graphic source unknown 

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Author's Method of Studying and Comparing  the  Simple-Shunt and  the Negative-FeedBack Limiters

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*** Below :   Normalized to the standard 1 V.  Vsignal is 3V, and Vdiodes is actually in the .23 to .60 range. 
*** In a real circuit the Vout is always normalized to some standard, such as 1V out Peak, by the user. 
*** Each type of Diode has a different Vout curve, and here they are Normalized for easier visual comparison. 
*** Black = Shunt only   = soft knee 
*** Blue = Shunt + Negative Feedback  = softest knee and least IMD.
*** Green = Negative Feedback only = sharp knee and greatest IMD.
Traces are optimized to normal so that the curve shape can be visually compared.

   

Above plots are normalized to the standard 1 V.  Vsignal is 3V, and Vdiodes is actually in the .23 to .60 range. 

******************************************************************************************************** 
Below:
Real world Turn-On curves are relative to actual current flowing in circuit and through diode.  
(The authors have designed working circuits, using negative-feedback,
which work in the 10uA to 60uA range, using Germanium diodes. )

*** Blue is Sine Sig at 3 V input, for reference.
*** Red is Germanium Diode gain controlled to compare with the Sine Sig.
*** Green is Germanium Diode at the standard 1 V ready for Vout to next circuit.
*** Black is Germanium Diode at real turn-on level.of 0.189 V in this circuit. 
  

*** Results: Blue-trace Vinput from 0 to 0.250 V is handled in a linear manner Red-Trace.
*** Then the Diode Limiter turn-on curve is entered , causing progressive limiting .
*** Vinput from 0. V up to 0.250 V is very linear.  
*** Vinput from 0.250 V up to max V is on the Log Curve.  

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Note:   The Author did a study of  Simple-Shunt Limiting using  Schotty Diode array, for "softer" log curves.  
Various modules were  inserted  /  disconnected.  
Schotty show a very soft curve but were difficult to match.

Schotty method discarded since the goal of the project was also to produce a Club Kit Project
with guaranteed results for any level of kit builder. . 

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