[sdiy] Clipping Amplifier questions...

[Oren Leavitt]

Hi Bret,

As for the 12 volts on the output, you may have left the (+) input 
floating with nothing connected to it.
In that case, the op amp output will drift towards a rail.
It should always be connected to something or grounded.
The inverting configuration Ryan suggested sounds like a better choice, 
if you don't mind the inverted signal.

Ryan wrote:

> Someone correct me if i'm wrong...
>
> I've got that book. I see the circuit but I don't think that will 
> work.  It seems to me that the output would be about 4V higher than 
> the input when the input is positive, 4V less with negative input. Is 
> that what you are measuring?
>
> The opamp will keep its output at just the right voltage so that both 
> of the inputs are at the same voltage, and the zeners cause a 4V drop 
> so, in this configuration the output would have to be 4V higher/lower 
> than the input (depending on the sign) at all times.
>
> You can make it work by changing it to an inverting amplifier, just 
> switch the ground with the input you have and it should work, but 
> you'll need another opamp to invert it again.
>
> ..Ryan
>
>>
>> Does anyone happen to have the book "Practical Electronics for 
>> Inventors"?  In it, there's a very nice looking circuit that they 
>> call the "Noninverting Clipper Amplifier" (page 245).  If you don't, 
>> I made a copy by hand (http://www.voxglitch.com/images/clip_amp.jpg)
>>
>> I can't, for the life of me, get this wee-little circuit to work.  
>> The circuit is a normal op-amp based amplifier, but an addition of 
>> two zener diodes accross the feedback loop was supposed to limit the 
>> voltage (both positive and negative) to the cutoff voltages of the 
>> zener diodes.
>>
>> I intended to use this amplifier circuit for one of the inputs to a 
>> module.  The module has an imbeded effects board, which has a 4V 
>> maximum rating on it's audio input.
>>
>> I had _thought_ that I had it working at one point and I created a 
>> PCB with the circuit on it.  However, it doesn't seem to be doing the 
>> job.  The zener diodes are 3.3V, but I can measure a full 12V on the 
>> output line of the op-amp.  I wired up the op-amp circuit again on a 
>> breadboard and got the same results.  Something is afoul.  I'm using 
>> TL082 op-amps.
>>
>> If anyone can take a moment to look at the circuit in question, I 
>> would be very happy.  Or, if I'm approaching this in the wrong way, 
>> let me know.  My intention was to have an input that I could control 
>> the level on (via potentiometer), but also have the input voltage 
>> clipped below 4 volts to protect the imbedded effects unit.
>
>
>
>

-- 
Oren Leavitt
oleavitt at ix.netcom.com