[sdiy] DC Offset Injector?

[Scott Gravenhorst]

Tim Parkhurst <tparkhurst at siliconbandwidth.com> wrote:
>
>> -----Original Message-----
>> From: Peter Grenader [mailto:peter at buzzclick-music.com]
>> Sent: Wednesday, March 17, 2004 4:53 PM
>> To: Oren Leavitt; synth-diy at dropmix.xs4all.nl
>> Subject: Re: [sdiy] DC Offset Injector?
>> 
>> This will do it:
>> 
>> http://www.buzzclick-music.com/dc_offsetter.jpg
>> 
>
>But this circuit will invert the incoming signal, right? A second inverting
>amp will get it back in phase with the original. Would it be okay to use a
>non-inverting summer in this case? 

that would work, you could also just put the reference on the inverting input
and the LFO output on the non-inverting input.  Then the phase is the same
and the opamp simply level shifts.

>BTW, what are the advantages/disadvantages of non-inverting stages? I'm
>trying to get this
>straight in my mind, and I believe I had read somewhere that a non >inverting
>stage can't have a gain of less than 1, meaning that everything (including
>noise and HF stuff) right up to the frequency limit of the op amp gets
>amplified along with the signal. Is this right or am I wrong (again)? 

Right, everything presented to the input, signal and noise is "copied" to the
output.  And yes, the opamp gain cannot be below 1.  Shorting the inverting
input to the output forms a voltage follower, meaning a gain of one.  If you
need a gain of less than one, you would use an attenuator (really just a
voltage divider which can be fixed or made from a pot).

>Also,
>does the 'virtual ground' concept work the same in inverting and non
>inverting configurations? 

Only the inverting input exhibits this behavior and only when the
noninverting input is grounded.

>The vast majority of the op amp circuits I see in
>SDIY use the inverting configuration, so there must be a good reason for
>that.

The virtual ground is actually the summing node.  The currents of all the
inputs are summed at that point.  Since the output supplies an equal but
opposite current through the feedback resistor, the voltage at the summing
node, or virtual ground, will be zero.  But this only works as a virtual
ground when the noninverting input is held at zero volts.

>Tim (virtually grounded) Servo
> 
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-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
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