[sdiy] Current sink schematic/calculations help.

[nicolas]

Perhaps the +/- inputs on that opamp are round the wrong way?


--- On Sat, 17/12/11, ASSI <Stromeko at nexgo.de> wrote:

> From: ASSI <Stromeko at nexgo.de>
> Subject: Re: [sdiy] Current sink schematic/calculations help.
> To: synth-diy at dropmix.xs4all.nl
> Received: Saturday, 17 December, 2011, 8:14 AM
> Am Freitag, 16. Dezember 2011,
> 17:35:15 schrieb Justin Owen:
> > I set up this schematic: http://www.sdiy.org/juz/current_sink.pdf
> based on
> 
> You haven't set up for biasing Q1 correctly in that
> schematic, which 
> certainly explains some of the wierdness you see. 
> Also, your op-amps all 
> need to be unity-gain stable with good phase margin. 
> The emitter of Q1 in 
> your circuit will be exactly at the CV input (0..5V), but
> you'll want it to 
> sit at some negative potential if you want to produce the
> Iabc for a diff 
> pair.
> 
> You might try an active V-I with current mirror along the
> following sketch:
> 
>  buffered CV o--------+         
>        +--------o Iabc
>                
>       |         
>        |
>                
>     +-+-+           
>    |
>                
>     |   |     
>          |
>                
>     |   | R1     
>       |
>                
>     |   |     
>          |
>                
>     |   |     
>          |
>                
>     +-+-+           
>    |
>                
>       |    \     
>       |
>                
>       |    |\     
>      |
>                
>       |    | \     
>     |
>                
>       o----|+ \     
>    |
>                
>       |   
> |   >--+     |
>              
> +-------|----|- /   | 
>    |
>               | 
>      |    | /   
> |     |
>               | 
>      |    |/ 
>    |     |
>               | 
>      |    /   
>   |     |
>          
>    ___    +-+-+     
>    |     |
>                
>      \|/   D1 
>    |     |
>                
>     __V__     
>    |     |
>                
>       |       
>    |     |
>                
>       |       
>    |     |
>                
>       \ |     
>    |   | /
>                
>        \|     
>    |   |/
>                
>         +---------o---+
>                
>        /|     
>        |\
>                
>       v |         
>    | v
>                
>       |         
>        |
>                
>       |         
>        |
>                
>       +-----------+-----+
>                
>                
>   |
>                
>                
>   |
>                
>                
>   o -V
> 
> D1 shifts the collector potential, a single diode assumes
> that the 
> differential pair will be biased at 0V and hence the
> emitter node will be a 
> diode drop below 0V (e.g. Iabc must be pulled from a source
> that is at 
> -0.7V).  If you leave D1 out, the precision of the
> current mirror degrades, 
> but it will still work.  The opamp keeps the voltage
> drop over R1 identical 
> to the CV (by keeping the "foot" at 0V), so that I = V/R =
> Iabc.
> 
> 
> Regards,
> Achim.
> -- 
> +<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda
> XTk Blofeld]>+
> 
> Wavetables for the Waldorf Blofeld:
> http://Synth.Stromeko.net/Downloads.html#BlofeldUserWavetables
> 
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