[sdiy] Problem with DCO

[Tom Wiltshire]

Thanks both, that's sorted it. I simply swapped the polarity going  
into the integrator and as if by magic everything became hunkydory.

What I was missing was that Roland were evidently using a negative  
voltage. That isn't immediately clear from the schematic.

Instant solutions on this list...great suff!

T.


On 13 Feb 2009, at 18:49, Eric Brombaugh wrote:

> Tom Wiltshire wrote:
>> Hi All,
>> I've been experimenting with a DCO design based on the Roland JX8P  
>> circuit:
>> http://www.electricdruid.com/DCOTestDesign.gif
>> Broadly speaking, it works, in that I can get a ramp wave out of  
>> it if I adjust the 47K pot on the left. This pot and op-amp buffer  
>> will be replaced by a DAC when I get it working better. I've got  
>> reset pulses generated by a PIC being fed into the base of the  
>> transistor at 440Hz for the purposes of the test. That part is fine.
>> The problem is that the output goes from 0V down to -0.5V and then  
>> seems to saturate. What's going on? How can the op-amp output be  
>> saturated at only -0.5V when it's running on +/-15V rails?  
>> Everything I've read suggests that the output from an integrator  
>> ought to carry on as far as the op-amp output can take it, so I  
>> was expecting a ramp from 0V down to -15V or thereabouts. I'm sure  
>> there's something I'm missing here...
>
> The ramp is negative going, so Vcb is negative. That causes the  
> base collector junction of your reset transistor to forward bias  
> and limit the output voltage. You'll need to rethink the way you've  
> got that reset transistor hooked up, or reverse the direction of  
> the ramp so it's positive going (hint - invert the input to the  
> integrator).
>
> Eric
>