[sdiy] Problem with DCO

[Eric Brombaugh]

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