Newbie question about capacitor discharge through BJT

Thread Starter

manthura_assemblies

Joined Feb 11, 2014
11
Hi everyone,

I am working on a ramp wave generator using this chain of components:

  1. an op-amp integrator (with a capacitor C), which generates the ramp
  2. an op-amp comparator, which detects if the step 1. integrator output reaches a fixed threshold and switches from 'high' to 'low'
  3. a saturated BJT. The base is connected with 2. output
I want to pose the Vce output in parallel with the capacitor C to discharge it, but all my efforts are vain.

Here below is an image of the schematics:synth_20160121.JPG

Thank you in advance for your help!!!!
 

Thread Starter

manthura_assemblies

Joined Feb 11, 2014
11
Your drawing has an error. Too many ground lines on the voltage source.
Thank you for the reply.
I've deleted a line betweeen ground and Ve.
Talking about V3, what I want to do is draw the negative rail, which feeds my op-amps along with positive rail (expressed by V1).

Here is the new version:

upload_2016-1-22_9-11-3.png
 

Dodgydave

Joined Jun 22, 2012
9,310
V3 is shown as as a positive supply the +/- signs are wrong,as it stands its ok the output will go from zero to positive,

If you connect your emitter to the negative V3 rail, and take your output from ground and the collector, your output will swing negative and positive through zero.
 

Thread Starter

manthura_assemblies

Joined Feb 11, 2014
11
V3 is shown as as a positive supply the +/- signs are wrong,as it stands its ok the output will go from zero to positive,

If you connect your emitter to the negative V3 rail, and take your output from ground and the collector, your output will swing negative and positive through zero.

This is what i get with the modifications you suggested:

upload_2016-1-22_11-42-1.png

Maybe I am missing something... (i.e. the connection of Ve with negative rail). Still I miss how to connect BJT to capacitor...
 

wayneh

Joined Sep 9, 2010
16,399
That looks better. What is the problem?
Note that the base voltage of the transistor will turn it on at any voltage above -8.3V.
 

Thread Starter

manthura_assemblies

Joined Feb 11, 2014
11
That looks better. What is the problem?
Note that the base voltage of the transistor will turn it on at any voltage above -8.3V.
What if I connect Emitter to ground? Anyway, putting cpacitor in parallel with Vce seems not to work (I don't get the desired oscillation). Maybe it's a timing problem...
 
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