# Triangular signal cutting off

#### ceulen101

Joined Mar 9, 2020
2
I made a circuit to demonstrate the work of a 555 and a constant current regulator and wanted to add triangle wave input. I've done that by integrating the output of a 555 timer, and at the node 'tri' I do get the desired triangle wave of ~ +-8V. Since I want the the voltage positive at all times (for the triangle to go from 0 to 8V), I added 8 Volts DC to the triangle signal. The node 'Vin' should be $Vin=\frac{V_{tri}}{2}+\frac{8V}{2}$. However, what I get is a cut off triangle wave at node 'Vin', at about 6V, even though the output of the 555 at the point 'tri' is correct. I can't seem to figure out what is the cause of this, I could use some help.

And the voltages at nodes tri and Vin, as well as the collector current (not relevant, that part works):

*Rthres and Q2 are there to limit the current through the diode if need be, but since Rthres=0 they don't play a part in this case. Opamps are 741s with +-15V supply.

#### AlbertHall

Joined Jun 4, 2014
11,302
When the output of X6 is at 6V, there is 2V across 200Ω (R4 + R5) and that needs X6 to sink 20mA which it seems it can't. Change R4 and R5 to 1k or even 10k.

#### ceulen101

Joined Mar 9, 2020
2
Would you look at that, it works now! And I spent 2 days trying to get it right. Thanks! Though, if you find time, could you explain a bit more why was that the problem? Why can't it sink 20mA, and how did you get the 2V?

#### AlbertHall

Joined Jun 4, 2014
11,302
The 2V is the 8V supply to R4 minus the 6V at the opamp output.
Every chip has a maximum current that it can sink or source that the datasheet should indicate and it is usually wise to not get too near those limits for reliable operation.

Joined Mar 10, 2018
4,057
R4 & R5 at 100 ohms means X6 has to sink 80 mA -pk of tri, is
the part capable of that ?

Regards, Dana.

#### crutschow

Joined Mar 14, 2008
27,170
What part number is the op amp?

#### Audioguru again

Joined Oct 21, 2019
3,171
The opamp part number and its supply voltages are not shown.
All resistor values around the opamps are way too low.

#### Kjeldgaard

Joined Apr 7, 2016
451
Is C2 missing a parallel resistance for DC balance?

#### AlbertHall

Joined Jun 4, 2014
11,302
The opamp part number and its supply voltages are not shown.
Shown at the bottom of post #1

#### Audioguru again

Joined Oct 21, 2019
3,171
Even the output voltage swing of the Cmos 555 is reduced by the very low 2.3k ohms it drives, and the loss of each IC will be different.
Why is there a 10k resistor at the CONT pin if the Cmos 555? This resistor will cause the threshold voltage to change with temperature changes. The threshold voltage will be different with each IC and instead should be a filter capacitor if needed.

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