# Schmitt trigger with three transistors

I usually understand well transistors circuits, but I just can't get to work with this one: (Note the circuit is powered with +/- 12V)

It is supposed to be a schmitt trigger circuit, and I'm trying to get a square wave at output by putting a triangle wave at the input. To do so, I'd like to detect when my triangle (oscillating from +12 to -12V) goes under -10V (negative trigger, output should go falling edge) and above +10V (positive trigger, output should go rising edge).

I could find an approximative formula for negative trigger, which is triggered when Q1 become cut-off:

$$V_{N} = (Vcc-0.3)[\frac{R2}{R1+R2}-1]$$

Now I'm trying to figure out what will be the formula for positive trigger, and what will be the voltage level at the output.

I'm also open to other circuits, if you mind something simpler (not using op amps, please).

• Why not use a free simulation tool? Do you understand the "long-tailed pair"? If not go google it. – Andy aka Feb 27 '18 at 10:19
• I do not agree with your $V_{cc} - 0.3$, I would use $V_{cc} - 0.7$ as the highest voltage at the emitter of Q3 is $V_{be_Q3} = V_{cc} - 0.7$. For the low threshold: Imagine what happens when Q1 is on, pulling the base of Q3 down. It depends on Rc1, Re and the input voltage what the voltage at the base of Q3 will be. Realize that a very high input voltage can pull up the emitter of Q1. Also realize that the values of the resistors matter here. If all resistors were 1 ohm, the circuit would never work. Their value is important but you do not show the values. – Bimpelrekkie Feb 27 '18 at 12:40