analog logic of shmidt trigger bjt circuit

[john23]

Hello, the following circuit turns slow pulse into fast one.
how the BJT makes it happen?
Thanks.

[Andy Watson]

Positive feedback.

The emitters of the transistors are coupled together. When one of the transistors begins to conduct it robs some drive from the other transistor ... which causes the original transistor to conduct more, etc., until the circuit has flipped state.

[magic]

With Q4 turned off, R7 turns on Q5 and the output is driven low. R7 + R8 currents flow through R9, dropping almost 200mV.
With Q4 turned on, Q4 saturates and turns off Q5. Only R7 current flows through R9, dropping less than 100mV.

Reminder: bipolar transistor behaves very much like a diode, in that its current increases sharply when base-emitter voltage crosses some 0.6~0.7V, depending on transistor type, temperature and magnitude of current being switched.

Hence Q4 switches when its base is about 650mV above R9 voltage.
The low to high threshold is 200mV + 650mV or 0.85V.
The high to low threshold is 100mV + 650mV or 0.75V.
This explains the hysteresis.

During transition, R9 voltage begins to change in the direction which accelerates the transition.
This gives the fast output edges.

[john23]

Hello,how did you get  100mv 200mv values?

"Hence Q4 switches when its base is about 650mV above R9 voltage.
The low to high threshold is 200mV + 650mV or 0.85V.
The high to low threshold is 100mV + 650mV or 0.75V."

[magic]

It's the voltage at the top of R9, so Ohm's law

In low state, there is over 4V across R7 and almost 5V across R8, and both of those currents (almost 10mA) flow into R9.
In high state, there is almost 5V across R7 and this current flows into R9.

[john23]

Hello,how this feedback mechanism works in this circuit?

[john23]

Hello Andy, yes both currents go into the same R9.Why why Q4 has more current then Q5 then it will cause it to take even more current?

"The emitters of the transistors are coupled together. When one of the transistors begins to conduct it robs some drive from the other transistor ... which causes the original transistor to conduct more, etc., until the circuit has flipped state."

[Andy Watson]

Why Q4 has more current then Q5 then it will cause it to take even more current?

As Q4 turns on, the voltage on its collector will start to drop. When there is less than approx 0.6V across Q4, the drive for Q5 is reduced. Less current through Q5 causes more current in Q4 because it is trying to maintain the voltage on the common emmiter resistor. Go back to the start of this paragraph and repeat until the circuit has flipped state.