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How does this circuit work?

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separate the opamp circuit from the comparator circuit

remember the behavior of RC circuits when switches close or open
the transistor q1 acts like a switch

google AN-31
open the pdf listed under TI (texas instruments) Amplifier circuit collection Rev C
it was a national semiconductor data book i had on my shelf
it is a collection of opamp circuits
very useful
 

What I wasn't clear (as novice) was the following

1. Threshold is set 2/3 Vref and 1/3vref. But is lower threshold not zero?
2. Why is tap taken from vin to positive terminal of op-amp and why is it half of VIN through voltage divider?
3. When MOSFET is ON, discharge timing is R4C1 and charging time is R1C1 (according to me). Does it really produce triangle wave?
4. Inverting Integrator always produces voltage output of opposite polarity, but threshold is positive, why?
5. In the frequency output equation, I do not find role of R4?

Sorry for by silly and basic questions?
 
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1. Threshold is set by the voltage at noninverting input of IC2. You can calculate it for both cases, square out = 0 and square out = 5V.
2+3. For symmetrical triangle wave, positive and negative currents into integrator capacitor C1 must have equal magnitude. You can calculate that the condition is fulfilled for R4=0.5*R1 and R2=R3.
4. You have opposite dV/dt rate, absolute voltage can be any value within power supply range. Thresholds set the voltage range. You could also implement a circuit with dual supply and bipolar output voltage.
5. R4=0.5*R1 is taken as granted.

discharge timing is R4C1 and charging time is R1C1
Click to expand...
No, integrator gain is +/- 1/(0.5*R1*C1)
 

how to select jfet/mosfet?
 

Needs n-channel enhancement type -> NMOSFET. Selected 2N7000 looks good, generally low output capacitance, reasonable Rdson (5 - 100 ohms). CMOS analog switch would be an alternative.
 

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