Currently, I'm trying to analyze a Schmitt trigger circuit using the 741 IC op-amp, and how it overcomes the problems faced by an open loop comparator circuit.
This is a typical positive feedback induced inverting Schmitt trigger:
The design specifications are as follows -
- \$ V_{CC} = +-8V \$
- \$ V_{threshold} = 2V \$
- Noise band tolerance \$= +- 0.3V \$
In this case, \$ V_{out} = +- V_{sat}\$ , and \$ V_{sat} = +- V_{CC} \$.
The noise tolerance refers to the hysteresis band, in my opinion, and that of width 0.6V. With the equations relating the design of the Schmitt trigger and its resistors R1,R2 , I can find R1 and R2 with the help of the lower and upper tripping voltages \$ V_{LTP} = -V_{sat}\frac{R2}{R1+R2}\$ and similarly for \$V_{UTP}\$, but all I've read about hysteresis loops and tripping voltages are symmetrically situated about 0V. Here, the threshold voltage is 2V, so as a result, \$V_{UTP} = 1.7V\$ and \$V_{LTP} = 2.3V\$.
I don't know how to set the reference around 2V, as I have only read about tripping voltages being of the same magnitude but different signs. Is there any possibility that I'm interpreting wrongly? Can someone help me out in this?
EDIT 1:
PS: Owing to helpful answers, I have figured out that I need to set up a reference voltage. The problem now is that I've only the dual power supply, and 3 resistors to make it happen, and I can't seem to think of any divider bias as such to set the reference voltage to 2V. It would be helpful if someone could atleast throw some hints so that I can proceed.
EDIT 2: Here are the equations -
Connecting the resistor R1 to +Vcc, R2 to ground and R3 as the feedback resistor, I get $$ \frac{V_{CC} - V_{+}}{R1} - \frac{V_{+}}{R2} - \frac{V_{out} - V_{+}}{R3} = 0$$
Therefore, I finally get - $$ V_{+} = (\frac{1}{R1+R2+R3})(\frac{V_{CC}}{R1} + \frac{V_{out}}{R3}) $$
So, to get 2.3V and 1.7 V, I can plug in the saturation values in high state and low state to get the respective resistor values. But, I don't understand how does this help me to create a reference of 2V. It would be helpful if someone could confirm if I'm proceeding in the right way.