Opamp Schmitt Trigger Ref Voltage Problem - Page 1
Forum section: electronics / projects

author: jaiminajmeri
author_rank: Newbie
timestamp: September 05, 2013, 06:14:29 am
content: See the pic of the ckt.I want know how the voltage @R3 changes to 500mv instead of 5V.please tell me equations too.

author: w2aew
author_rank: Super Contributor
timestamp: September 05, 2013, 01:17:59 pm
content: There are a few problems in your circuit diagram with respect to supply voltages. Do I see -10V on VCC for the op amp? If you are intending to show +/-10V (split supply) on the op amp, then you should have +10V on VCC and -10V on VEE. Also, you show -12V at the inverting input. This is beyond the supply rails of the op amp, and certainly beyond the common mode voltage range of the op amp. Since the op amp model was likely designed to give good results ONLY if it is biased and used correctly, it probably won't give you proper results (models typically give unpredictable results if used outside of normal/proper operating conditions).That being said, assuming you correct the supply and biasing issues, the simple way to calculate the two switching thresholds would be to simply write an equation that says that the sum of the currents entering/leaving node 2 are equal to zero, and solve for V2.0 = (V2-10V)/1K + (V2-Vout)/1K + V2/1Kwhere V2 is the voltage on node 2 that you are measuring, and Vout is the saturated output voltage of the op amp at it's extremes. In the datasheet, these are VOH and VOL - the highest and lowest voltages that the output can swing. You solve the equation twice, once with Vout=VOH and once with Vout=VOL.

author: jaiminajmeri
author_rank: Newbie
timestamp: September 05, 2013, 02:30:22 pm
content: Quote from: w2aew on September 05, 2013, 01:17:59 pmYes, Thank you I got it.But the output must saturate between +Vsat and -Vsat right ? here Vsat is supposed to be 9V and -9V. But I am getting 8.48 and -8.48.Can u tell me Why is this happening ? any equations to prove this too ?

author: Mr Smiley
author_rank: Frequent Contributor
timestamp: September 05, 2013, 02:51:12 pm
content: It's a schmitt.the output changes the reference voltage to give it it's hysteresis levels. You calculate your resistance values to give you your desired input switching levels.

author: jaiminajmeri
author_rank: Newbie
timestamp: September 05, 2013, 02:57:20 pm
content: Quote from: Mr Smiley on September 05, 2013, 02:51:12 pmCan I know how to do that ? equations please ?

author: Mr Smiley
author_rank: Frequent Contributor
timestamp: September 05, 2013, 03:11:35 pm
content: http://sim.okawa-denshi.jp/en/compkeisan.htm

author: Andy Watson
author_rank: Super Contributor
timestamp: September 05, 2013, 03:15:14 pm
content: Quote from: jaiminajmeri on September 05, 2013, 02:57:20 pmStart with assumptions about the op-amp, i.e. assume it's perfect. You can now workout the voltage levels at its positive input by working out the potential divider formed by R1 + R2//R3. Then you can add in the imperfections like Vsat of the output, and the output resistance, etc..

author: w2aew
author_rank: Super Contributor
timestamp: September 05, 2013, 03:38:06 pm
content: Quote from: jaiminajmeri on September 05, 2013, 02:30:22 pmThe +Vsat and -Vsat values will depend entirely on the op amp you use, and it's internal design. Also, note that simulation models for a given op amp may not accurately reflect the op amp's true behavior when driven to saturation. The output saturation voltages depend on the internal design of the op amp, the power supply rails used, and the load on the output. There are no simple equations for this, since every op amp is different.

author: jaiminajmeri
author_rank: Newbie
timestamp: September 10, 2013, 04:35:29 am
content: Another Problem.I have a seen a ckt with positive feedback with capacitor grounded, with common input. looks like a scmitt trigger. But is it really 1 ?I dont know wat happens to that (schmitt trigger) if we some how manage to set same Vref and Vin ??