I got confused about the positive feedback~~~

[ZengLei]

look at the Pic

i calculate it ,the get the result:

Vn=0

(Vin-Vp)/R2=(Vp-Vout)/R1

Vp*A=Vout
A is the gain of op and it is infinite

so i get the result:

(Vout/Vin)=-R1/R2;

R1=R2;so......Vout/Vin=-1;
so Vout=-5;

but someone said it's a SCHIMITT TRIGGLE,it will saturation!!!!!
it's starnge!!!
i think the result is -5,and Multisim add a very good evidence~~~~~~~~

But,they said it is not -5,it's a schimmit triggle~~~~~~~~~

why???????

i got confused about that~~~~~~~

 

[ZengLei]

and it's a positive feedback

Added after 49 minutes:

and another question is the signal is inputed in the positive terminal,maybe the output signal is positive becase of the positive input signal but the output signal is negative,why?

 

[RDRyan]

It is a positive feedbback. The input of the OP is not virtual shorted.
You should not use that formula to do calculation. The gain of OP is not infinite.

 

[bas]

this is not an inverting amplifier this is a comparator
to make your amplifier you have 2 methods : an inverting or a non-inverting amplifier

you must connect R1 and R2 from between the output ant the inveting input (-) and the input signal goes tthrough R2 to the non-inverting input
the non-inverting input is grounded
gain is Av=-R1/R2 (R1 is your feedback resistor)
this way you get your inverting amplifier

 

[ZengLei]

i did not think Vp and Vn are visual short,i use the module to calculate:

(Vin-Vp)/R4={Vp/(Rin+R5)}+{[Vp-(Vp-Vn)*A]/(R3+Rout)}

Vp*R5/(R5+Rin)=Vn,



A=1000G
Rin=1000G>>R5=5K
R3=10K>>Rout=1
==>(Vin-Vp)/R4=Vp/Rin+[Vp-(Vp-Vn)*A]/R3
Vp*R5/(R5+Rin)=Vn--->Vn=Vp*R5/Rin is 0！！！

then i get Vin-Vp)/R4=Vp/Rin+(Vp-Vp*A)/R3

Vp=(Vin*Rin*R3)/(R3*R4+Rin*R4-Rin*R4*A+Rin*R3)
then i get：Vp=0!!!
so：Vp=Vn=0;

the result is：-5V

and Multisim prove that i am right!!

but,??maybe there something wrong with the calculation????

 

[RDRyan]

If you connect the OP in positive feedback like your circuit, the OP will not in amplifer region, some MOS of OP will go to liner region, and the output of OP will go to VDD or GND because of the high gain.
In your formula, you have used Gain A=1000 that maybe is not correct. and the VCVS model is not suit in this situation. Maybe you can add some options in VCVS, such as (max=5 min=0).

 

[RDRyan]

Continued..

I have simulated this circuit, in EWB, the result is -5.

But, if you add a diode in series with a 5V battery connect to the output of OP to limite the max output, and add a reverse diode with a 1v battery connect to the output of OP to limite the min output. ( this may have the behaviour like the actrual circuit).
Change the input, use a sin wave amplitude=5 offset=5. then you will find this circuit works as a schimitt trigger.

 

[grittinjames]

when u power up a high gain amplifier initial out put can not be predicted . it will vary for each device .

in ur circuit opamp is going initialy to negative saturation.;
eventhough if ur giving 5v if opamp out put is -saturation input will be -ve 5 v and it will try to continue in its state

 

[Meridian]

But,they said it is not -5,it's a schimmit triggle~~~~~~~~~

... Schmitt Trigger ... (just for your information and no pun intended!)