Help with OpAmp Circuit Calculation

[~analoger~]

Hello

What's the output voltage Vout in terms of the input voltage Vg in this attached circuit? My answer I'm not sure about is Vout = 2*Vg - 5



Thanks.

 

[Junus2012]

Hello

First of all, please provide us with the op-amp voltage. However it seems that the op-amp is supplied with singleDC 5 V. if it is like this, the output voltage will have only positive swing with this expression

Vo= (10 K/5 K )*Vg-2.5 or 2*Vg-2.5

f the input is sinusoidal signal the negative part will not appear at the output. Remember also that adding or subtracting a DC value from the AC is nothing rather than a shift in the AC by theDC amount

Hello

What's the output voltage Vout in terms of the input voltage Vg in this attached circuit? My answer I'm not sure about is Vout = 2*Vg - 5

View attachment 86419

Thanks.

 

[~analoger~]

The supply voltage is 5 V. But how would this affect the analysis?

 

[barry]

Nope.

Here's what I got:

(Vg-2.5)/5K=(2.5-Vx)/10K ; Vx= op-amp output.

Rearranging: Vx= 7.5-2*Vg

Vout=Vx-2.5=5-2*Vg

One way to check this, is to think about what the output would look like if then non-inverting input were grounded-then you'd have a simple inverting amp, so the output would have to be negative with respect to the input (not what your equation shows)

 

[Junus2012]

if he is using single supply (VDD= 5 V and the ground), then the non inverting terminal should be at the middle of this supply, so it should be 2.5 to get maximum swing

Nope.

Here's what I got:

(Vg-2.5)/5K=(2.5-Vx)/10K ; Vx= op-amp output.

Rearranging: Vx= 7.5-2*Vg

Vout=Vx-2.5=5-2*Vg

One way to check this, is to think about what the output would look like if then non-inverting input were grounded-then you'd have a simple inverting amp, so the output would have to be negative with respect to the input (not what your equation shows)

 

[~analoger~]

How would you assign the voltages to input terminals? Both have to be equal as I understand from the ideal OpAmp rules, v+ = v-
But which one equals which voltage? So both are 20 or 10 V?

 

[pebe]

Output voltage = 2(3.75-Vg)
Maximum excursions of Vg = 2.5±1.25V = 1.25V to 3.75V

 

[barry]

How would you assign the voltages to input terminals? Both have to be equal as I understand from the ideal OpAmp rules, v+ = v-
But which one equals which voltage? So both are 20 or 10 V?

View attachment 86454

That circuit is a degenerate case. You can't analyze that using an ideal opamp model.

- - - Updated - - -

The supply voltage is 5 V. But how would this affect the analysis?

You are absolutely correct, for an ideal case. The power supply LIMITS the output excursions, but has no effect on your gain calculation.

 

[~analoger~]

That circuit is a degenerate case. You can't analyze that using an ideal opamp model.

I tried it with PSpice and it did not complain. The voltages on V+ V- and Vout are: 20V, 10V and -15V respectively.

How can I tell if it's a degenerate case?

 

[LvW]

I tried it with PSpice and it did not complain. The voltages on V+ V- and Vout are: 20V, 10V and -15V respectively.
How can I tell if it's a degenerate case?

Sounds fantastic. -15 volts output with a single +5 V supply.

 

[bigdogguru]

Sounds fantastic. -15 volts output with a single +5 V supply.

It's one of those new OPAMPs with a builtin SMPS.

 

[~analoger~]

Sorry, the second circuit is different it's 18 V supply.

 

[LvW]

-analoger-,

if you really expect a serious answer you should present a complete schematic with all supplies and signal sources (indicate dc or ac).
By the way - it looks surprising to see a voltage source connected to the opamp output. Your own invention?

 

[barry]

-analoger-,

if you really expect a serious answer you should present a complete schematic with all supplies and signal sources (indicate dc or ac).
By the way - it looks surprising to see a voltage source connected to the opamp output. Your own invention?

Yeah, I was wondering about that voltage source, too. But you never know what a professor is going to give you for homework....

- - - Updated - - -

I tried it with PSpice and it did not complain. The voltages on V+ V- and Vout are: 20V, 10V and -15V respectively.

How can I tell if it's a degenerate case?

The question was about v+=v- which obviously can't happen because you've got two ideal voltage sources driving your ideal opamp. SOMETHING has to give. In REALITY (big word, that) there IS a finite differential voltage between the inverting and non-inverting inputs.

 

[LvW]

~analoger~

Some further hints:
* The dc voltage of 2.5 volts at the noninv. input wiil be amplified by (1+10/5)=3 and, thus, appears at the output as +15 volts;
* If you like to suppress any dc voltage at he output simply use a capacitor.

 

[barry]

~analoger~

Some further hints:
* The dc voltage of 2.5 volts at the noninv. input wiil be amplified by (1+10/5)=3 and, thus, appears at the output as +15 volts;
* If you like to suppress any dc voltage at he output simply use a capacitor.

Oooo, new math: 3*2.5=15!!!

 

[LvW]

Oooo, new math: 3*2.5=15!!!

Barry, thanks - but you should take into account that here in Germany we are short before sleeping time. Nevertheless, blame my pocket calculator.

 

[barry]

Barry, thanks - but you should take into account that here in Germany we are short before sleeping time. Nevertheless, blame my pocket calculator.

Just having some fun, don't take it personally Gute Nacht