# simple opamp amplifier circuit

Started by June 18, 2012
```On 6/18/2012 11:05 AM, panfilero wrote:
> On Monday, June 18, 2012 9:37:27 AM UTC-5, Fred Bartoli wrote:
>> panfilero a &#65533;crit :
>>> I'm simulating on LTSpice, a simple amplifier circuit with a 2.5V offset, but if I don't include a capacitor on my input my output hits the rails... why is this?  The link shows what I'm doing in LTSpice
>>>
>>> thanks
>>>
>>> http://i555.photobucket.com/albums/jj477/panfilero/Capture-1.jpg
>>>
>>
>> Assuming your DC input is 0V, then the circuit has a gain of 3 WRT to
>> the opamp positive input and the 3x2.5V = 7.5V hits the rail.
>>
>> Do you really need simulation for that?
>>
>> --
>> Thanks,
>> Fred.
>
> what's WRT?

'With Respect To'
```
```bloggs.fredbloggs.fred@gmail.com a &#2013265929;crit :
> On Monday, June 18, 2012 10:37:27 AM UTC-4, Fred Bartoli wrote:
>> panfilero a &#65533;crit :
>>> I'm simulating on LTSpice, a simple amplifier circuit with a 2.5V offset, but if I don't include a capacitor on my input my output hits the rails... why is this?  The link shows what I'm doing in LTSpice
>>>
>>> thanks
>>>
>>> http://i555.photobucket.com/albums/jj477/panfilero/Capture-1.jpg
>>>
>> Assuming your DC input is 0V, then the circuit has a gain of 3 WRT to
>> the opamp positive input and the 3x2.5V = 7.5V hits the rail.
>>
>> Do you really need simulation for that?
>>
>> --
>> Thanks,
>> Fred.
>
> huh?

(huh?)?
Huh? :-)

--
Thanks,
Fred.
```
```"Jon Elson"
>>
>> http://i555.photobucket.com/albums/jj477/panfilero/Capture-1.jpg
>
> This circuit has a 2:1 DC gain ...

** Nope -  the schem as shown has a DC gain of zero.

Without C1 ( and R2 grounded) it has a DC gain of 3, so the 2.5 volt fixed
input drives to op-amp to the 5V rail.

The "pan fried sandwich" is a colossal idiot.

And you are no better.

....   Phil

```
```Try centering the input around 2.5V instead of 0 volts
```
```<rcmonsen@gmail.com> wrote in message
> Try centering the input around 2.5V instead of 0 volts

Good suggestion, bump the offset in the sig gen to 2.5V

```
```Dennis wrote:
> <rcmonsen@gmail.com> wrote in message
>
>>Try centering the input around 2.5V instead of 0 volts
>
>
> Good suggestion, bump the offset in the sig gen to 2.5V
>
>
Why not use a dual rail supply, virtual ground or
a (-) regulated DC-DC simple converter on board to
supply the (-) rail?

Jamie

```
```On 6/18/2012 2:59 PM, Phil Allison wrote:
> "Jon Elson"
>>>
>>> http://i555.photobucket.com/albums/jj477/panfilero/Capture-1.jpg
>>
>> This circuit has a 2:1 DC gain ...
>
>
> ** Nope -  the schem as shown has a DC gain of zero.
>
> Without C1 ( and R2 grounded) it has a DC gain of 3, so the 2.5 volt fixed
> input drives to op-amp to the 5V rail.
>
> The "pan fried sandwich" is a colossal idiot.
>
> And you are no better.
>
>
> ....   Phil
>
>
>
>
When learning, you need to start somewhere. I see no reason to insult
the original poster. Everyone else managed to be civilized.

```
```"miso = Cunthead"

>> The "pan fried sandwich" is a colossal idiot.
>>
>> And you are no better.
>>
>
> When learning, you need to start somewhere.

** When bullshitting  -  you need to know when to stop.

> I see no reason...

** That alone is true, the rest is purest bullshit.

Fuck off  -  you bloody troll.

...  Phil

```
```On Jun 18, 4:03=A0pm, panfilero <panfil...@gmail.com> wrote:
> On Monday, June 18, 2012 9:25:39 AM UTC-5, panfilero wrote:
> > I'm simulating on LTSpice, a simple amplifier circuit with a 2.5V offse=
t, but if I don't include a capacitor on my input my output hits the rails.=
.. why is this? =A0The link shows what I'm doing in LTSpice
>
> > thanks
>
> >http://i555.photobucket.com/albums/jj477/panfilero/Capture-1.jpg
>
> sorry, the input signal is a sine wave, with no dc offset, at 10kHz with =
0.25V amplitude

Input signal has zero offset, opamp has half Vrail offset. Problem.

NT
```
```On Tue, 19 Jun 2012 07:59:16 +1000, "Phil Allison" <phil_a@tpg.com.au>
wrote:

>
>"Jon Elson"
>>>
>>> http://i555.photobucket.com/albums/jj477/panfilero/Capture-1.jpg
>>
>> This circuit has a 2:1 DC gain ...
>
>
>** Nope -  the schem as shown has a DC gain of zero.
>
>Without C1 ( and R2 grounded) it has a DC gain of 3, so the 2.5 volt fixed
>input drives to op-amp to the 5V rail.
>
>The "pan fried sandwich" is a colossal idiot.
>
>And you are no better.
>
>
>....   Phil
>
>
>
On my planet, the DC gain relative to 2.5V  (without C) would be
-R1/R2. i.e. -2x.
I know, it is hard..
```