DC & Op Amps

[zero]

In PSPICE I created a circuit with a VDC = 6V feeding into a uA741 op-amp in
a inverting amplifier configuration, with both the input and feedback
resistors at 1K.

I expected Vout = -6V but instead got Vout to be about 0V. I also expected
the V- to be 0V because of virtual ground but it came out about 1/2 of Vin.

I was wondering why the op-amp doesn't amplify at DC.

 

[Paul Wagner]

"zero" <slave@discussions.com> schrieb im Newsbeitrag
news:M7P0e.36100$qn2.8080122@twister.nyc.rr.com...

In PSPICE I created a circuit with a VDC = 6V feeding into a uA741 op-amp
in
a inverting amplifier configuration, with both the input and feedback
resistors at 1K.

I expected Vout = -6V but instead got Vout to be about 0V. I also expected
the V- to be 0V because of virtual ground but it came out about 1/2 of
Vin.

I was wondering why the op-amp doesn't amplify at DC.

It does, but only in the given bounds. For the OPAmp to go to negative
voltages, it has to be supplied with a negative voltage, which means that
you have to use a symmetric power supply (two DC-sources in series, the
connection point being ground).

So use

+6V
|
|
-----
---
|
|
|
o--------+
| |
| GND
-----
---
|
|
|
-6V

instead of

+6V
|
|
-----
---
|
|
|
GND

and it will work. By the way, the 741 does reach its supply limits only
within a margin of about 2..3V, so if supplied with +/-6V, you can only
expect +/-4V at the output.

Paul