Op amp as buffer problems

[Rusttree]

I'm trying to use an LM741 as a buffer, but I'm getting peculiar behavior. I have the signal connected to the non-inverting pin and the output connected to the inverting pin. VCC (3.3V) and GND are connected to V+ and V-, respectively. I also have a 0.1uF capacitor from VCC to GND near the op amp's pins.

The output I'm getting is only a partial voltage following. See the figure attached. The blue trace is the input wave and yellow is the output of the buffer. It appears as though the output is clamping between an arbitrary upper and lower bound. Within that bound, however, it's following the input signal perfectly.

I've tried a couple different op amp chips and they all produce the same behavior. Any troubleshooting advice would be much appreciated. I'm not even sure where to start.

Thanks!
-Dan

 

[ericgibbs]

hi Dan,
The 741 is a VERY old OPA, with a poor specification.
The output will not swing lower than about +1V and less than Vcc -1.5V

Thats the limiting problem, approx +1V thru 2V

 

[Rusttree]

Ah, thank you Eric. That helps a lot. I scrounged around and found an LM324 and threw that in the circuit. It did better, but still clamped the upper end. I see in the datasheet there's a "Input common mode voltage range" specification of Vcc-1.5V. I assume that's what you're referring to. So I connected a 5V supply to the V+ pin and now it works perfectly.

So are there op amps that will allow the full range of voltage without needing an external power supply?

 

[ericgibbs]

Ah, thank you Eric. That helps a lot. I scrounged around and found an LM324 and threw that in the circuit. It did better, but still clamped the upper end. I see in the datasheet there's a "Input common mode voltage range" specification of Vcc-1.5V. I assume that's what you're referring to. So I connected a 5V supply to the V+ pin and now it works perfectly.

So are there op amps that will allow the full range of voltage without needing an external power supply?

hi,
In 'regular' OPA's the upper swing is about 1.5V lower than the Vss, also some OPA's will not get less than 1V.

What you really need is a rail to rail OPA, these swing very close to 0V and Vss, a inexpensive version is the MCP6002 a dual. Vss =5.5Vmax

 

[Rusttree]

I see. Thanks Eric. That is very informative.