Op am as a rectifier

[MrOmnos]

Hi, I was watching Dave's videos on op amps and was confused when he started talking about offsets. If I take an op amp in closed loop configuration with a gain of 1 and the negative power rail connected to ground and feed a sine wave, will it function as a half wave rectifier? I was thinking that since, the negative rail is connected to ground, the output can't swing to negative. So output will show only the positive part of the signal.

[danadak]

Yes and no.

1) No if OpAmp has phase reversal problems when its input is taken below ground.
Note you have to make sure your input R limits injection current(reverse current).

2) Yes if OpAmp is RRIO (preferred) and does not suffer 1) above.

One possibility - http://www.linear.com/solutions/1608

Regards, Dana.

[uncle_bob]

Hi

As you feed the (say 10V) sine wave into the input of the op amp, you will drive that input below ground (by say 5V). As with all semiconductors ... they don't like this. The most likely outcome is that you will blow the input of the op amp. There are circuits that will turn an op amp into an "ideal diode". You still use dual supplies and keep all the signals in bounds.

Bob

[Audioguru]

Take an opamp that has inputs and an output that go all the way down to ground when there is no negative supply like an LM358, LM324, MC3307x or a rail-to-rail opamp. Make it an inverting opamp. When the input to the input resistor goes negative then the output of the opamp goes positive and the inverting input of the opamp stays at 0V without going into a not allowed negative voltage. When the input to the input resistor goes positive and the inverting input pin also goes positive then the output of the opamp normally goes to ground. It makes a good rectifier.

[Carl_Smith]

Google "absolute value amplifier" or "absolute value circuit."   
Here's one:  http://www.ti.com/lit/an/sboa068/sboa068.pdf
It has several appropriate circuits with explanations.

[danadak]

LM324 or 358 will not drive to its negative rail, see attached.

Regards, Dana.

[munkeyman1985]

Yes, in one Dave's examples it was acting as a rectifier. Good job recognizing the wave form.  As Dave said the more common functions of an opamp are comparator, amplifier, and buffer. If wave shaping is needed, there are more efficient ways to do that.

[Audioguru]

LM324 or 358 will not drive to its negative rail

The typical output is 0.01V when it is sinking a few uA so do not let it sink current, instead let it source current like a normal load to ground.