Circuit challenge

I hope I'm not unduly helping you with your homework.

Use the op-amp as a comparator (it has to be a suitable op-amp for that purpose- on that will switch quickly and come out of saturation quickly) and use the output to switch between the A and A' inputs with an analog multiplexer (or build one out of discrete transistors if you must). Meets all your (stated) requirements.

You might have to add a series resistor and/or attenuate the comparator input if the op-amp can't tolerate differential voltage.

Best regards, Spehro Pefhany

Can you add an SPDT analog switch, or does it have to be just '1 op-amp'?

If the output impedance weren't specified low, there's the possibility of using the op amp to make a precision clamp, and resistor-sum the clamped signal with half the inverse signal.

If the inputs are actually defined that way, there's no added information from the inverted version, so discard it and amplify only the one phase. The output wil be indistinguishable from any circuit that would employ both phases.

Here's a sim of a TL071 driving an ADG513 with a 10K load, 60Hz 5V peak sine wave input.

Typical output impedance is < 200 ohms (assuming low source impedances).

Best regards, Spehro Pefhany

I meant, rectify only the one phase.

I think that's correct, but is that easier?

Best regards, Spehro Pefhany

bitrex schrieb:

Hello,

the circuit for a precision fullwave rectifier with ground referenced output needs two opamps. This circuit has only one input, not two.

What should be done with the two signals, do you want the sum or the difference of them?

Bye

The difference is zero, the sum is just double the signal from one, by definition.

I think that this is one of those problems designed for and by academia. They pose a problem with seemingly impossible constraints, then arbitrarily throw in a way to solve it by the unlikely presence of the inverted signal.

Because it's a rare real requirement, it's not as easy for the student to just look up a cookie cutter circuit in a book or on Google.

Best regards, Spehro Pefhany

only use ONE of the signals, since they're the same.??

Like this...

The switch opens/closes at zero crossing of V2.

I've used this very scheme to do synchronous detection of signals. ...Jim Thompson

See attachment!

wtf? I don't see my attachment.. Ok, so here is a screen shot of it!

Jamie

not precision

Like this...

The switch opens/closes at zero crossing of V2.

I've used this very scheme to do synchronous detection of signals. ...Jim Thompson

-- | James E.Thompson | mens | | Analog Innovations | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | San Tan Valley, AZ 85142 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

I seem to recall this full wave rectifier circuit from AoE that used one op amp (and diode) as an active half wave rectifier and then added that to the inverse (in a 2nd opamp). I went looking, but couldn't find it (in 2nd ed ition) If it's a 'requirement' that both phases be used.. then that might work... he'd have to add the two signals by some "clunky" method w/o a 2nd opamp.

George H.

Thanks, (I copied for later analysis.) Is your added 2nd diode there to reduce the maximum voltage of the first opamp? (Less slew rate limited performance at the cross over?)

George H.

The added diode keeps the first OpAmp from sticking against the rail, with concomitant recovery delay... my first use of the scheme was for

You can do gain-of-two and precision (diode) clamp with one op amp. Then, a differential output between the two-times-when-positive and the unamplified (negated) signal, is a fully rectified output. But, it's differential (not ground-referenced) and the op amp precision clamp isn't extremely low impedance. See

for a schematic