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Analysis help for full wave active rectifier - RF Cafe Forums

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Mr.Whatever

Post subject: Analysis help for full wave active rectifier Posted: Thu Oct 19, 2006 12:32 pm

Captain

Joined: Fri Apr 07, 2006 6:10 pm

Posts: 18

Hi all! I'm trying to see if I've done my analysis correctly.

For the first opamp, I have the following, assuming positive input:

VA1 = VIN * (R2 / (R1 + R2))

VA1 = VA2 = VOA (Voltage follower)

Therefore, VB1 = VOA = VB2.

Since I know VB2, I can calculate the current going thru the resistors.

-I3 + I4 + I5

I3 = (VS-VB2)/R3

I4 = (VB2 / R4)

I5 = (VB2 - VOB)/R5

Then, I combine all this and get a VOB / VIN relationship:

VOB = VIN * ( (R2/(R1+R2) + R2*R5/(R4*(R1+R2)) - R2*R5/(R3*(R1+R2)) - R5/R3)

However, when I try plugging in my resistor values, VOB is always equal to zero.

What am I doing wrong? My simulation clearly shows an active fullwave rectifier.

Thanks!

Top

fred47

Post subject: Active RectifierPosted: Thu Oct 19, 2006 3:40 pm

General

Joined: Wed Feb 22, 2006 3:51 pm

Posts: 104

Hi!

It's hazardous to rely on generic models, rather than specific SPICE models of specific circuits - especially when you're depending on nonlinear behavior of linear ICs (like OpAmps). Most opamp input active ranges are less than Vcc to Vee, likewise the outputs. The few for which that's not true are specially denoted by the term "Rail-to-Rail", for input or output or both. Even there, the outputs have some restrictions.

When you say, "when I try plugging in my resistor values", do you mean in a simulation or in a breadboard? It sounds like you mean in a breadboard, and that the physical results don't match the simulation. Reality wins over simulation every time.

My general rule for simulations (SPICE or other) is that if they tell you a circuit won't work, they're probably correct. If they tell you that a circuit will work, it doesn't mean there aren't a lot of hidden problems that might keep it from working.

Good Luck!

Fred

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Mr.Whatever

Post subject: Posted: Thu Oct 19, 2006 4:41 pm

Captain

Joined: Fri Apr 07, 2006 6:10 pm

Posts: 18

Thanks, fred47!

Actually, this all refers to a simulation environment. I just wanted to see how the configuration actually rectifies the input. I re-did my work and realized that I had a problem with one of my signs (e.g. treated it as a positive instead of a negative).

By the way, regarding your comments on SPICE, do convergence problems in the simulation account for the actual circuit not working?

I've been getting a lot of convergence errors. From what I found out so far, it's possible that it may be due to some settings in the simulation environment.

Top

fred47

Post subject: Convergence ErrorsPosted: Thu Oct 19, 2006 5:47 pm

General

Joined: Wed Feb 22, 2006 3:51 pm

Posts: 104

Hi!

Lots of things can cause convergence errors. One of the most common is positive feedback; you're right in thinking that sometimes it's simulation parameters or settings.

Active full-wave rectifiers usually require matched resistors.

Good Luck!

Fred

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