# Help with combination clipper

1. Sep 22, 2015

### waffle123

• Poster has been reminded to post schoolwork questions in the Homework Help forums
Ok so my teacher gave us this as an assignment:

and was tasked with drawing the output voltage. The input voltage is a sine wave with 10v and -10v as its peaks.

So I know that the positive cycle will have both diodes be forward biased and the negative cycle will have 3 parts. First is when Vin<2v, both diodes are forward biased; Second is when 2v<Vin<5, D1 is reverse biased and D2 is Forward biased so Vo=-5v; And the last part is when Vin>5v, both diodes are reverse biased so Vo=Vin.

Now what confuses me is when both diodes are forward biased so the diodes turn into a short, leaving 2 cells with different voltages in parallel which I don't think is good/posibble. If it is ppossible then I'm not sure what is Vo is. I'm thinking it's -5v, -3v or -7v.

Anyways, can someone help me out here?

2. Sep 22, 2015

### Jeff Rosenbury

You are correct. This is a poorly drawn pedantic circuit. There are obvious real world problems with it.

So what is it trying to teach? Reworded, what are you studying?

Think about the current flow. Where does it go?

If there is no current flow (since there's no current path) what happens?

Of course real circuits have both current and voltage, which makes this an odd duck. I'm sure it's supposed to illustrate a lesson, but without knowing your coursework, I can't guess what that is.

In any case, you can't go wrong with KVL and KCL. Apply them. Find numbers.

3. Sep 22, 2015

### waffle123

Sorry about the drawing. I did it with a mouse.

Right now we just started with learning about clippers. We've talked about the different types of clippers(positive, negative, combination, etc) and illustrated the output wave of each if the input wave was a sine wave.

I do believe the resistor is unimportant right now since it wasn't used in the examples he gave us.

I don't exactly know what the purpose of this is. I think he's trying to see if we can apply what we learned to an unfamiliar case.

I'm sorry if this post isn't of any help. I'm rather tired right now and need to sleep so I can't really think well.

4. Sep 22, 2015

### Jeff Rosenbury

So what happens if the output voltage is -10V? What about -6V? What about -4V? What about 0V?

Give it some thought. Trace out the current flow and voltage levels.

5. Sep 22, 2015

### waffle123

I believe the 5v cell would discharge onto the 2v cell so that they'd be at the same voltage which would be 3v. If that is the case then that would mean that the output voltage for when the diodes are forward biased would be -3v.
Am I right?

6. Sep 30, 2015

### CWatters

I don't see a problem with the circuit :-) Can both diodes ever be forward bias?

Set the input to -10v and redraw it with +ve nodes at the top of the page. Then gradually increase vin to -5v. Plot vout vs vin. Can Vout ever get much higher than say -5v + vbe ?