Re: Voltage follower with a step function (need a design for one)

On 17 Apr 2006 06:26:49 -0700, "Rocky" <forums@xxxxxxxxxxxxxxxx>
wrote:

John Fields wrote:
On Sun, 16 Apr 2006 10:20:13 -0500, John Fields
<jfields@xxxxxxxxxxxxxxxxxxxxx> wrote:


---
I think this is better:

+V>----------+---------------------+
| |
Vin--+-------|------[10k]--+------|+\
| | | | >------+-->Vout
| [10k] | +--|-/LM392 |
| | | | | OPAMP |
[7500] | +--[4M7]---+ +---|--------+
| | | | |
+-------|--+--|+\ | |
| | | >----+ |
[12.1k] +-----|-/LM392 |
| | COMPARATOR |
| [LM385-1.2] |
| | |
GND>-+-------+---------------------+----------->GND


--
John Fields
Professional Circuit Designer
Hi John,
I was just about to implement your suggested circuit when I came across
something I don't understand. When Vin >2V, does the output from the
LM392 comparator swing to the comparator's positive supply rail?

---
No, it goes open collector, unclamping the + input to the opamp.
---

I
thought that's how a comparator worked - basically an opamp that allows
rail to rail output swing.

---
It's more like an opamp with all the compensation removed to make it
fast, and an open-collector transistor output.
---

If however the comparator's output impedance went high (like a
transistor when switched off) when its positive input voltage >neg.
input voltage then that would make sense, but because I can't see how
it would, then surely we need the opamp to drive a transistor as Tony
suggested above.

---
It's precisely because it _does_ work that way that we don't need
the transistor; there's one already in the comparator.
---

While I think about it, is there any reason why I need to use
comparator IC instead of say an LM158/LM358 opamp ?

---
It makes life easier, but if you can't use a proper comparator you
could do this:

+V>----------+-------------------------------------+
| |
Vin--+-------|------[10k]-----------------+-------|+\LM358
| | | | >--+-->Vout
| [10k] | +--|-/ |
| | | | | |
[7K5] | +--[1M]--+ | +---|----+
| | | | | |
| +--[1k]--+--|+\ | C |
| | | >--+--[1k]--B Q1 |
+-------|-----------|-/LM358 E |
| | | |
[12k1] |K | |
| [LM385-1.2] | |
| | | |
GND>-+-------+----------------------------+--------+------->GND

Choose Q1 to give you the lowest Vce(sat) you can get or, if you
choose a MOSFET, the lowest Rds(on).

--
John Fields
Professional Circuit Designer
.

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