Re: Simple amplifier: simulation & real world do not agree

On Wed, 27 Sep 2006 13:39:30 -0400, John Popelish wrote:

Jenalee K. wrote:
I wanted to amplify a TTL-level 1MHz square wave a bit so that it can
be used as a clock for a CMOS counter running from 9V. According to
LT-Spice the circuit below works fine, but the real world does not
agree (at all). Actually I measured a cut-off frequency of 165kHz and
at 1MHz not much is left. Why is the simulation so wrong? 1MHz isn't
that high is it?

VCC
+
|
.-.
| | R4
| | 2K4
'-'
|
o----- out
R1 470 |
TTL Level ___ |/
in ----o--|___|---o-----| 2N2222
| | >
+----||----+ |
cap: ~100 pf. |
===
GND

I suggest you simplify your circuit to make it more of a switching
output than a linear amplifier. TTL sources generally are a lot
better at pulling to ground than they are at pulling up to the
positive rail. So you might try eliminating the input capacitor,
lowering R1 to about 470 ohms, get rid of R2, and R3, short out R5 and
possible lower R4 to between 1k and 4.7k. This should give you a
fast, full output swing.

This is almost word-for-word what I was going to say, but I'd add a
speedup cap across R1.

Do I win a prize? ;-)

Cheers!
Rich


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