M
Mac
Guest
On Tue, 18 Jan 2005 18:05:48 +0000, Joerg wrote:
--Mac
Thanks! You seem to be the king of cheap discrete solutions. ;-)Hello Mac,
I had to redesign a few video circuits because they would occasionally
become unstable. To tell you the truth, I kicked the opamps out of
almost all of them and replaced them with transistors of the 10 cent
category.
This is very interesting. How many transitors and in what configuration?
Do you rely on relatively high voltage rails (like +/- 12) or can it be
done easily with, say, +/- 5?
Being a consultant I usually took whatever voltages the client's system
offered. +/-12V is nice, so is a +/-5V situation. But there were
occasions when I had to make do with +5V alone. That requires a lot of
quiescent current to be able to properly drive a 75Ohm cable with source
termination. On +5V only systems there was plenty of power available
though, thanks to the widespread use of guzzlers such as PALs, Thevenin
terminators etc.
Gain was achieved with the usual simple two-transistor stages and I used
AC coupling a lot. You just have to make sure that the 3dB limit at the
low end is low enough for the allowed droop at the end of a horizontal
line which usually will be in the millivolt range. The same goes for any
clamping but here FETs such as the BSS84 and BSS123 come to the rescue
which can be had for around five cents a pop. These would also make good
video buffers but, being a penny pincher, that was delegated to bipolar
versions at half the cost.
If you need lots of gain, for example after elaborate filtering, there
is the uA733 (Texas) which is a blazingly fast amp for its day and age.
That was always my friend because of its great performance and low cost.
The strange thing was that hardly anybody knew about it. I don't know
how long it is going to be around considering that this chip design
dates back to the early 70's. But it was transferred to SMT which
surprised me. Then again, for under 40 cents it is a true bargain.
Regards, Joerg
http://www.analogconsultants.com
--Mac