At 12:46 PM 8/25/01 +0300, you wrote:
>How about this (temp. compensated buffer):
>Select fixed width font (terminal)
>
>      VCC   VCC
>      |     |
>      |     R2
>      C     |
>IN---B      E---OUT
> NPN  E----B  PNP
>      |     C
>      R1    |
>      |     |
>      GND   GND
>
>Voltage gain is about 1 works to within 0.8V of the rails and can achieve
>current gain of 100 or more if necessary. Speed range includes video (>10
>MHz).

I bench tested this one yesterday, and I don't really like it much. With the
values I picked (300K and 10K) and using mid-gain-bin cheapie transistors
(2SC8050C/2SC8550C) the offset was near zero with a 1.5M source Z. There
will be some TCvos, however, because of the change in beta with temperature
and there is significant drop due to the base currents. Worse, it went
within 0.5V of one rail, but only within 1.2V of the other (at room
temperature).

A single 2N7000+resistor (source follower) would probably be better, or the
bipolar op-amp circuit I posted yesterday. They would cost about $0.05 more
each, however.

Best regards,

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Spehro Pefhany --"it's the network..."            "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com
Contributions invited->The AVR-gcc FAQ is at: http://www.bluecollarlinux.com
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