Spehro wrote:

>>
>>      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.
>


Also, with your selection of Re = 10K for the 2nd follower,
you are providing a significant effective load in parallel
with the 300K R.

I suspect, with the source impedances being discussed here,
that no BJT circuit will be found adequate. Probably have to
go with either MOSFET or CMOS opamp.

--
http://www.piclist.com hint: PICList Posts must start with ONE topic:
[PIC]:,[SX]:,[AVR]: ->uP ONLY! [EE]:,[OT]: ->Other [BUY]:,[AD]: ->Ads