Re: differential amplifier

On 29 Dec 2006 00:55:04 -0800, "Tom Bruhns" <k7itm@xxxxxxx> wrote:



On Dec 28, 10:48 am, "kell" <kellrobin...@xxxxxxxxxxxx> wrote:
Hi guys
I have to amplify a small differential voltage near the top rail.
This voltage has some indeterminate impedance (like between 100k and 1
Meg),
represented by R1 and R2 on the diagram.
I plan to use the dual jfet op amp TL082:
__________
| + Vx - |
|__________|
| |
R1 R2
| |
__ | |
/+ |--' |
,--< | |
| \_-|--, |
| | |
+---------' |
| |
R3 |
| |
+-------------, |
| | |
| __ | |
p- '-| / -|--' |
channel ||---< | |
mosfet ,-| \_+|-----'
|
+--v out
|
R4
|
gnd

V out goes to a voltage-controlled oscillator, and I
want Vout to get within 200 mV of the rails. In the application Vx is
going to be very small (millivolts) and I'll use a lot of gain, so
offset has to be eliminated.
Now the op amp doesn't have any nulling pins. I wouldn't want to
install two pots for two amps anyway. So I was thinking about
nulling it like this:
R5=R3

__________
| + Vx - |
|__________|
| |
R1 R2
| |
__ | |
/+ |--' |
,--< | |
| \_-|------, |
| | |
+----R5-------+ |
| | |
| \ |
| / |
| ,---->\ |
R3 | / |
| | \ |
| gnd / |
| | |
+-------------+ |
| | |
| __ | |
p- '-| / -|--' |
channel ||---< | |
mosfet ,-| \_+|-----'
|
+--v out
|
R4
|
gnd

Is this correct?
I'm also contemplating adding a couple of fixed resistors in series
with the legs of the potentiometer, to improve the resolution of the
nulling adjustment.

Since the voltages are near the upper rail, and you have the amps
configured as followers, their outputs will be near the upper rail.
Make sure your amplifier of choice will handle positive rail input AND
output gracefully. And for offset adjustment, I would recommend NOT
tying the pot center to ground, unless the voltages are pretty stable.
Even then, the pot also adjusts gain where you have it shown. Also,
note that the current in the upper section of the pot works to drive
the upper op amp output more positive: be careful that it doesn't try
to drive it beyond the rail. Why not apply the offset to the output
side? Since you want a lot of gain, and the gain will be proportional
to the ratio R4:R3, presumably R4 will be fairly large, and you may be
able to just use a 100-ohm pot tied between, say, +1 and -1 volts, and
the +/- 12.5 ohms variability in the net value of R4 as you adjust the
pot might be insignificant. (Can't tell; you didn't post enough info
about it.)

Cheers,
Tom

Correct! I was going to comment when the OP said TL082 would work...
inputs will go to +rail, but output won't.

...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona Voice:(480)460-2350 | |
| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
| http://www.analog-innovations.com | 1962 |

I love to cook with wine. Sometimes I even put it in the food.
.

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