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Cranky Op-Amp

A

Ancient_Hacker

Jan 1, 1970
0
[email protected] wrote:

Okay, if you only need 20KHz bandwidth, I'd definitely roll off the
gain with some small capacitors. Try 220pf across each 470k resistor
for starters.

Ans since it's coming from a coil, there's no DC that you could ever
want, so please put a Dc blocking capacitor between the stages. That
will get rid of 83 times of your offset problem.

A ground place near the metal detector coil is a no-no.


You definitely need to put the amplifier in a metal box, so you should
run your coil thru a short cable to the amplifier box. the IC's should
be separated by at leasty an inch or so from each other, each one
decoupled from the plus and minus supplies with 100 ohm resistors and
100 uF capacitors.
 
T

Terry Given

Jan 1, 1970
0
Ancient_Hacker said:
[email protected] wrote:

Okay, if you only need 20KHz bandwidth, I'd definitely roll off the
gain with some small capacitors. Try 220pf across each 470k resistor
for starters.

Ans since it's coming from a coil, there's no DC that you could ever
want, so please put a Dc blocking capacitor between the stages. That
will get rid of 83 times of your offset problem.






You definitely need to put the amplifier in a metal box, so you should
run your coil thru a short cable to the amplifier box. the IC's should
be separated by at leasty an inch or so from each other, each one
decoupled from the plus and minus supplies with 100 ohm resistors and
100 uF capacitors.

I had a problem a while back, with 80dB total gain, and it was PSRR that
killed me - output was 4vpp into a couple of k, and the resultant psu
noise, with TL064's having -10dB of PSRR at 100kHz (thats what I
measured, bloody gain), was enough to kill me. then of course I had to
try and find a fast opamp with low current, that was cheap. I couldnt
decouple each amplifiers psu, because there were 40 of them and I
couldnt spend the dosh....besides, at 100kHz, I needed a humungous cap, $$.

Cheers
Terry
 
W

Winfield Hill

Jan 1, 1970
0
Fred Bloggs wrote...
Just throw it in the trash...

I agree. I liked the LM318 when it first came out, about
a million years ago (erpp, over 35 years ago), but it was
finicky to keep from oscillating, etc., and I was glad to
see the last of them a few years later (did'ja notice the
funny glitch in the curves at 300kHz?). Even tho it was
a serious high-speed amplifier (many companies made their
own second-source versions), especially with its fabulous
miracle feedforward compensation with 150V/us slewing, or
add overcompensation for the other side...

Despite all this, Paul should take your advice, Fred, and
give 'em the old heave-ho. I did. <sigh!>
 
Winfield said:
Fred Bloggs wrote...

I agree. I liked the LM318 when it first came out, about
a million years ago (erpp, over 35 years ago), but it was
finicky to keep from oscillating, etc., and I was glad to
see the last of them a few years later (did'ja notice the
funny glitch in the curves at 300kHz?). Even tho it was
a serious high-speed amplifier (many companies made their
own second-source versions), especially with its fabulous
miracle feedforward compensation with 150V/us slewing, or
add overcompensation for the other side...

Despite all this, Paul should take your advice, Fred, and
give 'em the old heave-ho. I did. <sigh!>

Thanks, I'll take Fred's advice. They also have a lot of latch-up. Oh
well, there's always that dusty LM741 in the back office, lol. Just
kidding and many thanks for the great help!

Paul
 
K

Ken Smith

Jan 1, 1970
0
The amplifier input capacitance may be reduced if the NPN base is
bootstrapped from the JFet source, causing the NPN emitter to
back-drive the input through the JFet Cdg.

Im not sure I followed the description. Is this a case of a JFETs input
capacitance being reduced by bootstrapping?


Basically:

constant
current
!
! ---[Z1]----
! ! !
!---+-+--!-\X1 !
In---->! ! >--+--+---
!---+----!+/ !
! !
----------

Obviously X1 isn't really an op-amp. It would have to have a couple of
volts of offset voltage if it was etc. The overall feedback is from the
output to the JFETs source so all of the terminals of the JFET have the
same AC (within the limits of gain) voltage on them so no current flows in
the device capacitance.
 
J

Jon Elson

Jan 1, 1970
0
[email protected] wrote:



Very interesting. I'll try it. I know that an R across output helps a
little.
Much better is a series resistor. You add, say, 100 Ohms from the op-amp's
output to the cable. Connect the feedback network to the far end of the
resistor, so the loop stays closed with respect to what is sent out on
the cable.
This will certainly tame the effect of output capacitance.

That may, or may not, be the real problem, however. 58 dB is a LOT of gain,
and extremely careful circuit layout practices are going to be necessary.

If you want a really GOOD diff amp, see the Analog Devices OP620. It is
truly
spectacular. I used it in a servo drive where there was a tens of
millivolts current
sense signal superimposed on a 100 V 100 KHz square wave common mode signal,
and with a little matched resistor attenuator, it handled it with total
grace! In another
application, I was having trouble until I realized that a
differential-mode remnant
of the local AM broadcast station was coming through the AD620 without
distortion,
and being rectified by the next stage. Reducing the bandwidth of the
AD620 was the
cure in that case.

Jon
 
J

Jon Elson

Jan 1, 1970
0
Hmm, I've been looking for a replacement. I don't need high frequency--
< 20KHz. Although most op-amps that NTE sells are > 4MHz bandwidth. I'm
wondering if there's any particular parameters to look for in a
datasheet to identify if the chip is prone to unwanted oscillations and
such?
You don't get 4 MHz bandwidth at the 58 dB gain (from another post). It is
gain-bandwidth PRODUCT. As you turn up the gain, the bandwidth
necessarily is reduced.

Jon
 
P

Phil Hobbs

Jan 1, 1970
0
Winfield said:
I agree. I liked the LM318 when it first came out, about
a million years ago (erpp, over 35 years ago), but it was
finicky to keep from oscillating, etc., and I was glad to
see the last of them a few years later (did'ja notice the
funny glitch in the curves at 300kHz?). Even tho it was
a serious high-speed amplifier (many companies made their
own second-source versions), especially with its fabulous
miracle feedforward compensation with 150V/us slewing, or
add overcompensation for the other side...

Yeah, that was an integrated version of the even older trick of
bypassing the lateral PNP level shifters by putting a cap from the
inverting input to one of the compensation pins, in the LM301. I
remembered this from a prehistoric NS app note (LB2).

That phase whoop-de-do was what made the settling behaviour of the 318
so crappy for its bandwidth.

Cheers,

Phil Hobbs
 
J

Jim Thompson

Jan 1, 1970
0
Yeah, that was an integrated version of the even older trick of
bypassing the lateral PNP level shifters by putting a cap from the
inverting input to one of the compensation pins, in the LM301. I
remembered this from a prehistoric NS app note (LB2).

That phase whoop-de-do was what made the settling behaviour of the 318
so crappy for its bandwidth.

Cheers,

Phil Hobbs

See Figure 17 of the MC1494 Data Sheet for how to handle the Lateral
PNP compensation.

...Jim Thompson
 
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