I am new to posting here ('cept for selling stuff) but am ready to take the plunge!
been lurking for ages.
I have scoped out the OPA 2134 for my first stage. total gain (over several stages) must be ? 80? There seems to be quite a range of problems when working at 1 meg input. (I got, allegedly, 2.0 mV @ 1 megohm to amplify)
The OPA spec sheet says to correlate your input imp with the resistance in your feedback loop, but that might mean 1.5 meg//250K (to ground) which seems a bit out of whack.
Also it would appear difficult to get much gain in this stage, also. that i could live with, but 9 or 10 sure would be nice.
Do i need a Jfet or complementary Jfet input buffer here?
I have no design skills to speak of --just cookbook competence.
Advice, please. I'll happily use a 6DJ8 if I have to. Would i know what Im doing? No.
I am also correct that the inverting input is out of the question?
been lurking for ages.
I have scoped out the OPA 2134 for my first stage. total gain (over several stages) must be ? 80? There seems to be quite a range of problems when working at 1 meg input. (I got, allegedly, 2.0 mV @ 1 megohm to amplify)
The OPA spec sheet says to correlate your input imp with the resistance in your feedback loop, but that might mean 1.5 meg//250K (to ground) which seems a bit out of whack.
Also it would appear difficult to get much gain in this stage, also. that i could live with, but 9 or 10 sure would be nice.
Do i need a Jfet or complementary Jfet input buffer here?
I have no design skills to speak of --just cookbook competence.
Advice, please. I'll happily use a 6DJ8 if I have to. Would i know what Im doing? No.
I am also correct that the inverting input is out of the question?
You can use SE JFET buffer (see attachment). Input impedance is 1M and you can load this buffer with 5k or more. P1 (multiturn pot) sets DC offset.
Mouser sells BF862 JFETs.
Mouser sells BF862 JFETs.
Attachments
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that is very useful AND I see that there are no boutiqe parts!
It would be good to expand my knowledge of fet's
I have an excellent text (Horowitz & Hill) but nothing clicks
until I put it into wire and bits.
Doug
It would be good to expand my knowledge of fet's
I have an excellent text (Horowitz & Hill) but nothing clicks
until I put it into wire and bits.
Doug
Hi,
a gain of 80 times (+39dB) for an opamp is not too much to ask.
A FET input opamp can easily achieve 1M for Zin.
The opa2134 meets both requirements and it is unity gain stable.
a gain of 80 times (+39dB) for an opamp is not too much to ask.
A FET input opamp can easily achieve 1M for Zin.
The opa2134 meets both requirements and it is unity gain stable.
Low level input--Low noise-- OPA 2134
Oh good I was waiting to hear that.
I will try setting that up. I would have to guess why Burr Brown warns against excess noise & distortion. Would you have a suggestion for R1//R2? something like 500K/ 62K?
Oh good I was waiting to hear that.
I will try setting that up. I would have to guess why Burr Brown warns against excess noise & distortion. Would you have a suggestion for R1//R2? something like 500K/ 62K?
Long delayed thanks to Juma! I see that Q8 is set up as a current source??
Done some more reading in Horowitz et al. I had previously no real notion of Jfets.
Am I to assume that distortion is not an issue? Normally i think of any amplifier stage as being enclosed in a feedback loop--
My text sez "you can build discrete Jfet stages, but normally one looks at Op-amps that use Jfets in the input stage." Hello TL072?
Done some more reading in Horowitz et al. I had previously no real notion of Jfets.
Am I to assume that distortion is not an issue? Normally i think of any amplifier stage as being enclosed in a feedback loop--
My text sez "you can build discrete Jfet stages, but normally one looks at Op-amps that use Jfets in the input stage." Hello TL072?
Yes, Q8 represents a working load for Source Follower Q7.
Not at all.
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