Re: low noise amplifier for high impedance source
- From: "Phil Allison" <philallison@xxxxxxxxxx>
- Date: Mon, 16 Oct 2006 12:48:37 +1000
<archiees@xxxxxxxxx> wrote in message
news:1160963431.490482.294350@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
** Groper alert.
I am a student who has been working on a low noise preamplifier for a
high impedance current source. I have put the model of the detection
circuit here:
http://img146.imageshack.us/img146/9748/detectmodelid8.jpg
Its a differential ac current source with an instrinsic capacitance ~
20pF on each side. I use 1M ohm resistors to bias my input JFETs.
The bandwidth i need is only from 10 Khz to couple of MHz.
** Do you really have a current source ???
Or is it more like a voltage source with small series capacitance ?
Makes a big difference.
Assuming the latter is the case, the dominant noise source in your circuit
is the bias resistors. A 1 megohm resistor has 130 nV / rt Hz of noise while
a good J-FET will have only 100th of that.
The trick is to make the bias resistors as high in value as possible and so
allow the capacitance of your source attenuate their noise contribution. For
each doubling the resistor value, the attenuation factor increases by 6dB
while noise increases by only 3 dB.
To maximise available signal voltage, make the input capacitance of the
pre-amp as low as possible, ie install the pre-amp right next to the sensor.
......... Phil
.
- Follow-Ups:
- Re: low noise amplifier for high impedance source
- From: archiees
- Re: low noise amplifier for high impedance source
- References:
- low noise amplifier for high impedance source
- From: archiees
- low noise amplifier for high impedance source
- Prev by Date: Re: TV sounds like dripping faucet when off
- Next by Date: Re: PCB Drill
- Previous by thread: low noise amplifier for high impedance source
- Next by thread: Re: low noise amplifier for high impedance source
- Index(es):
Relevant Pages
|
