jfet preamp - does a jfet exist that'll work at just 5V supply?

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peskywinnets

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#1
Subject says it all really...I'm constrained to only using a jfet, but I really want to use the single 5V rail I intend using in this circuit ...else I'm faced with having to add in an additional supply just to feed the jfet!

The other constraint is that the jfet must be self biased, with it's gate grounded via a high resistor (which rules mosfets and opamps out)

Top tips on any jfets that'll operate ok at 5V supply welcomed!
 
#2
actually several CMOS op amps work with input CM V at the rail or even ~100 mV beyond - in fact its a easily searchable class: "rail-to-rail" input (and often output)

sw-C V inverter chips let you invert supplies cheaply, there is even op amp incorporating the sw-C inverter internally
 
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#3
Thanks but like I say ....I *must* use a jfet ...opamps are out (I tried a dual rail opamp & therefore mid point biasing to ground, but it wasn't any good).....I want to keep this with the lowest jfet count possible...hence seeking a single jfet to perform the task.

Now I'm fairly new to jfets...I understand how they work etc, but not experienced enough to know of a particular jfet that will operate as low as 5v rail...and wondered if anyone on here did?
 
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#4
You'll need a jfet with low Vp and high gm0 like the BF246A or 247A or some japanese types specially designed for electret microphone amplifiers. Select them for low Vp. Then even a SRPP is possible at 5V.
2SK170 (Vp= -0,2...-1,5V), 2SK190 (-0,15..-1,5V)
 
#7
Just has a rummage in my components tray and found a PMBFJ110....a quick check of the datasheet, reveals it might be a contender, but I can't see the gm0 figure you mention?

Tekko said:
Why are opamps out of the question ?
Click to expand...

Opamps are out the question as I want the input to be biased at 0V with no DC blocking cap, which means a dual rail supply...I don't want to go there (& the testing I did with opamps wasn't encouraging vs a single jfet & two resistors)
 
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#10
What is the application ? Is it an amplifier or just a inverter ? Even the single jfet amplifier needs its gate biased somewhere between GND and Vcc = coupling cap needed.

If its just a inverter, theres ttl and cmos logic which is designed for 3.3 and 5 volts.
 
#12
Tekko said:
What is the application ? Is it an amplifier or just a inverter ? Even the single jfet amplifier needs its gate biased somewhere between GND and Vcc = coupling cap needed.

If its just a inverter, theres ttl and cmos logic which is designed for 3.3 and 5 volts.
Click to expand...

It's an audio preamplifier.

If I use a self biased jfet, then the gate biased to ground through a high value resistor, but will actually be negative wrt to the source, therefore no input DC blocking cap needed.

scott wurcer said:
BF862 costs almost nothing and is widely stocked, and rarely has Vp over .5V or so.
Click to expand...

Thanks...how can I work out what gain can be coaxed out of a jfet...I realise it's related to transconductance, but I'm having diffs gleaning which part of the datasheet that's etracted & what formula to use? I'm looking for a voltage gain of about 3x ...doable?
 
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#14
zinsula said:
What is the voltage swing at the input?
How would you use it? as a buffer, i.e. common drain, or should it amplify, i.e. common source?
Click to expand...

voltage swing is about 1V max peak to peak (but averaging about 300mv)....I seek a voltage gain of about 3x out of this circuit.

Just experimenting right now to establish the pinch off & the IDS of the J110 I have here....then I can work out the biasing!

let's see if I have this right....

gate grounded
source grounded
drain 100R

VCC= 5V

voltage drop across the 100R resistor is 0.489V ...therefore an IDS of 4.89mA?
 
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#15
2N3819 from Vishay is easily available. Check the Datasheet, if you set ID with appropriate source resistor at about 5mA, you should have enough headroom at the input.
But...if you have 1Vpp and you want 3Vpp, this will be difficult. There isn't just enough headroom.
And how do you handle the output? Midpoint will be above ground....
 
#17
zinsula said:
2N3819 from Vishay is easily available. Check the Datasheet, if you set ID with appropriate source resistor at about 5mA, you should have enough headroom at the input.
But...if you have 1Vpp and you want 3Vpp, this will be difficult. There isn't just enough headroom.
And how do you handle the output? Midpoint will be above ground....
Click to expand...

You make a good point...I'll have to reign the gain expectations back a bit.
 
#18
zinsula said:
Yes.
But if you are going to ground the source, you might be able to swing the negative input half wave, but not the positive....more than 0V G-S will only forward bias the G-S...no good.
Click to expand...

But the source will be sitting at a DC level higher than 0V ...so it won't be forward biased on the positive signal swings. So long as the source is biased higher than the largest voltage input swing, then I should be ok?
 
#19
Why is DC coupling a requirement ?

Its a common misconception that dc coupling improves bass response.
Also in most audio applications you dont even want frequency response down to DC.

Besides any imaginable audio source will have a dc blocking cap on the outputs anyways.
 
#20
"not using op amps" is not an engineering design criteria - maybe for a homework assignment

the better way to use engineering design skills is to specify the problem in performance terms, define input, output signals, levels, available supplies, area, cost , power requirements that the application demands

specifying device tech from the start is not a proper performance related constraint - you rule out too many options you may not be aware of
 
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