Question about JFETs

B

Bill Bowden

Guest
I noticed the JFET input impedance at the gate appears to change as
the resistance in the drain connection changes. If I connect the JFET
as a source follower with the drain connected to Vcc, the gate
impedance is much higher than if there is some load in the drain
connection.

I can see this with a LTspice example where the drain resistance (R4)
is 1 ohm and the input impedance is high and there is not much drop
across the series gate resistor (R1). However, changing the drain
resistor from 1 ohm to 10k ohms causes about a 50% drop in voltage
across the input resistor R1 (150K) indicating a much lower input
resistance. You can see this by changing the drain resistor from 1 ohm
to 10k and probing both sides of the 150K resistor

Any idea what is going on?

-Bill

Version 4
SHEET 1 880 680
WIRE 112 0 -128 0
WIRE 272 0 192 0
WIRE -128 64 -128 0
WIRE 112 64 0 64
WIRE 224 64 192 64
WIRE 272 192 272 176
FLAG 272 192 0
FLAG -128 144 0
FLAG 0 144 0
SYMBOL njf 224 0 R0
WINDOW 3 57 62 Left 0
SYMATTR InstName J1
SYMATTR Value 2N4416
SYMBOL voltage 0 48 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
WINDOW 0 -80 139 Left 0
WINDOW 3 -29 139 Left 0
SYMATTR InstName V1
SYMATTR Value SINE(0 .1 500K)
SYMBOL voltage -128 48 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
WINDOW 3 60 12 Left 0
WINDOW 0 14 11 Left 0
SYMATTR Value 12
SYMATTR InstName V2
SYMBOL res 208 -16 R90
WINDOW 0 -5 86 VBottom 0
WINDOW 3 -33 28 VTop 0
SYMATTR InstName R4
SYMATTR Value 10k
SYMBOL res 208 48 R90
WINDOW 0 75 90 VBottom 0
WINDOW 3 47 30 VTop 0
SYMATTR InstName R1
SYMATTR Value 150k
SYMBOL res 256 80 R0
WINDOW 3 74 39 Left 0
SYMATTR InstName R2
SYMATTR Value 3k
TEXT -152 232 Left 0 !.tran 50u.1m
 
On Fri, 19 Aug 2011 20:39:44 -0700, Bill Bowden wrote:

I noticed the JFET input impedance at the gate appears to change as the
resistance in the drain connection changes. If I connect the JFET as a
source follower with the drain connected to Vcc, the gate impedance is
much higher than if there is some load in the drain connection.

I can see this with a LTspice example where the drain resistance (R4) is
1 ohm and the input impedance is high and there is not much drop across
the series gate resistor (R1). However, changing the drain resistor from
1 ohm to 10k ohms causes about a 50% drop in voltage across the input
resistor R1 (150K) indicating a much lower input resistance. You can see
this by changing the drain resistor from 1 ohm to 10k and probing both
sides of the 150K resistor

Any idea what is going on?
Click to expand...
The gate to drain capacitance coming into play. I suspect that the gate
looks more capacitive than resistive, but the gate admittance is going to
be much greater at 500kHz than at DC unless you ground the drain.

--
Tim Wescott
Control system and signal processing consulting
www.wescottdesign.com
 
On Fri, 19 Aug 2011 20:39:44 -0700 (PDT), Bill Bowden
<bperryb@bowdenshobbycircuits.info> wrote:

I noticed the JFET input impedance at the gate appears to change as
the resistance in the drain connection changes. If I connect the JFET
as a source follower with the drain connected to Vcc, the gate
impedance is much higher than if there is some load in the drain
connection.

I can see this with a LTspice example where the drain resistance (R4)
is 1 ohm and the input impedance is high and there is not much drop
across the series gate resistor (R1). However, changing the drain
resistor from 1 ohm to 10k ohms causes about a 50% drop in voltage
across the input resistor R1 (150K) indicating a much lower input
resistance. You can see this by changing the drain resistor from 1 ohm
to 10k and probing both sides of the 150K resistor

Any idea what is going on?
Click to expand...
It's an AC effect. The gate capacitance is loading the 150K resistor.
Some of the gate capacitance is the Miller capacitance, namely Cd-g
multiplied by 1+G, where G is the voltage gain. With a small drain
resistor, G is close to zero. If you add a big cap from the drain to
ground, you can force G to be zero, and see the effect.

Try it at a lower frequency, like 10 KHz, and you'll see very little
drop in R1.

Another effect is that the actual gate capacitance drops at higher DC
drain voltages, as you get with a lower drain resistor. You can vary
V2 to see that effect.

John


-Bill

Version 4
SHEET 1 880 680
WIRE 112 0 -128 0
WIRE 272 0 192 0
WIRE -128 64 -128 0
WIRE 112 64 0 64
WIRE 224 64 192 64
WIRE 272 192 272 176
FLAG 272 192 0
FLAG -128 144 0
FLAG 0 144 0
SYMBOL njf 224 0 R0
WINDOW 3 57 62 Left 0
SYMATTR InstName J1
SYMATTR Value 2N4416
SYMBOL voltage 0 48 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
WINDOW 0 -80 139 Left 0
WINDOW 3 -29 139 Left 0
SYMATTR InstName V1
SYMATTR Value SINE(0 .1 500K)
SYMBOL voltage -128 48 R0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
WINDOW 3 60 12 Left 0
WINDOW 0 14 11 Left 0
SYMATTR Value 12
SYMATTR InstName V2
SYMBOL res 208 -16 R90
WINDOW 0 -5 86 VBottom 0
WINDOW 3 -33 28 VTop 0
SYMATTR InstName R4
SYMATTR Value 10k
SYMBOL res 208 48 R90
WINDOW 0 75 90 VBottom 0
WINDOW 3 47 30 VTop 0
SYMATTR InstName R1
SYMATTR Value 150k
SYMBOL res 256 80 R0
WINDOW 3 74 39 Left 0
SYMATTR InstName R2
SYMATTR Value 3k
TEXT -152 232 Left 0 !.tran 50u.1m
Click to expand...
 
On Aug 19, 10:50 pm, John Larkin
<jjlar...@highNOTlandTHIStechnologyPART.com> wrote:
On Fri, 19 Aug 2011 20:39:44 -0700 (PDT), Bill Bowden



bper...@bowdenshobbycircuits.info> wrote:
I noticed the JFET input impedance at the gate appears to change as
the resistance in the drain connection changes. If I connect the JFET
as a source follower with the drain connected to Vcc, the gate
impedance is much higher than if there is some load in the drain
connection.

I can see this with a LTspice example where the drain resistance (R4)
is 1 ohm and the input impedance is high and there is not much drop
across the series gate resistor (R1). However, changing the drain
resistor from 1 ohm to 10k ohms causes about a 50% drop in voltage
across the input resistor R1 (150K) indicating a much lower input
resistance. You can see this by changing the drain resistor from 1 ohm
to 10k and probing both sides of the 150K resistor

Any idea what is going on?

It's an AC effect. The gate capacitance is loading the 150K resistor.
Some of the gate capacitance is the Miller capacitance, namely Cd-g
multiplied by 1+G, where G is the voltage gain. With a small drain
resistor, G is close to zero. If you add a big cap from the drain to
ground, you can force G to be zero, and see the effect.

Try it at a lower frequency, like 10 KHz, and you'll see very little
drop in R1.

Another effect is that the actual gate capacitance drops at higher DC
drain voltages, as you get with a lower drain resistor. You can vary
V2 to see that effect.

John
Click to expand...
Yes, that explains it a bit. I added a capacitor from drain to gate
and managed to make things worse. It seems the same as an op-amp with
negative feedback, out of phase, that lowers the input resistance.

-Bill
 
"Bill Bowden" <bperryb@bowdenshobbycircuits.info> schrieb im Newsbeitrag
news:0f4cac39-4f48-490e-95a4-e4ea9b7e0653@e20g2000prn.googlegroups.com...
I noticed the JFET input impedance at the gate appears to change as
the resistance in the drain connection changes. If I connect the JFET
as a source follower with the drain connected to Vcc, the gate
impedance is much higher than if there is some load in the drain
connection.

I can see this with a LTspice example where the drain resistance (R4)
is 1 ohm and the input impedance is high and there is not much drop
across the series gate resistor (R1). However, changing the drain
resistor from 1 ohm to 10k ohms causes about a 50% drop in voltage
across the input resistor R1 (150K) indicating a much lower input
resistance. You can see this by changing the drain resistor from 1 ohm
to 10k and probing both sides of the 150K resistor

Any idea what is going on?

-Bill
Click to expand...
Hello Bill,

This is called the Miller Effect.

http://www.odyseus.nildram.co.uk/RFIC_Theory_Files/Miller_Effect.pdf

The effective input capacitance is Cdg*(1+|gain|) with an inverting
amplifier.

You can find more examples with
Google: Miller effect capacitance

Best regards,
Helmut
 
Helmut Sennewald wrote:
"Bill Bowden" <bperryb@bowdenshobbycircuits.info> schrieb im Newsbeitrag
news:0f4cac39-4f48-490e-95a4-e4ea9b7e0653@e20g2000prn.googlegroups.com...

I noticed the JFET input impedance at the gate appears to change as
the resistance in the drain connection changes. If I connect the JFET
as a source follower with the drain connected to Vcc, the gate
impedance is much higher than if there is some load in the drain
connection.

I can see this with a LTspice example where the drain resistance (R4)
is 1 ohm and the input impedance is high and there is not much drop
across the series gate resistor (R1). However, changing the drain
resistor from 1 ohm to 10k ohms causes about a 50% drop in voltage
across the input resistor R1 (150K) indicating a much lower input
resistance. You can see this by changing the drain resistor from 1 ohm
to 10k and probing both sides of the 150K resistor

Any idea what is going on?

-Bill


Hello Bill,

This is called the Miller Effect.

http://www.odyseus.nildram.co.uk/RFIC_Theory_Files/Miller_Effect.pdf

The effective input capacitance is Cdg*(1+|gain|) with an inverting
amplifier.

You can find more examples with
Google: Miller effect capacitance

Best regards,
Helmut


We used that effect once years ago to vary the cap value for a oscillator.
Click to expand...

The gate was connected to the tank circuit via a small R and we simply
varied the DRAIN. It was a neat idea at the time but I guess one that
isn't used that much.

Jamie
 
On 2011-08-21, Jamie <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:

We used that effect once years ago to vary the cap value for a oscillator.

The gate was connected to the tank circuit via a small R and we simply
varied the DRAIN. It was a neat idea at the time but I guess one that
isn't used that much.
Click to expand...
it seems kind of like a lossy varicap diode.

--
⚂⚃ 100% natural

--- Posted via news://freenews.netfront.net/ - Complaints to news@netfront.net ---
 
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