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Question about JFETs

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

Reply to
Bill Bowden
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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
Reply to
Tim

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

Reply to
John Larkin

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

Reply to
Bill Bowden

"Bill Bowden" schrieb im Newsbeitrag news: snipped-for-privacy@e20g2000prn.googlegroups.com...

Hello Bill,

This is called the Miller Effect.

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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

Reply to
Helmut Sennewald

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.

Jamie

Reply to
Jamie

it seems kind of like a lossy varicap diode.

--
?? 100% natural

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Reply to
Jasen Betts

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