PSpice problem - circuit doesn't work as expected

[David Wooff]

Hi,
I've tried to simulate the circuit posted here :
http://www.muzique.com/amz/mini.htm.
The circuit is a JFET mu amp which is supposed to provide a large amount of
gain, but when I tried to model it I got attenuation not gain. I'm not sure
why this is but I'm pretty sure I have entered the circuit properly. I
could attach a zip with the files but I wasn't sure whether or not this was
a good idea.

I'm using PSpice/Orcad capture 9.1 student edition.

Any help appreciated.

 

[Jim Thompson]

Hi,
I've tried to simulate the circuit posted here :
http://www.muzique.com/amz/mini.htm.
The circuit is a JFET mu amp which is supposed to provide a large amount of
gain, but when I tried to model it I got attenuation not gain. I'm not sure
why this is but I'm pretty sure I have entered the circuit properly. I
could attach a zip with the files but I wasn't sure whether or not this was
a good idea.

I'm using PSpice/Orcad capture 9.1 student edition.

Any help appreciated.

The circuit relies on a mono plug pushed into the stereo jack thus
providing a ground path for the 9V battery. Does you simulation have
that grounded?

Also the FET-as-a-load looks rather "novel" ;-)

...Jim Thompson

 

[Mike Engelhardt]

David,

I've tried to simulate the circuit posted here :
http://www.muzique.com/amz/mini.htm.
The circuit is a JFET mu amp which is supposed to provide a large amount of
gain, but when I tried to model it I got attenuation not gain. I'm not sure
why this is but I'm pretty sure I have entered the circuit properly. I
could attach a zip with the files but I wasn't sure whether or not this was
a good idea.

I'm using PSpice/Orcad capture 9.1 student edition.

There's a number of pitfalls you might be having.
You should enter the 1M resistors as 1Meg not
1M so that MegaOhm is understood, not milliOhm.
Also, capacitance is measured in Farads.

Below is an LTspice schematic of your drawing
and it shows 42dB gain from 34Hz to 2Meg.

--Mike

--- amp.asc ---
Version 4
SHEET 1 880 680
WIRE -256 304 -176 304
WIRE -112 304 -80 304
WIRE -80 304 -80 336
WIRE -80 416 -80 464
WIRE -80 304 16 304
WIRE 16 464 16 400
WIRE 16 336 16 304
WIRE 16 304 80 304
WIRE 128 336 128 352
WIRE 128 448 128 480
WIRE 128 240 128 208
WIRE 128 64 128 16
WIRE 128 16 192 16
WIRE 320 16 320 32
WIRE 320 112 320 128
WIRE -80 112 -80 128
WIRE -80 224 -80 240
WIRE 80 128 16 128
WIRE -80 128 -80 144
WIRE 16 128 16 208
WIRE 16 128 -80 128
WIRE 16 208 48 208
WIRE 112 208 128 208
WIRE 128 208 128 160
WIRE -80 32 -80 16
WIRE -80 16 128 16
WIRE 272 16 320 16
WIRE -256 464 -256 416
WIRE -256 336 -256 304
WIRE 128 208 320 208
WIRE 128 352 240 352
WIRE 128 352 128 368
WIRE 240 352 240 384
WIRE 240 448 240 480
FLAG 128 480 0
FLAG 16 464 0
FLAG -80 464 0
FLAG 320 128 0
FLAG -80 240 0
FLAG -256 464 0
FLAG 240 480 0
SYMBOL njf 80 64 R0
SYMATTR InstName J1
SYMATTR Value 2N4416
SYMBOL njf 80 240 R0
SYMATTR InstName J2
SYMATTR Value 2N4416
SYMBOL res 112 352 R0
SYMATTR InstName R1
SYMATTR Value 1K
SYMBOL cap -112 288 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C1
SYMATTR Value .05µ
SYMBOL res -96 320 R0
SYMATTR InstName R2
SYMATTR Value 1Meg
SYMBOL cap 0 336 R0
SYMATTR InstName C2
SYMATTR Value 10p
SYMBOL voltage 320 16 R0
SYMATTR InstName V1
SYMATTR Value 9
SYMBOL res -96 128 R0
SYMATTR InstName R3
SYMATTR Value 1Meg
SYMBOL res -96 16 R0
SYMATTR InstName R4
SYMATTR Value 1Meg
SYMBOL cap 112 192 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C3
SYMATTR Value 3.3µ
SYMBOL res 288 0 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R5
SYMATTR Value 100
SYMBOL voltage -256 320 R0
SYMATTR InstName V2
SYMATTR Value ac 1
SYMBOL cap 224 384 R0
SYMATTR InstName C4
SYMATTR Value 10µ
TEXT 344 424 Left 0 !.ac oct 10 1 10Meg

 

[David Wooff]

Mike,
I feel such a fool. You correctly pointed out that 1M is 1milli-ohm.
Thanks a million. I'll put that one down to bitter experience.

Thanks again,
Dave W.

 

[Frank Bemelman]

The circuit relies on a mono plug pushed into the stereo jack thus
providing a ground path for the 9V battery. Does you simulation have
that grounded?

Also the FET-as-a-load looks rather "novel" ;-)

Did you scroll down?

"derived from an application note in the National Semiconductor Linear
Applications book published in the late 1970s"

and:

"If you are still reading at this point you might be interested in knowing
that the National Semiconductor circuit is a jfet derivative of an expired
patent of a vacuum tube circuit for a "Series balanced Amplifier", US Patent
2,310,342 issued Feb. 1943"

 

[Tim Wescott]

Did you scroll down?

"derived from an application note in the National Semiconductor Linear
Applications book published in the late 1970s"

and:

"If you are still reading at this point you might be interested in knowing
that the National Semiconductor circuit is a jfet derivative of an expired
patent of a vacuum tube circuit for a "Series balanced Amplifier", US Patent
2,310,342 issued Feb. 1943"

So in 1943 it was novel -- what's your problem with that?

 

[Tim Wescott]

Mike,
I feel such a fool. You correctly pointed out that 1M is 1milli-ohm.
Thanks a million. I'll put that one down to bitter experience.

Thanks again,
Dave W.

-- snip --

Don't feel too bad; I make that one all the time. There's a permanent
impression of my palm on my forehead just from that one alone.

 

[Frank Bemelman]

So in 1943 it was novel -- what's your problem with that?

No problem at all. It's nice to see that an idea from 60 years
back is still seen as 'rather novel'.

 

[David Wooff]

Tim,
Thanks for the sympathy, haven't we all (at one time or another) wondered
why the damn circuit isn't working when we come back from tea break and
waste half an hour tracing the "fault" from the output all the way back
through the circuit to the power supply to find that the bench supply output
was switched off - doh!
Dave W.

 

[David Wooff]

Did you scroll down?

"derived from an application note in the National Semiconductor Linear
Applications book published in the late 1970s"

and:

"If you are still reading at this point ...

Nice put down ! )

 

[Winfield Hill]

Jim Thompson wrote...

Also the FET-as-a-load looks rather "novel" ;-)

Yes, although it has an old pedigree (old NSC app note and
U.S. patent 2,310,342 issued Feb. 1943), one doesn't often
run into it; I don't recall having seen it before. It's an
interesting solution to the issue of placing two current
sources in series without saturation resulting (the upper
JFET is a DC source follower, and an AC current source).
A third JFET used as output source-follower would be good.


Thanks,
- Win

whill_at_picovolt-dot-com (use hill_at_rowland-dot-org for now)