SillyAmp

Jim started a cute thread on a.b.s.e. with a pdf post. Many
if not most of you can't read binary groups, and the messages
are not archived by Google so they soon disappear. I feel like
a bit of ASCII art, so I'll try translating the thread here.
---------------------------------------------------------------

Newsgroup: alt.binaries.schematics.electronic
Title: SillyAmp - SillyAmp.pdf
Author: Jim Thompson
Date: Thu, 15 Mar 2007 15:20:19 -0700
Message-ID: <>

Just a silly playing around. [pdf image]
Has 9nV/rt-Hz input-referred noise... not as good as compound
NPN/PNP. But has 0.03% Distortion with no overall feedback loop.

.. ________________________ +17
.. | | ,---- 6.8k ----,
.. 2.2k 2.2k | __ |
.. | | | | \ gnd
.. | +----------+---|+ \
.. | | | >-----+----
.. +------------- | ---------+---|- / | OUT
.. IN | | | |__/ TL071 |
.. --------- | ------------ | ----, | |
.. | | | '---- 6.8k ----'
.. \_|_ TL431 (2) \_|_ |
.. ---+-----/ \ / \----+
.. | --- --- |
.. 3k | 470u | 3k
.. | +-- 12 ---|(---+ |
.. gnd | | gnd
.. 6.8k 6.8k
.. | |
.. ____|______________|____ -17


ed: http://focus.ti.com/docs/prod/folders/print/tl431a.html


Author: John Larkin
Date: Mon, 19 Mar 2007 15:46:04 -0700

Hey, I can do silly amps, too. [jpeg image]

.. +200
.. +12 |
.. | |
.. R |
.. | |/
.. _V_ --> | npn
.. | |\e
.. IN | | OUT
.. --------+ +---------
.. | |
.. | |/e
.. _V_ --> | pnp
.. | |\
.. R |
.. | |
.. -12 |
.. -200


Author: Jim Thompson

Ooooooh! That's gross ;-)


Author: John Larkin

How else are you gonna get 400 volts p-p swing with two active
parts?


Author: Tim Williams
Date: Tue, 20 Mar 2007 14:15:43 -0600

Easy, you do it with one.

Warning: input is at -400V [gif image]

.. +400
.. |
.. RL
.. |
.. +---- OUT
.. |
.. IN |-'
.. ----+---|| 1500V
.. | |-, MOSFET
.. 100k |
.. | Rs
.. __|______|_____
.. -400


Author: John Larkin

Fine, if your input doesn't mind floating on -400.


Author: Fred Bartoli
Date: Wed, 21 Mar 2007 23:45:45 +0100

That one is 800Vpp, and 400Vpp isn't necessary symmetrical.
And you're having a bit of crossover distortion with the optos.

Speaking of dreadful amplifier: [ascii "image"]

..
.. .-------+-----< +400
.. | |
.. .-. |
.. | | |
.. | | |
.. '-' |
.. | |
.. .-------+----||-+
.. | | ||->
.. | | ||-+
.. | | |
.. --- --- |
.. --- --- |
.. | | |
.. | | |
.. >-------+ +-------+----+---->
.. | | | |
.. | | | .-.
.. | | | | |
.. --- --- | | |
.. --- --- | '-'
.. | | | |
.. | | | |
.. | | ||-+ ===
.. | | ||<- GND
.. '-------+----||-+
.. | |
.. .-. |
.. | | |
.. | | |
.. '-' |
.. | |
.. '-------+----< -400


Author: John Larkin

That's the famous KTFG (kiss the fets goodbye) circuit.


Author: Jim Thompson

Yep, Just prior to the KYAG ritual ;-)

 

Author: John Larkin

One could ac couple, unless it's a DC amplifier, then
back to two parts is better. The pnp is an mpsa92.

+275
IN |
--47k--, RL
| |
|/e +---- OUT
,---| |
| |\ |
gnd | |-'
+---|| 600V
| |-, MOSFET
100k |
| Rs
__|______|_____
-275

 

Winfield Hill a écrit :

That one has a highly optimized 2nd order distortion.
Should sound good and satisfy the most exigent tube aficionados

 

How about a feedback resistor from "OUT" to PNP(e) ??

But there's probably not enough loop gain to reduce the distortion by
much.

...Jim Thompson

 

Jim Thompson a écrit :

Before doing that, you'll need to properly bias the input stage.
Exercise left to the student

 

Are there any "students" here? They certainly don't step forward and
attempt to learn :-(

...Jim Thompson

 

Mini-nits: both my optocouplers were the same, npn's, and my
input-side supplies were +-2, to keep idle current down:

Is not! You're just jealous that you don't have 400 volt optocouplers
in any of your ic libraries.

John

 

Actually, it's not so bad with a significant Rs. That's
because the power high-voltage MOSFET will be operating
in its sub-threshold region, where its gm is nearly as
high as a BJT at the same current. With the high supply
voltage and a modest ratio RL/Rs = 50, for 500V out with
10V in, Rs > 1/gm will keep the distortion under control.

Actually, as mentioned above, with Rs = 0, the loop
gain could be sizeable, and rightly, done it could work
reasonable well. The pullup capabillty is rather wimpy
and there are several DC offset sources.

+325
ZERO |
-- Rx --, RL = 150k
IN | | 5W
-- 20k --+-- 1M --+
-/+5V | |
|/e +---- OUT
,---| | +/- 250V
| |\ | G = -50
gnd | |--'
+----|| 600V
| |--, MOSFET
2.2M |
__|________|_____
-275

A real circuit would be more complicated, like some
I've posted before, including an opamp, a totem-pole
pullup transistor, current limiting, and compensation.

 

Winfield Hill a écrit :

I was just referring to the absence of bias for the PNP, assuming the
input signal was symmetric

I'd probably reduce the 2.2M to 22k or such, which is about 200uA and
60mW for the PNP, for a better gate positive slew rate and GBW product.

 

Yep, but be careful, that 200uA would have to come from
the PNP summing junction, making a high output offset to
correct. Maybe 2.2M is too high, but consider, Crss is
only 3pF for an FQP1n60 above Vds = 25 volts, and with
Vgs about 3 volts at Id = 2mA, we get a rising slew rate
of S = I/C = Vgs/RC = 0.5V/us. 220k would give us 5V/us.
If this stage is placed inside an opamp feedback loop,
then we can begin to get aggressive about the currents.
Unless a pullup transistor is added, external capacitive
loads will seriously slow down this wannabe beast anyway.

+300
ZERO | QUICK-n-DIRTY
-- Rx --, RL = 150k HV AMPLIFIER
IN | | 5W
-- 20k --+-+-- 1M --+
-/+5V | |
|/e +---- OUT
,---| | +/- 250V
| |\ MPSA92 | G = -50
gnd | |--'
+------|| 600V MOSFET
| |--, FQP1N60
220k |
__|__________|_____
-260

 

I'd feel better if there was a gate zener, and maybe a resistor in
series with the PNP collector; I hate all-silicon paths from -260 to
ground. Then go ahead and add a source resistor, to get some current
limiting and improve linearity.

If you want silly, how about an optocoupler/mosfet cascode? Or better,
ie sillier, an opto+depletion fet cascode!

Actually, that's getting interesting.

John

 

John Larkin a écrit :

I thought about that last one as a replacement to the HV optos when you
first 'published' your idea here a couple of years ago.
But now that's 4 active parts. Well six, including the leds.

 

The zener is mandatory. Diode protection for the PNP,
yep. Safety current limiting is wise, although after
making hundreds of such amps with mpsa92 trannies, I
had only one failure and it blew the mpsa92 wide open.
Lots of energy stored in the supply caps after all.

+300
ZERO | QUICK-n-DIRTY
-- Rx --, RL = 150k HV AMPLIFIER
-/+5V IN | | 5W
-- 20k -----+--+-- 1M --+
| |
,--|>|--+ |
| | |
| |/e +-- 1k --- OUT
+-----| | +/- 250V
| |\ MPSA92 | G = -50
gnd | |
47k |
| |--'
+----+--|| 600V MOSFET
| \|_ |--, FQP1N60
220k /_\ |
| | 12V |
__|____|______|_____
-260

 

v+
|
|
r
|
+-----------out
|
|
d
in---------+ g ---+
| s |
| | |
a c |
k ---> b |
| opto e |
| | |
gnd +-----+----- v-




3 parts total! And you can get low-voltage optos as duals, so you can
make it full totem-pole, class B, 500 volts p-p out, with three parts!

Apex, cry your heart out.


John

 

v+
|
|
r
|
+-----------out
|
| Supertex
d depletion-mode
in--/\/\---, g ---,
| s |
| | |
a c |
k ---> b |
| opto e |
| | |
gnd +-----+----- v-

Awesome. Don't forget a resistor for the opto.

 

Another silly KYAG circuit ;-)

...Jim Thompson

 

Jim Thompson <wrote:

?? Really? KYAG? How so? ?? That looks like a relatively-well
behaved circuit, with excessive gain. But it does cry out for a
feedback loop.

 

Concern for opto NPN punch thru during power-up... I could go for a
zener or some-such in there.

I've been musing really retro... maybe a mixed toob/silicon amplifier
stage ;-)

...Jim Thompson

 

You want silly? How about a parametric amplifier using the
nonlinearity of ceramic caps!

John

 

Bring it on! I'd love to see anything about parametric amplifers-- the only
thing I can find on them is "they work by varying some part of the
circuit"...drrrrr...okay...

Ceramic caps would be a very accessible way to demonstrate one.

Tim