Don's source follower

[Michael Lloyd]

Don:

Sorry for the long delay - had to do some real work for a while :-)

You're right, my CD301 deck uses the same circuit as your 401. I was always
told the eyes were the first to go, looks like it's the memory.

Anyway, I did manage to find where I did actually see a circuit that looks
like what you described.
It's from an article in the Journal of the Audio Engineering Society
September 1975, Volume 23, Number 7, pages 530 - 535. The article title is
"A Complementary Source Follower Circuit for Condenser Microphone
Preamplifier by T. Ken Matsudaira and Osamu Kono of Sony Corp.

For most of their applications in the paper, they feed the buffer output
into a source follower (biased with a current sink) to drive a transformer.

Your 12 points were right on, too.

Thanks to Don for making me dig in the basement to find this paper :-)  I
found some other stuff I wanted to reread down there as well!

Michael
mlloyd1 at enteract.com

----- Original Message -----
From: "Don Tillman" <don at till.com>

Sent: Tuesday, June 20, 2000 12:54 AM
Subject: Re: Don's source follower


>    From: "Michael Lloyd" <michaellloyd at lucent.com>
>    Date: Mon, 19 Jun 2000 11:40:23 -0400
>
>    Nifty circuit! You asked if any has seen it before. I've seen it in
>    my old (from about 1982) harmon kardon CD301 cassette deck. The
>    audio circuits (except dolby, of course) used discrete devices
>    instead of the monolithic op amps.
>
> Heh-heh, I happen to own a 1982 vintage Harman Kardon CD401 cassette
> deck.  It may or may not use the same circuit; I'll check the
> schematics... Okay, the line amp uses something similiar, on top is a
> NFET/PNP compound and on the bottom is a PFET/NPN compound.  That's
> the same basic idea.  Is that what the CD301 does?
>
>    I'm curious; what are the ten features. I don't think I could come
>    up with ten :-)
>
> 1.
>
> With no load the thing is pretty much distortionless, at least as far
> as a basic FET model goes.  Compare that to the standard source
> follower (imagine an illustration here) where the square-law FET
> transfer curve shows up in the output.  A standard source follower
> with a current sink on the drain terminal (imagine that illustration
> here) is also distortionless with no load.  But...
>
> 2.
>
> Compared to a source-follower-with-a-current-source circuit, running
> with the same quiescent current, this circuit will put out several
> times more drive current and have a pretty low offset.
>
> 3.
>
> When there is a load, the even order distortion products cancel.  And
> since an FET is close to a perfect square law device, almost all the
> distortion cancels.
>
> 4.
>
> If the devices aren't matched exactly, the remaining distortion is
> probably the least annoying possible.  Perhaps even preferred.
>
> 5.
>
> Clipping is graceful.
>
> 6.
>
> With an input resistor, current limiting into low impedance loads
> appears to be graceful too.  (I haven't given it a rigorous test yet
> though.)
>
> 7.
>
> No common-mode range issues.  No crazy behavior like when you run a
> TL071 with the inputs within 1.5 volts of the supply rails.
>
> 8.
>
> It's easy to cascode the FET drains if you think it's a source of
> nonlinearity.  I don't know if it is or not in real life situations,
> but that could be a potential improvement.
>
> 9.
>
> You can replace the two resistors with a pot and zero out any
> remaining offset by hand.
>
> 10.
>
> The usual "I hate opamps argument":
>
> In the classic opamp voltage follower the signal has to go through
> about 6 transistors, each with an exponential characteristic, each
> exponential characteristic generating distortion products of all
> harmonics, and the transistor stages generally alternate polarity so
> the exponential characteristics multiply, and than a lot of feedback
> is applied around the whole thing.  Lots of high order intermodulation
> distortion.
>
> Here there's one transistor stage between input and output.
> Distortion is removed by the basic nature of the devices and by
> parallel symmetry.
>
> 11.
>
> The circuit requires hand matching of the FETs.  I used to consider
> hand matching an engineering wart, but lately I've realized that hand
> matching enables many interesting topologies that wouldn't be possible
> otherwise.
>
> 12.
>
> The output impedance isn't real low, about 1000 ohms.  Not good for
> any precision control type of applications, but for audio it's
> fine.
>
>
> Like I said, I'm just starting to use this in a number of designs.  I
> don't have enough experience with it to declare it the best thing ever
> constructed with four components, but it seems to have a lot of
> potential.
>
> I'm imagining having to build up a test device with 50 complementary
> source followers in a row to check signal degradation.  But I'm not
> really in the mood to do that, so I was wondering if there's any
> experience with it around.
>
>   -- Don
>
> --
> Don Tillman
> Palo Alto, California, USA
> don at till.com
> http://www.till.com
>
>