Differential-pair amplifer

[Reg Edwards]

If it is true that a pair of transistors, a differential pair
amplifier, has a Tanh transfer characteristic, then at audio
frequencies it behaves as a sort of amplitide or volume compandor.

At large signal inputs the output approaches a squarewaveform and
hopeless distortion occurs. At intermediate inputs audio distortion
increasingly appears.

What other so-called useful applications does the circuit have? I
can't think of any myself?

 

[colin]

If it is true that a pair of transistors, a differential pair
amplifier, has a Tanh transfer characteristic, then at audio
frequencies it behaves as a sort of amplitide or volume compandor.

At large signal inputs the output approaches a squarewaveform and
hopeless distortion occurs. At intermediate inputs audio distortion
increasingly appears.

What other so-called useful applications does the circuit have? I
can't think of any myself?

you can convert a triangle wave to a pretty good aproximation of a sinewave
with it.

Colin =^.^=

 

[John Popelish]

(snip)

you can convert a triangle wave to a pretty good aproximation of a sinewave
with it.

Two properly adjusted stages can produce a damn good approximation of
a sine wave from a triangle.

 

[John Popelish]

If it is true that a pair of transistors, a differential pair
amplifier, has a Tanh transfer characteristic,

That is my understanding.

then at audio
frequencies it behaves as a sort of amplitide or volume compandor.

Well, the compressor part of the compander, anyway. You need another
one in a feedback path to do the equivalent expanding, downstream.

At large signal inputs the output approaches a squarewaveform and
hopeless distortion occurs. At intermediate inputs audio distortion
increasingly appears.

What other so-called useful applications does the circuit have? I
can't think of any myself?

I think it is useful to compress a wide range AC signal to be
digitized by an A/D converter, having high resolution at small
amplitudes, and lower resolution for large amplitudes, where the
resolution is mostly wasted the A/D result can then be linearized with
a look up table.

I think audio that any exceed the swing limits of a power amplifier
sounds a lot better if the hard clipping is prevented by predistorting
the audio with a tanh compressor so that the power stage cannot reach
clipping level, or if it does, clips gently, so it doesn't generate
such high harmonics of the signal frequencies.

 

[John Larkin]

If it is true that a pair of transistors, a differential pair
amplifier, has a Tanh transfer characteristic, then at audio
frequencies it behaves as a sort of amplitide or volume compandor.

At large signal inputs the output approaches a squarewaveform and
hopeless distortion occurs. At intermediate inputs audio distortion
increasingly appears.

What other so-called useful applications does the circuit have? I
can't think of any myself?

As you observe, differential pairs are totally worthless. Nobody uses
them any more.


John

 

[Jim Thompson]

As you observe, differential pairs are totally worthless. Nobody uses
them any more.


John

ROTFLMAO! Dummies are good for business... except when they insist
they know what they are doing :-(

...Jim Thompson

 

[Ken Smith]

At large signal inputs the output approaches a squarewaveform and
hopeless distortion occurs.

That distortion is not hopeless if it is what you need. In that case it
is very nice distortion. Take a look at the circuit of a comparitor IC
like the LM339. It distorts like crazy just like you would want it to.

What other so-called useful applications does the circuit have? I
can't think of any myself?

If you combine a few in tricky ways, you can make an analog multiplier and
or, a circuit that finds the RMS of an applied waveform.

You can use them to perform logical operations.

 

[Ol' Duffer]

What other so-called useful applications does the circuit have?
I can't think of any myself?

I guess you don't consider a circuit whose output is a
function of the difference between two inputs useful,
but a lot of op-amp makers find it convenient...

 

[Kevin Aylward]

As you observe, differential pairs are totally worthless. Nobody uses
them any more.

Not at all. You just need to know how to design with them correctly

http://www.anasoft.co.uk/EE/MicAmpDesign/MicAmpDesign.html

If you want really high CMRR, they are the *only* way to go. And as we
can see from the above, in the correct configuration, distortion can be
very low indeed.

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[martin griffith]

If it is true that a pair of transistors, a differential pair
amplifier, has a Tanh transfer characteristic, then at audio
frequencies it behaves as a sort of amplitide or volume compandor.

At large signal inputs the output approaches a squarewaveform and
hopeless distortion occurs. At intermediate inputs audio distortion
increasingly appears.

What other so-called useful applications does the circuit have? I
can't think of any myself?

It s useful in analogue video when you are doing cross mix between 2
video source ISTR, (from a video servicing course at Ampex years ago)
Sorry Reg, I cant remember the theory, I fell asleep during this part
of the course


martin

After the first death, there is no other.
(Dylan Thomas)

 

[Winfield Hill]

Kevin Aylward wrote...

Not at all. You just need to know how to design with them correctly

Kevin, Kevin... John was merely speaking tongue-in-cheek!

 

[Kevin Aylward]

Kevin Aylward wrote...

Kevin, Kevin... John was merely speaking tongue-in-cheek!

Indeed. But we don't want any newbies to get the wrong idea

Tell you what though, this discrete design lark is getting really
difficult. All the components are disappearing. Its really hard to find
*small* signal mosfets. They are all 1A high Cgs jobs. So far, I'm back
to the 25+ year old SD211 for its low capacitance as a part of a current
source. Still looking for a pmos with a spice model. I get lazy now, I
found one pmos, but cant be bothered to make the model as one wasn't
given.

Kevin Aylward
[email protected]
http://www.anasoft.co.uk
SuperSpice, a very affordable Mixed-Mode
Windows Simulator with Schematic Capture,
Waveform Display, FFT's and Filter Design.

 

[Winfield Hill]

Kevin Aylward wrote...

Tell you what though, this discrete design lark is getting really
difficult. All the components are disappearing. Its really hard to find
*small* signal mosfets. They are all 1A high Cgs jobs. So far, I'm back
to the 25+ year old SD211 for its low capacitance as a part of a current
source. Still looking for a pmos with a spice model. I get lazy now,
I found one pmos, but cant be bothered to make the model as one wasn't
given.

Now Kevin, I'm slaving away making accurate low-current MOSFET
spice models, you can join the fray, too. The more the merrier.
Also, just because a MOSFET model is available, don't assume it's
accurate below say 3 to 5% of the FET's absolute-maximum current.
In other words, it's likely to be inaccurate in the subthreshold
linear-circuit region that interests us. However, it's not too
hard to fix the models with a current-controlled current source
feeding a diode directly in series with the FET's gate. Using n=3
works pretty good for a 2n7000, big power MOSFETs are around n=5.

See the Steven M. Sandler and Charles E. Hymowitz article in the
May 2005 issue of Power Electronics Technology magazine. OK,
I know it's not out yet, but... In the meantime, try this link,
http://www.edacafe.com/technical/papers/Mosfet_paper.php

Sandler's approach works well, after correcting his parameters.

 

[Fred Bloggs]

If it is true that a pair of transistors, a differential pair
amplifier, has a Tanh transfer characteristic, then at audio
frequencies it behaves as a sort of amplitide or volume compandor.

At large signal inputs the output approaches a squarewaveform and
hopeless distortion occurs. At intermediate inputs audio distortion
increasingly appears.

Hello- ever hear of something called feedback- can be used to
pre-distort the input drive of the diff amp as well as maintain
operation in small signal region. Is there such a thing as a monolithic
op amp without a differential amplifier front-end?

 

[Jim Thompson]

Kevin Aylward wrote...

Now Kevin, I'm slaving away making accurate low-current MOSFET
spice models, you can join the fray, too. The more the merrier.
Also, just because a MOSFET model is available, don't assume it's
accurate below say 3 to 5% of the FET's absolute-maximum current.
In other words, it's likely to be inaccurate in the subthreshold
linear-circuit region that interests us. However, it's not too
hard to fix the models with a current-controlled current source
feeding a diode directly in series with the FET's gate. Using n=3
works pretty good for a 2n7000, big power MOSFETs are around n=5.

See the Steven M. Sandler and Charles E. Hymowitz article in the
May 2005 issue of Power Electronics Technology magazine. OK,
I know it's not out yet, but... In the meantime, try this link,
http://www.edacafe.com/technical/papers/Mosfet_paper.php

Sandler's approach works well, after correcting his parameters.

What? You don't want to deal directly with the device model ?

As I showed a while back, the Supertex Level=3 model was pretty close,
without adding on subcircuit elements.

I'm always leery of add-ons based on limited measured data, covers one
particular case, but may bomb out on the next.

How does this model behave in regard to modeling gate charge?

Win, I don't receive PET magazine. Could you get me a copy of the
Sandler & Hymowitz article when it comes out? Thanks!

...Jim Thompson

 

[John Larkin]

Hello- ever hear of something called feedback- can be used to
pre-distort the input drive of the diff amp as well as maintain
operation in small signal region. Is there such a thing as a monolithic
op amp without a differential amplifier front-end?

Current-mode opamps have a different topology.

John

 

[Fred Bloggs]

Current-mode opamps have a different topology.

John

Most people understand op amp to mean voltage mode- current mode only
made a difference because people were too lazy to invent a different
circuit symbol.

 

[John Larkin]

Most people understand op amp to mean voltage mode- current mode only
made a difference because people were too lazy to invent a different
circuit symbol.

You'd better explain that to all the semiconductor companies; they
keep calling them 'opamps'.

John

 

[Winfield Hill]

Jim Thompson wrote...

What? You don't want to deal directly with the device model ?

As I showed a while back, the Supertex Level=3 model was pretty
close, without adding on subcircuit elements.

Where'd you show that? I'd love to take a look at it.

Bring it on.

I'm always leery of add-ons based on limited measured data, covers
one particular case, but may bomb out on the next.

Actually, in this instance, it's general, because at the low
subthreshold currents the MOSFET reverts to diffusion-driven
n VT = n kT/q performance, very predictable and very general
for all MOSFETS, with a little variation in n, natch. Takes
over nicely in the region where Id otherwise drops like a rock
because it's below Vthreshold.

How does this model behave in regard to modeling gate charge?

No problem, you add the gate-voltage-adjusting diode *inside* the
complex charge-modeling circuitry, stuff that's already carefully
worked out by the manufacturer and is retained.

Win, I don't receive PET magazine. Could yo get me a copy of the
Sandler & Hymowitz article when it comes out? Thanks!

It's a free mag and I get it, but they also put up the articles
on the web. The May issue must be overdue, but I'm watching for
it and will alert you when it hits. Meantime Sandler's notes
give the idea. It's a better approach than the successful one
I had developed with components in the MOSFET's source. I had
gotten that one working well over a 50pA to 100mA range, but I've
now exceeded that with the new one from 50pA to 2A. Recommended.

 

[Jim Thompson]

Jim Thompson wrote...

Where'd you show that? I'd love to take a look at it.

Bring it on.

[snip]

Originally posted to A.B.S.E on 4/6/2005, re-posted at....

Newsgroups: alt.binaries.schematics.electronic
Subject: Win Hill - 2N7000 Spice Model Comparisons -
Compare2N7000Models.pdf
Message-ID: <[email protected]>

...Jim Thompson