active collector loads (audio freq)

No that loads thing down.

Say it's an NPN input stage. When the currents thru the bjts become unbalanced, the following stage either gets the surplus current from the active load or gives some up to the controlling bjt.

You want to mirror an extra transistor off the diode connected reference.

--
Best Regards,
Mike

Looks to me like Kev got overwhelmed with his own BS ;-)

130dB feedback... ROTFLMAO!

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

I read in sci.electronics.design that snipped-for-privacy@yahoo.com wrote (in ) about 'active collector loads (audio freq)', on Sun, 26 Dec 2004:

No, it's not brushed over. You are missing what I tend to agree is at first a not-so-easy visualization of what is happening in the circuit.

You *can* look at the output of an actively-loaded gain stage with a scope, but the input impedance of the scope will change the *voltage* gain. A very good active load might have an impedance of 100 Mohms, so even a 10 Mohm scope probe will greatly reduce the *voltage* gain.

If your next stage is a low impedance, there is not a lot of point in using an active load, unless you need to squeeze the last drop of

*current* out of the first stage. If you do, you are not at all interested in the *voltage* gain of the first stage but its *current* gain or transconductance, depending on which of the input signal parameters you are interested in. >

--
Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Interesting circuit. You mean the common mode feedback going to the active loads on the input pair?

I'd like to hear your explaination of the next stage. I'm only halfway understanding its operation. Looks like emitter followers with an improved Widlar current mirror at the emitters. Cascode outputs with all that surrounded by the miller caps. Looks like the feedback current out the bottom of all that bucks the current in its respective diff input branch.

--
Best Regards,
Mike

This is why I use a common mode feedback feeding extra emitter followers in

The two stage gain amounts to the order of 160db gain. The 130db of feedback at low frequencies gets one rather low distortion:-)

Kevin Aylward snipped-for-privacy@anasoft.co.uk

SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design.

thanks, this is helping. which one is the controlling bjt? the one that has the diode-connected transistor in it's collector circuit? and if so, what is the other bjt of the input pair doing if the controlling bjt and the mirror are doing all the work?

the

gain

can you describe the connections? what do you do with the diode connected bjt? seems like you'd have to remove it from the collector circuit of the 1st input bjt to get the gain up. where does it go?

also, generally, what's the clue for when one should think "current operation" and when one should think "voltage operation"? Thanks!

Sean B

I take back that part about the Wilson mirror. The other half has the same circuit so it's just 3 emitter followers.

How many MOSFET models in SS are accurate in the sub-threshold region? Surely none you've just dropped in from the manufacturer. If it's as bad as Win suggests, I may as well forget expecting any Spice results to be accurate.

I'd like to find some good models for audio power.

--
Best Regards,
Mike

Both input NPNs control current in their own branch. You can cut the circuit down the middle to simplify it and I was explaining things in relation to one side only.

active load current source | | | |

+------- current difference | | | NPN current, controlled

KCL, dude. | |

leave it as a reference

off to the side out of the way or in between. Wherever you want to draw it.

Uh, who wants to start the list? Topology is one consideration if you're talking about analysis. In this case, you can figure out the currents. You might want to convert to a voltage and hit an emitter follower with an active load on its emitter. You might not.

Scroll thru 7MANUAL.pdf and designanalogchips.pdf from

for a pile of examples.

--
Best Regards,
Mike

Nonsense.

It does do 130db of feedback at LF, according to spice. Its two stages of cascode amplification. 160 db of gain is 2 of 80db. This is 2 of

10,000. A non cascode stage might do Va/Vt = 50/25m = 2000. So, a cascode can certainly get 10,000 in one stage.

Why don't you actually spice it before opening your mouth and inserting ones foot in it.

Kevin Aylward snipped-for-privacy@anasoft.co.uk

SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design.

I read in sci.electronics.design that Kevin Aylward wrote (in ) about 'active collector loads (audio freq)', on Sun, 26 Dec 2004:

Until the signal reaches the end-stop, of course. x volts out, distortion 0.00...1%. (x + 0.1) V out, distortion 1%. (x + 1) V out, distortion 30%. (All voltages r.m.s, of course)

--
Regards, John Woodgate, OOO - Own Opinions Only. 
The good news is that nothing is compulsory.
The bad news is that everything is prohibited.
http://www.jmwa.demon.co.uk Also see http://www.isce.org.uk

Yes. It keep a low common mode impedance impedance to set the common mode bias voltage, but a high impedance for differential signals.

Its just a diff amp with emitter followers that have a current source. Its not a widlar.

Bog standard diff amp with current mirror load.

Kevin Aylward snipped-for-privacy@anasoft.co.uk

SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design.

Yes and no. BSim3 models are usually quite accurate in all regions, if you can get the correct models. However, small signal distortion analyis is not supported for them in spice.

One cheats.

SS has two set of models for the EC10N20 and EC10P20 range. One is a kludged BSim3 (EC10N20 and EC10P20) one is a level 1 (EC10N16 and EC10P16). Both are set up to approximately match each other especially the capacitances. I do runs with each type to check stability, and use the level 1 for small signal distortion.

If one is not in subthreshold, the level 1 will give results that are "reasonable". Who cares if its 100% in error if one is designing say at

0.01%. What one is trying to do is know that the distortion is not 0.1% if one is after 0.01%. So, if one wants say 0.002%, one tries to get spice to show its better by a factor of, say two. Engineering is not exact, but one can still work around this with guestimates.

I can send you the SS files for the amp if you want.

Kevin Aylward snipped-for-privacy@anasoft.co.uk

SuperSpice, a very affordable Mixed-Mode Windows Simulator with Schematic Capture, Waveform Display, FFT's and Filter Design.

Thanks. I think I could learn something from that. "mcol" should bring me up in your address book. If not let me know.

--
Best Regards,
Mike

.. In this case, you can figure out the

how does one convert? thanks for the cool web reference!

SB

[snip]

Barf ;-)

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.

Personally, I always think of a bjt as a voltage controlled device, so I don't think in terms of converting a diff stage current to a voltage, but it's referred to as a current source AFAIK because of the high impedance of the source.

I read somewhere that it's best to not convert to a voltage at all and I don't know wtf the author was talking about.

Those pdfs cover diff stages and opamps. One of them covers output circuits and level shifters. Study that.

What you do is follow the diff stage with a CE stage, sometimes called the 2nd stage, gain stage, or often, the Miller stage (google that - also google it in the archives of this group.)

Anyway, the Miller stage does the "current" to voltage conversion though the output of the diff stage does have a voltage wiggle to it. The Miller stage gives you lots of voltage gain and a good place for a compensation cap. At this point, your voltage swing is going to be closer to one rail than the other and you might level shift it down prior to the output. The output stage would be some kind of emitter follower, either 1 bjt and a current source/sink or a push-pull emitter follower. That gives you extra current drive. Of course, you'll see from the schems of existing chips that there's other neat stuff in there.

You can find a few versions of the 741 opamp schem on the web. The National linear data book has schems for opamps, too.

--
Best Regards,
Mike

Ok. I learned a lot from the h-param equiv circuit with beta and all.

Maybe you could comment on my confusion over that author saying it's best to never convert to a voltage after the diff stage. What could he have meant?

--
Best Regards,
Mike

Was that us you're referring to? Where'd we say that?

--
 Thanks,
    - Win

That's where slew-rate and "power-bandwidth" come into play in OpAmp descriptions.

...Jim Thompson

--
|  James E.Thompson, P.E.                           |    mens     |
|  Analog Innovations, Inc.                         |     et      |
|  Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
|  Phoenix, Arizona            Voice:(480)460-2350  |             |
|  E-mail Address at Website     Fax:(480)460-2142  |  Brass Rat  |
|       http://www.analog-innovations.com           |    1962     |
             
I love to cook with wine.      Sometimes I even put it in the food.