Early Voltage problems

ed

0, 50

Nice effort. Not too many explore this; and you may be finding part of the reason.

You say that you set the "base current" - but which? The output transistor alone?

If it is just the output transistor (else all bets are off)- the process used may be optimized for its optical response characteristics

- and have really soft saturation characteristics (which you see as a current-dependent Early voltage). Try running the same tests on a known good transistor (say, 2N2222) to see if there's something wrong with your test setup, calculations, or if it is the part.

HTH--

Post a schematic of, or a link to, your fixture.

You're probably getting some saturation or don't fully have the driver transistor off. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
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                   Spice is like a sports car... 
     Performance only as good as the person behind the wheel.

I got non-convergent tangents for Q2, giving VAF ranging from 70~~170. After much hair-tearing and deep searching, I've learned that this is a 'well-known' issue in devices with small areas and/or self-heating.

In these conditions, the Early voltage is current dependent, and does not conform to Gummel-Poon model.

;-( So, I'll probably leave Q2 VAF at the default 100. For Q1, I'll use VAF =

17 (seventeen) from Avago's published 6n136 model...

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Does not compute. For a single chip Darlington, VAF of BOTH devices should be same. I question validity of characterization. ...Jim Thompson

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| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

                   Spice is like a sports car... 
     Performance only as good as the person behind the wheel.

"I question validity of characterization."

Jim, I was surprised, too !!

In the absence of a manufacturer's model, I was able to directly determine hFE for Q2 as it had accessible EBC. My Peak Atlas (PA) mini component tester gave 25 results with an average of 101.1 and an SD(n-1) of 5.0

I'm fairly confident of the LED==>photo-diode CTR being ~0.0015 (0.15%) as this was quoted in H-P Opto-electronics Applications Manual (c)1977, p3.7, as valid for a range of devices and it matches the ~0.16% of related

6n136's data-sheet and spice model.

Connecting LED cathode to Q2 emitter and commoning Q1~Q2 collectors let the PA recognise entire device as a Darlington. That gave an overall mean hFE of 7.3

Given those, a Q1 hFE of ~45 just falls out of the mix...

D'uh, like the Early Voltage being non-convergent for small-area devices

--Due to 'current crowding' ??-- is it improbable that 'buffer' Q1 is optimised for speed and sensitivity, while Q2 carries the bulk of the gain ?

Unfortunately, Avago (was Agilent, was H-P) aren't talking...

FWIW, I did Q2's 'Early' curves on one chip at 5,6,7,8,9,10, 20, 30, 40,

50, 60, 70, 80, 90 & 100uA iBase and 3, 4.5, 6, 7.5, 9 & 12 VCC. The 100uA curve had both a marked 'soft shoulder' and a hint of approaching breakdown. With or without dubious end-points, the tangents did not converge. At least I've found a likely basis for that anomaly...

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If you're seeing current crowding, parameter "IKF", then get Early effect "VAF" from low currents, make model, then go back and determine IKF. If your simulator can handle avalanche parameters, add those as well. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

                   Spice is like a sports car... 
     Performance only as good as the person behind the wheel.

Thank you. Nik

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I should have also pointed out that BF will be the same for both devices, but each device will have its own IKF, IS and NE, and other area (and geometry) dependent terms. ...Jim Thompson

-- | James E.Thompson, CTO | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at

| 1962 |

Spice is like a sports car... Performance only as good as the person behind the wheel.

FWIW, here are the resistor values I used to set 5~~100 uA iBase.

My supplier only stocked low-value metal-film 1% resistors in E12 series and an LM 334 has a nominal +/- 3% error, so I measured the delivered current prior to connecting the opto-coupler.

LM334 mA = 67.7 / R

TargetmA R_step R_Cum I_nominal Error % Actual uA

0.100 680.0 680.0 0.100 -0.4 99.9 0.090 82.0 762.0 0.089 -1.3 89.2 0.080 91.0 853.0 0.079 -0.8 79.6 0.070 110.0 963.0 0.070 +0.4 70.5 0.060 160.0 1123.0 0.060 +0.5 60.4 0.050 240.0 1363.0 0.050 -0.7 49.8 0.040 330.0 1693.0 0.040 -0.0 40.1 0.030 560.0 2253.0 0.030 +0.2 30.1 0.020 1100.0 3353.0 0.020 +1.0 20.2 0.010 3300.0 6653.0 0.010 +1.8 10.2 0.009 910.0 7563.0 0.009 -0.5 9.0 0.008 910.0 8473.0 0.008 -0.1 8.0 0.007 1200.0 9673.0 0.007 -0.0 7.0 0.006 1600.0 11273.0 0.006 +0.1 6.0 0.005 2200.0 13473.0 0.005 +0.5 5.0

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Jim, I seem to have goofed with that BF split. The photo-diode, Q1 & Q2 are surely monolithic hence have similar junctions in all but size. FWIW, a comment in Zetex model info (see below) was only reference Google could find for Darlington gain splits...

Yes, I know that transistors on-chip may be very well matched, it is what makes long-tail pairs and current mirrors so predictable, but how do TO220 Darlingtons contrive dissimilar Driver and Power stages ??

Okay, I've a Plan_B for driving my 6n139s' LED with known current from LM334 and measuring Q1's collector current. Take a few days to source the dropper chain parts...

===(c)Zetex=== Darlingtons These are subcircuits using a standard transistor model. A Darlington model

is shown as follows:

* *Zetex FZT605 Spice Model v1.0 Last revision 27/04/05

• .SUBCKT FZT605 1 2 3

• C B E Q1 1 2 4 SUB605 Q2 1 4 3 SUB605 3.46

• .MODEL SUB605 NPN IS=4.8E-14 BF=170 etc. .ENDS FZT605

* *$ Note: Because Zetex Darlingtons are monolithic, the two transistors used are identical in all respects other than size. (The number at the end of the Q2 line multiplies the size of the SUB605 transistor by 3.46 - the ratio of the areas of the input and output transistors for this device).

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I knew that :-)

What issues are you having? Matching Input (LED) current to output curves? Perhaps both driver and output device are exposed to the light?? Seems odd, but that could really !@#$% up your results.

Way back when I did optical chips, we masked off everything but the desired opto-active device with an extra layer of metal.

Modern processes actually have an opto-blocking layer in the process, and layers that enhance specific wavelengths (see XFAB for examples). ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

                   Spice is like a sports car... 
     Performance only as good as the person behind the wheel.

The basic problem with modelling the 6n139 is that Q1's base is not accessible, so *only* the product LED==> photo-diode CTR x Q1 hFE is available...

(CTR = Current Transfer Ratio ;-)

Or so I thought...

Last night, I had a rush of blood to the brain: What if I fed the LED a few milliamps and put my DVM across Q1's emitter and collector ? No Vcc, just the photo-diode's output ? Would its photo-current flow through Q1's base/emitter PN ?

So, today, I tried it. I bridged Q1 emitter to Q2 emitter as latter is connected to substrate. I put an LED in series with the opto's LED to see current was flowing. A 9V battery and 680R, 1K0 or 1K8 fed 9.05, 6.21 or

3.49 mA by DVM. I measured 53.4, 35.4 and 18.6 uA photo-current across Q1 emitter and collector.

These gave CTR of 0.0059, 0.0057 & 0.0053, equivalent to ~0.5 %.

This is approx three (3) times the CTR from the old H-P reference, thrice the quoted figure for the data-sheet and model of 6n136...

If this ratio is representative, it means that a Q1's hFE is only ~15, in marked contrast to the Q2s' measured hFE of ~100...

D'uh, can you see any hole in my methodology or logic ??

--------------------------------------- Posted through

Yes. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

    Democrats are Living Proof that Government IS the Problem!

I've just been reminded that part of photo-current will flow via Base-Collector junction. Why so much went via Base-Emitter is probably down to the 'small' geometry...

D'uh, my bad...

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The way I read a 6N139 data sheet...

Tie VCC to GND

Treat VO as Collector

Treat VB as Base

Treat GND as Emitter

Measure BF

This BF will apply to both devices ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

          Democrats are best served up prepared as a hash
           Otherwise the dogs will refuse to eat them :)

After another deep search, I found a Spice model of related 6n140 at...

The sub-circuit uses a near-generic LED, two generic 2n2222 Qn (BF=100, VAF ~75), a 5k Q2 E-B resistor and a polynomial 'G' function to match the product's curves. It does not pretend to be a manufacturer's physical model but I'm left with scant choice. Although my measurements imply that 6n139's Q1 & Q2 are dissimilar, I'll have to model them as twins and adjust the CTR to suit...

FWIW, there are models for some rare and unusual parts at that site, including the classic 4n49 opto. No warranty intended or implied.

Per site's terms & conditions, I'll only post list of their models.

• Models included:

• UC1845 - UC1845 PWM controller

• 555 - 555 timer - fixes problems with PSpice library model

• HCPL5201 - Optocoupler

• HCPL5230 - Dual Optocoupler

• UC1707 - Dual Channel Power Driver IC

• MAX707 - µP Supervisory IC

• TC4427 - Buffer/driver

• UCC1802 - PWM controller IC

• UCC1804 - PWM controller IC

• D1N3611 - 1N3611 diode

• OPAMPIB - near-ideal op amp macromodel

• OPAMPIC - near-ideal op amp macromodel

• BUFST - Schmitt trigger digital buffer macromodel

• VREGD - 3-terminal ideal regulator macromodel

• COMPI - near-ideal comparator

• ILOADB - current load macromodel

• EILIM - voltage source with current limit macromodel

*

• Models included:

• LM117S - LM117 voltage regulator improved model

• 4N49N - 4N49 Optocoupler

• 6N140 - 6N140 Darlington Optocoupler

• PS2502 - PS2502 Optocoupler

• PS2501 - PS2501 Optocoupler

• 2823 - High voltage driver IC

• Q2N6058 - 2N6058 Darlington Power Transistor subcircuit

*

• Models included:

• Q2N3585 - 2N3585 NPN transistor

• Q2N3868 - 2N3868 PNP transistor

• Q2N3506 - 2N3506 NPN transistor

• Q2N6032 - 2N6032 NPN transistor

• Q2N5794 - 2N5794 NPN transistor

• === Most models look generic, but some are $nn format eg...

• UC1845 PWM controller IC *

• connections: pins 1 - 8 sequential (8-pin pkge.) .SUBCKT UC1845 COMP VFB CS RTCT GND OUT VCC VREF R_R1 VCC GND 1.4k D_D7 GND VCC DZ34 R_R12 VCC $N_0001 10K R_R13 $N_0001 VREF 60K V_V1 $N_0002 GND DC 7.22V X_U21 $N_0003 $N_0004 $G_DPWR $G_DGND BUFST X_U7 $N_0004 $N_0005 $G_DPWR $G_DGND INV R_R8 etc.

--------------------------------------- Posted through

That's just a subcircuit that uses other part or subcircuit models... DZ34, BUFST and INV.

How do you download? The download button stays gray :-( ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
Nine years ago, on September 11, around 6AM, 4 telephone calls in
rapid succession, "Mom, Where's Dad, Is he flying?"
My wife woke me and turned on the television.
                               ****
                       We will NEVER forget!
                         We WILL get even!
                  Destroy All Islamic Barbarians!
            Start by applying salt to the Obama-slug ;-)

Never mind... it suddenly decided to work ?? ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
Nine years ago, on September 11, around 6AM, 4 telephone calls in
rapid succession, "Mom, Where's Dad, Is he flying?"
My wife woke me and turned on the television.
                               ****
                       We will NEVER forget!
                         We WILL get even!
                  Destroy All Islamic Barbarians!
            Start by applying salt to the Obama-slug ;-)

[snip]

Size scaling won't change the CTR _until_ some kind of crowding or saturation occurs. What it mostly will affect is timing... due to junction capacitances.

Did you try my suggestion:

Message-ID:

The way I read a 6N139 data sheet...

Tie VCC to GND

Treat VO as Collector

Treat VB as Base

Treat GND as Emitter

Measure BF

This BF will apply to both devices ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
Nine years ago, on September 11, around 6AM, 4 telephone calls in
rapid succession, "Mom, Where's Dad, Is he flying?"
My wife woke me and turned on the television.
                               ****
                       We will NEVER forget!
                         We WILL get even!
                  Destroy All Islamic Barbarians!
            Start by applying salt to the Obama-slug ;-)