# LT Spice diode C-V graph

Started by June 27, 2014
```
What's an easy way to plot the C-V curve of a back-biased diode?

Maybe a constant-current ramp, then a differentiator, and a little
math block?

Or, flip that over, constant voltage ramp and current sensor, less
math.

Low level AC voltage on top of a DC sweep, and some sort of AC current
detector? (Fake Boonton)

--

John Larkin         Highland Technology, Inc

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

```
```On Fri, 27 Jun 2014 16:20:18 -0700, John Larkin <jlarkin@highlandtechnology.com>
wrote:

>
>
>What's an easy way to plot the C-V curve of a back-biased diode?
>
>Maybe a constant-current ramp, then a differentiator, and a little
>math block?
>
>Or, flip that over, constant voltage ramp and current sensor, less
>math.
>
>Low level AC voltage on top of a DC sweep, and some sort of AC current
>detector? (Fake Boonton)

This is rude and crude but looks OK. Horizontal is 1 ns per volt, vertical is 1
volt per pF. That 1N914 keeps depleting past 1 kilovolt!

Version 4
SHEET 1 896 680
WIRE 48 80 0 80
WIRE 176 80 48 80
WIRE 320 80 240 80
WIRE 416 80 320 80
WIRE 512 80 464 80
WIRE 560 80 512 80
WIRE 0 144 0 80
WIRE 320 144 320 80
WIRE 464 144 464 80
WIRE 416 160 416 80
WIRE 0 272 0 224
WIRE 320 272 320 224
WIRE 416 272 416 208
WIRE 464 272 464 224
FLAG 0 272 0
FLAG 320 272 0
FLAG 416 272 0
FLAG 464 272 0
FLAG 512 80 CAP
FLAG 48 80 RAMP
SYMBOL diode 240 64 R90
WINDOW 0 -57 29 VBottom 2
WINDOW 3 -52 29 VTop 2
SYMATTR InstName D1
SYMATTR Value 1N914
SYMBOL res 304 128 R0
WINDOW 0 -58 44 Left 2
WINDOW 3 -61 74 Left 2
SYMATTR InstName R1
SYMATTR Value 1m
SYMBOL voltage 0 128 R0
WINDOW 0 28 111 Left 2
WINDOW 3 -11 192 Left 2
WINDOW 123 0 0 Left 2
WINDOW 39 0 0 Left 2
SYMATTR InstName V1
SYMATTR Value PULSE(0 2000 0 2000n)
SYMBOL e 464 128 R0
WINDOW 0 64 53 Left 2
WINDOW 3 58 85 Left 2
SYMATTR InstName E1
SYMATTR Value 1e6
TEXT 16 104 Left 2 ;RAMP 1 V/nS
TEXT 496 112 Left 2 ;1 V / pF
TEXT 538 315 Left 2 !.tran 500n
TEXT 360 -40 Left 2 ;Capacitor C:V Curve
TEXT 352 0 Left 2 ;J Larkin  June 27, 2014

--

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com

Precision electronic instrumentation
```
```On 6/27/2014 9:31 PM, John Larkin wrote:
> On Fri, 27 Jun 2014 16:20:18 -0700, John Larkin <jlarkin@highlandtechnology.com>
> wrote:
>
>>
>>
>> What's an easy way to plot the C-V curve of a back-biased diode?
>>
>> Maybe a constant-current ramp, then a differentiator, and a little
>> math block?
>>
>> Or, flip that over, constant voltage ramp and current sensor, less
>> math.
>>
>> Low level AC voltage on top of a DC sweep, and some sort of AC current
>> detector? (Fake Boonton)
>
>
> This is rude and crude but looks OK. Horizontal is 1 ns per volt, vertical is 1
> volt per pF. That 1N914 keeps depleting past 1 kilovolt!

Grekhov should have lived to see this day.  Spherical cows rule. ;)

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net
```
```On Friday, June 27, 2014 4:20:18 PM UTC-7, John Larkin wrote:
> What's an easy way to plot the C-V curve of a back-biased diode?

Easy, is to buy the source/meter Keithley solution.  They'd love
to explain it all to you...
<http://www.keithley.com/promo/lp/semiconductor>

> Low level AC voltage on top of a DC sweep, and some sort of AC current
> detector? (Fake Boonton)

This  one, I think; the small-signal capacitance versus voltage is a very good indicator
of the doping profile inside the junction (as the depletion region boundary moves
through the various dopant concentrations).  Classic work on this
was done at Bell Labs, by Lawrence and Warner
Diffused Junction  Depletion Layer Capacitance Caculations, BSTJ vol. 34, 1955
```
```On Sun, 29 Jun 2014 01:15:08 -0700 (PDT), whit3rd <whit3rd@gmail.com> wrote:

>On Friday, June 27, 2014 4:20:18 PM UTC-7, John Larkin wrote:
>> What's an easy way to plot the C-V curve of a back-biased diode?
>
>Easy, is to buy the source/meter Keithley solution.  They'd love
>to explain it all to you...
><http://www.keithley.com/promo/lp/semiconductor>

I bought an expensive Keithley source-meter. Crap. Sent it back.

But I meant in LT Spice, which is why I titled the post "LT Spice..."

I can read the C-V curve off the data sheet. What I want to do is make sure (or
force) my Spice sim to behave like the actual diode, so I want to do a
simulation of the diode c-v curve, to make sure I have everything right.

--

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com

Precision electronic instrumentation
```
```On Sun, 29 Jun 2014 07:29:40 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

>On Sun, 29 Jun 2014 01:15:08 -0700 (PDT), whit3rd <whit3rd@gmail.com> wrote:
>
>>On Friday, June 27, 2014 4:20:18 PM UTC-7, John Larkin wrote:
>>> What's an easy way to plot the C-V curve of a back-biased diode?
>>
>>Easy, is to buy the source/meter Keithley solution.  They'd love
>>to explain it all to you...
>><http://www.keithley.com/promo/lp/semiconductor>
>
>I bought an expensive Keithley source-meter. Crap. Sent it back.
>
>But I meant in LT Spice, which is why I titled the post "LT Spice..."
>
>I can read the C-V curve off the data sheet. What I want to do is make sure (or
>force) my Spice sim to behave like the actual diode, so I want to do a
>simulation of the diode c-v curve, to make sure I have everything right.

I mistakenly also thought you were trying to devise an instrument.

There's a better way in a Spice simulator, using a gimmick I devised
to measure input capacitance in amplifiers... extracts both real and
imaginary parts.

If I have time today, I'll post it.

...Jim Thompson
--
| James E.Thompson                                 |    mens     |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

I love to cook with wine.     Sometimes I even put it in the food.
```
```On Sun, 29 Jun 2014 07:29:40 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

>On Sun, 29 Jun 2014 01:15:08 -0700 (PDT), whit3rd <whit3rd@gmail.com> wrote:
>
>>On Friday, June 27, 2014 4:20:18 PM UTC-7, John Larkin wrote:
>>> What's an easy way to plot the C-V curve of a back-biased diode?
>>
>>Easy, is to buy the source/meter Keithley solution.  They'd love
>>to explain it all to you...
>><http://www.keithley.com/promo/lp/semiconductor>
>
>I bought an expensive Keithley source-meter. Crap. Sent it back.
>
>But I meant in LT Spice, which is why I titled the post "LT Spice..."
>
>I can read the C-V curve off the data sheet. What I want to do is make sure (or
>force) my Spice sim to behave like the actual diode, so I want to do a
>simulation of the diode c-v curve, to make sure I have everything right.

I don't know how to post-process data in LTspice, but here's how I do
it in PSpice...

An alternative is to do it like the Keithley does, superimpose a small
sinusoidal signal on the DC, and measure the co-sinusoidal current.

...Jim Thompson
--
| James E.Thompson                                 |    mens     |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

I love to cook with wine.     Sometimes I even put it in the food.
```
```On Sun, 29 Jun 2014 09:58:04 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:

>On Sun, 29 Jun 2014 07:29:40 -0700, John Larkin
><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:
>
>>On Sun, 29 Jun 2014 01:15:08 -0700 (PDT), whit3rd <whit3rd@gmail.com> wrote:
>>
>>>On Friday, June 27, 2014 4:20:18 PM UTC-7, John Larkin wrote:
>>>> What's an easy way to plot the C-V curve of a back-biased diode?
>>>
>>>Easy, is to buy the source/meter Keithley solution.  They'd love
>>>to explain it all to you...
>>><http://www.keithley.com/promo/lp/semiconductor>
>>
>>I bought an expensive Keithley source-meter. Crap. Sent it back.
>>
>>But I meant in LT Spice, which is why I titled the post "LT Spice..."
>>
>>I can read the C-V curve off the data sheet. What I want to do is make sure (or
>>force) my Spice sim to behave like the actual diode, so I want to do a
>>simulation of the diode c-v curve, to make sure I have everything right.
>
>I don't know how to post-process data in LTspice, but here's how I do
>it in PSpice...
>
>
>An alternative is to do it like the Keithley does, superimpose a small
>sinusoidal signal on the DC, and measure the co-sinusoidal current.
>
>                                        ...Jim Thompson

The voltage ramp thing that I did seems OK.

It does report the initial capacitance of a 1N914 as 4 pF, which is high, but
that's the value in the LT Spice model.

--

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com

Precision electronic instrumentation
```
```On Sun, 29 Jun 2014 11:24:05 -0700, John Larkin
<jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:

>On Sun, 29 Jun 2014 09:58:04 -0700, Jim Thompson
><To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:
>
>>On Sun, 29 Jun 2014 07:29:40 -0700, John Larkin
>><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:
>>
>>>On Sun, 29 Jun 2014 01:15:08 -0700 (PDT), whit3rd <whit3rd@gmail.com> wrote:
>>>
>>>>On Friday, June 27, 2014 4:20:18 PM UTC-7, John Larkin wrote:
>>>>> What's an easy way to plot the C-V curve of a back-biased diode?
>>>>
>>>>Easy, is to buy the source/meter Keithley solution.  They'd love
>>>>to explain it all to you...
>>>><http://www.keithley.com/promo/lp/semiconductor>
>>>
>>>I bought an expensive Keithley source-meter. Crap. Sent it back.
>>>
>>>But I meant in LT Spice, which is why I titled the post "LT Spice..."
>>>
>>>I can read the C-V curve off the data sheet. What I want to do is make sure (or
>>>force) my Spice sim to behave like the actual diode, so I want to do a
>>>simulation of the diode c-v curve, to make sure I have everything right.
>>
>>I don't know how to post-process data in LTspice, but here's how I do
>>it in PSpice...
>>
>>
>>An alternative is to do it like the Keithley does, superimpose a small
>>sinusoidal signal on the DC, and measure the co-sinusoidal current.
>>
>>                                        ...Jim Thompson
>
>The voltage ramp thing that I did seems OK.
>
>It does report the initial capacitance of a 1N914 as 4 pF, which is high, but
>that's the value in the LT Spice model.

The PSpice model has the same CJ0.

...Jim Thompson
--
| James E.Thompson                                 |    mens     |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| San Tan Valley, AZ 85142     Skype: skypeanalog  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |

I love to cook with wine.     Sometimes I even put it in the food.
```
```On Sun, 29 Jun 2014 11:48:52 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:

>On Sun, 29 Jun 2014 11:24:05 -0700, John Larkin
><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:
>
>>On Sun, 29 Jun 2014 09:58:04 -0700, Jim Thompson
>><To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:
>>
>>>On Sun, 29 Jun 2014 07:29:40 -0700, John Larkin
>>><jjlarkin@highNOTlandTHIStechnologyPART.com> wrote:
>>>
>>>>On Sun, 29 Jun 2014 01:15:08 -0700 (PDT), whit3rd <whit3rd@gmail.com> wrote:
>>>>
>>>>>On Friday, June 27, 2014 4:20:18 PM UTC-7, John Larkin wrote:
>>>>>> What's an easy way to plot the C-V curve of a back-biased diode?
>>>>>
>>>>>Easy, is to buy the source/meter Keithley solution.  They'd love
>>>>>to explain it all to you...
>>>>><http://www.keithley.com/promo/lp/semiconductor>
>>>>
>>>>I bought an expensive Keithley source-meter. Crap. Sent it back.
>>>>
>>>>But I meant in LT Spice, which is why I titled the post "LT Spice..."
>>>>
>>>>I can read the C-V curve off the data sheet. What I want to do is make sure (or
>>>>force) my Spice sim to behave like the actual diode, so I want to do a
>>>>simulation of the diode c-v curve, to make sure I have everything right.
>>>
>>>I don't know how to post-process data in LTspice, but here's how I do
>>>it in PSpice...
>>>
>>>
>>>An alternative is to do it like the Keithley does, superimpose a small
>>>sinusoidal signal on the DC, and measure the co-sinusoidal current.
>>>
>>>                                        ...Jim Thompson
>>
>>The voltage ramp thing that I did seems OK.
>>
>>It does report the initial capacitance of a 1N914 as 4 pF, which is high, but
>>that's the value in the LT Spice model.
>
>The PSpice model has the same CJ0.
>
>                                        ...Jim Thompson

Various data sheets have typs from 4 to 0.85. Not the thing you'd want to use as
a varicap.

I do need a "power varicap" for a weird thing I'm considering. I figured I'd
fudge up some standard LT library parts, series and parallel or whatever, to see
if my circuit might work. If it does, then I can try to find real diodes with
the required CV curves.

--

John Larkin                  Highland Technology Inc
www.highlandtechnology.com   jlarkin at highlandtechnology dot com

Precision electronic instrumentation
```