# Measuring output impedance in LTSpice

• posted

I'm wondering what method folks here use to measure amplifier output impedance in LTSpice? One method I recall reading about is, IIRC, hanging a resistor off the output and stepping it, looking for the closed loop gain*input voltage*output voltage divider halfway point, but that seems kind of kludgey. Any other methods? Thanks.

• posted

Several ways:

(1) Transient load the output (IPULSE) and observe voltage. This will show output "impedance" and exhibit any ringing due to poor compensation.

(2) Do a .AC analysis using an AC current source. You can get the real and imaginary parts. Caution: this is "small-signal" linearized at the operating point.

I tend to do both, plus various loop-gain and forward transient examinations. ...Jim Thompson

```--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |```
• posted

Version 4 SHEET 1 880 680 WIRE 64 0 -64 0 WIRE 64 32 64 0 WIRE -64 80 -64 0 WIRE 0 80 -64 80 WIRE 64 144 64 128 WIRE 272 144 64 144 WIRE 384 144 272 144 WIRE 64 176 64 144 WIRE 384 176 384 144 WIRE -64 288 -64 80 WIRE 64 288 64 256 WIRE 384 288 384 256 FLAG -64 288 0 FLAG 64 288 0 FLAG 384 288 0 FLAG 272 144 PLOT_ME SYMBOL current 384 176 R0 WINDOW 123 24 91 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName I1 SYMATTR Value "" SYMATTR Value2 AC 1 SYMBOL npn 0 32 R0 SYMATTR InstName Q1 SYMATTR Value 2N3904 SYMBOL current 64 176 R0 WINDOW 123 0 0 Left 0 WINDOW 39 0 0 Left 0 SYMATTR InstName I2 SYMATTR Value 1m TEXT 220 40 Left 0 !.ac dec 1000 1e6 100e6 TEXT 496 160 Left 0 ;-->10^(28.33/20)\n ans =\n \n 26.091557

• posted

You don't need to measure at the half-way point. You can also drive the output with a source and measure the I/V across a resistor (any value). Doesn't LTSpice have an AC mode?

• posted

As Phil Hobbs pointed out, if you ATTACH your LTspice .asc, it can be launched directly from most readers.

When you put the .asc into the body of the message you have to copy and paste to create a file, then launch LTspice, slo-o-o-o-ow :-(

...Jim Thompson

```--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |```
• posted

Thank you, that's much clearer. Suck current, measure voltage. I kept thinking about applying a voltage and measuring current, the way one might do it in circuit analysis, but I don't think that will work properly with SPICE.

• posted

Minor problem: The vast majority of participants in NGs no longer has access to news servers that support binaries. Thus no attachments. Slow? C'mon, a bit spoiled? Remember the days when one had to wait for the arrival of the 11:00am flight and the tapes to be unloaded?

Bitrex: The way you describe it is how it's usually done, almost. Sometimes a load or current is modulated to see how things are at higher frequencies. It is also customary to check switch-mode converters for load change behavior via pulsed loads.

[...]
```--
Regards, Joerg

http://www.analogconsultants.com/```
• posted

It does work if you apply the voltage via a resistor. IOW try to "wiggle" the output.

```--
Regards, Joerg

http://www.analogconsultants.com/```
• posted

Are text attachments forbidden?

Certainly ;-)

Ha! I used to walk mailing tubes to the AA counter and the pilot would carry it to San Jose ;-)

Back when I could drive up to the curb, leave my car and walk to the counter maybe 30' away ;-)

TSA has made things so much better :-(

...Jim Thompson

```--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |```
• posted

No binaries, but ASCII attachments seem to work fine, though. For instance, here's a class-B driver for a Peltier cooler.

Cheers

Phil Hobbs

```--
Dr Philip C D Hobbs
Principal```
• posted

Yup! Thanks for discovering that, Phil!

Now, all we need to do is train the naysayers ;-) ...Jim Thompson

```--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |```
• posted

Yup. Any and all attachments. I've asked the guys at my (paid) server if they couldn't open 100k for attachments since they allow max 100k in the message body. The answer was a resounding "no way". Wouldn't make much sense anyhow, it doesn't do any good if only 1% of the others can read it.

In Europe we had a really nice system, not sure if still available: We had the schedules of all those Intercity fast trains hanging on an office wall. If something urgent needed to get to Munich we'd sprint to the station, hand it over along with some cash, goes onto train. Called the guy in Munich to be at their station at around 4:00pm. Even with air freight today that's impossible unless you charter a plane.

Nowadays many biz folks actually drive if the destination is within 300 miles. Air travel has lost the advantage over short hauls because of the security wait.

[...]
```--
Regards, Joerg

http://www.analogconsultants.com/```
• posted

Well, shazam! It sure worked. Beats me why because the folks at the news server said it wouldn't. And in the past it hadn't. Probably the Hobbs coefficient was too low on all the others :-)

```--
Regards, Joerg

http://www.analogconsultants.com/```
• posted

And here it is autogenerated to ascii:

-7V

|

---

- V1

--- -7

-
|
|

gnd

R9

,--------------------/\/\-,---,

100k | |

| |

| |

R10 | |

,-------/\/\-' |

100k || |

,-----||-|

| || |

|---+ ,-+------------,

| gn+| | / R8

| U2 /| | \ 10k

R3 | /-|--' /

| \+|-----------+

LT| \| |

| -| \

| | / R7

|---' | \ 20k

| /

| |

'------------+----------------+----------' -7V

Not entirely pretty, but the ASCII generator does what it can understand to do.

Jon

• posted

So you subscribe to a weenie server ?:-) Giganews works just ducky with Hobb's recent test post to s.e.d

Your weenie server support people probably don't even know what you are talking about :-)

I do that. My current driving range to the west is Californica from Mexico to Santa Barbara, east to Albuquerque and San Antonio / Austin / Dallas.

Maybe further if the stay is a week or longer. ...Jim Thompson

```--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |```
• posted

-7V

|

---

V2 - V1

-9 --- -7

-
|
|

gnd

R9

,--------------------/\/\-,---,

100k | |

| |

| |

R10 | |

,-------/\/\-' |

100k || |

,-----||-|

| || |

|---+ ,-+------------,

| gn+| | / R8

| U2 /| | \ 10k

R3 | /-|--' /

75 | \+|-----------+

LT| \| |

| -| \

| | / R7

|---' | \ 20k

| /

| |

'------------+----------------+----------' -7V

And it's so-o-o-o readable ;-) ...Jim Thompson

```--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |```
• posted

hanging

No, it provided the attachment. I know for sure that a year ago it didn't because we tried it. Question is, how many other here in this exquisite club of soldering iron wielders are able to read the attachment?

[...]

I only drive such distances when there is no other reasonable connection. Many destinations north of here can be a problem, where you have no real choice other than taking the Interstate.

```--
Regards, Joerg

http://www.analogconsultants.com/```
• posted

hanging

Huh? Are you in there? LTspice is FREE. Who would it be that can't read it?

Anything other than snow and rain and uber-liberals north of you ?:-) ...Jim Thompson

```--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |```
• posted

The orginal wasn't that pretty either.

Cheers

Phil Hobbs

```--
Dr Philip C D Hobbs
Principal```
• posted

hanging

seems

In the old days, about a year ago, such attachments were visible as such but no contents were transmitted. Now LTSpice has a minor problem running a simulation off of hot air. Although Mike Engelhardt could take an internship with a few politician's offices, they'll show him how that's done :-)

[...]

Once you get to about 300 miles from here it is actually very pretty country. Like this:

```--
Regards, Joerg

http://www.analogconsultants.com/```

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