I'd like to do some feedback analysis of op-amps using Mathematica, i.e. doing those bode plots of open loop gain and beta, etc. This is to check for stability and other stuff. Problem is that most spec sheets don't give the open loop output impedance. What can I do?
Typically the open-loop output impedance would be something like looking into the emitter of a Darlington with its base having 10-30pF on it.
I've never tried modeling it that exactly, but you certainly have broached an interesting question!
...Jim Thompson
--
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| E-mail Address at Website Fax:(480)460-2142 | Brass Rat |
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I love to cook with wine. Sometimes I even put it in the food.
Or often a single-transistor emitter follower. Following your suggestion, that's something like Zo = beta * Xc = beta / 2pi f C, but at the higher frequencies beta = f / f_T, so that in the end Zo looks more or less resistive. E.g., we get 40 ohms for 20pF and an output transistor f_T = 200MHz. And even tho we're being rather crude, we should still add on r_e, which can be another 20 to 100 ohms, depending on the class-AB quiescent current.
It's when we put this resistive Zo inside the feedback loop that the opamp's overall Zo looks like an inductor, as Alan will see.
--
Thanks,
- Win
Open drains or collectors would give open-loop output impedances well in excess of 1K, UNLESS there was a local loop around the output stage.
Does anyone own a network analyzer? It would be interesting data to see open-loop output impedance versus frequency for various popular OpAmps... under no load, sinking current and sourcing current conditions.
...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 think the open-loop output impedance of 4558 type amplifiers at moderate frequencies is surprisingly high. Had trouble with this years ago with low-level amplfiers in extremely noisy situations
Best regards, Spehro Pefhany
-- "it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com
Why? You are usually far better using an electronic simulation program for this sort of job. Some actually have hooks to do loop gain plots directly, i.e. no fiddling about:-)
Use a good spice model, guess, measure, or calculate from the known circuit.
Typically open loop output impedances are 1-200 ohms.
Kevin Aylward snipped-for-privacy@anasoft.co.uk
"Jim Thompson" schrieb im Newsbeitrag news: snipped-for-privacy@4ax.com...
Hello all, this has been true for the non rail to rail amplifiers. The RRIO and RRO amplifiers have the drains or collectors connected to the output. That's the reason why they have an open loop output impedance of a few hundred ohms to one kOhm.
Best Regards, Helmut
RRIO: Rail to Rail Input and Output RRO: Rail to Rail Output
My HP Agilent network analyzer starts at 300kHz. I have an old HP network analyzer that goes from analog frequencies to 13MHz, but its digital interface is rather obsolete. So now I'm looking for eBay to present me with an HP 4192A 5Hz-to-13MHz vector impedance analyzer at an affordable price sometime soon. If not, I may have to consider the $3.5k to $8.5k asking price in the surplus-instrument markets.
In the meantime, I may be able to employ my 10kHz to 10MHz HP 4275A LCR meter for some of these tasks, with an appropriate test fixture.
--
Thanks,
- Win
Are we defining "open loop" as having the compensation circuit disabled or just breaking the external feedback loop.
With compensation enabled:
The output devices are typically either driven by the last point that the compensation circuit encloses or are them selves enclosed in the loop so i'd expect:
With an op-amp like an LT1498, I'd expect the output impedance to look somewhat capacitive at lowish frequencies. At, lets say about 100Hz, the impedance will start to look almost purely resistive and somewhere in the
50 to 100 ohms range. As the frequency gets near the top of the range of the op-amp, I'd expect to see a slight swing into the inductive side of the phase curve.With the very high speed op-amps, I'd expect the output to not start looking resistive at a higher frequency. The swing into the inductive side to be either missing or very slight.
-- -- kensmith@rahul.net forging knowledge
Paul Kiciak, N2PK, has an interesting VNA design using a DDS and a novel method of measuring phase. Here is some info
"This is a homebrew VNA capable of both transmission and reflection measurements from 0.05 to 60 MHz, with about 0.035 Hz frequency resolution and over 110 dB of dynamic range. Its transmission measurement capabilities include gain/loss magnitude, phase, and group delay."
"Its reflection measurement capabilities include complex impedance & admittance, complex reflection coefficient, VSWR, and return loss."
The main interest for rf work is extending the high frequency capability, but it might be possible to go lower in frequency.
Mike Monett
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