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mgm2000ro 30th May 2006 12:50 PM

Help! Thd + N.
Please help me to determin the Total Harmonic Distortion of this amplifier

This design is drawn in Orcad 9.2.
Orcad can determin this, and how?

jcx 30th May 2006 03:37 PM

If I remember correctly the free demo version won't handle that many parts

a full licensed version of ORCAD could sim the circuit and in principle calc thd #, noise modeling is also possible but you'll need realistic transistor noise spice models

Linear Technology's LtSpice is a free Spice that I use exclusively in preference to the limited OrCad demo version, you do have to hunt down part models and add to the small lib distributed with LtSpice

but as someone who likes simulating amplifiers I haven't ever tried to calc thd # because it gives little insight into circuit performance

even the simulated intermodulation harmonic distortion spectrum analysis that I prefer are far from predicting real amplifier harmonic distortion due to the lack of realistic transistor models and the inability to easily handle realistic thermal effects in the output Qs in Spice

A Spice model of a circuit is a useful tool, when you can get paper calcs and approximations to reconcile with sim results - and understand the limitations of both then you are in a better position to debug and verify operation of the hardware prototypes

simulation should show you some problems with the bias levels in the circuit you show - although a common spice transistor model flaw is a ridiculously high hfe (and always matched between same model #) which could hide the bias problems

mgm2000ro 30th May 2006 11:01 PM

I have a full licensed version of ORCAD 9.2
How can simulated the circuit and THD & noise.

clem_o 31st May 2006 04:55 AM

Run the simulator of course... :-)

If memory serves:

Click on Transient Analysis, put in a decent analysis time (start at 0, simulate for say 20mS).

Go over to the waveform display tab, and select the node number that represents the output of your amplifier. You should see a sine-wave (since your input was a sine wave).

This is where my memory is really fuzzy, but there should be something in the waveform display that you can select to make it do a Fourier or spectrum analysis...


Limhes 31st May 2006 12:52 PM

The problem is that PSpice simulations are too ideal to 'measure' THD, because negative feedback is too ideal I guess and ground points are real 0V points with zero impedance, etc. etc.

clem_o 1st June 2006 12:19 AM

Ah, well that's another issue - which I believe is present in all simulators as they'd all (more or less) be all based on Berkley's Spice engine... Simulation will get you some figures, but in the end, always build the circuit as a prototype and 'really' test it.


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