Confusion: NFB, OLG, Unity Gain

[sreten]

Originally posted by corrieb
Hi,

In the equation HF Gain = gm/2*pi*Fq*cdom, are they using the net gm or only of the one transistor?

What is a good guide for the gm of a perdictable stable amplifier?

corrieb

Hi,

HF gain is set by the input stage gm and Cdom of the VAS.

For a typical output stage and Cdom stabilisation you can get away
with up to ~ 30dB excess gain, or feedback factor at ~ 20Khz. The
exact numbers are always imprecise, the question being usually
how much phase margin is enough ? never an exact answer ....

/sreten.

[corrieb]

Hi,

Thank you for all the replies.

I have designed a spreadsheet that calculates my values/components required for a input & vas stage that will be stable based on the recommendations throughout this threat and another one which should produce a very linear design but I am concerned with the stability.

Attached you will find the circuit, graph 1 (OLG=Red, CLG=Blue and Output=Green); (Phase=Green), graph 2 (3rd Distortion measured in volts); graph 3 (3rd Harmonic measured in dB) and a printout of the spreadsheet.

Can someone please look at the ouput of the input&vas and advise whether I can pursue this design based on my current configuration.

Thank you,
CorrieB

[maouna]

examples loopgain.asc and loopgain2.asc in ltspice are used to plot the open loop gain.
example audioamp.asc is refered as to look the gain and phase margins.

which one is better to be used?

if we have a 30db gain amplifier ,can we plot the open loop gain based on the formula on loopgain example,then go at 30db at the open loop gain graph and look at the phase margin corresponding there??