How much CM range does a differential need?

[Kevinbd]

Hello,

Just wondering if you can help me with something. How much common mode range (voltage) does a differential need in a power amplifer ?

In other words, what is the minimum voltage I can feed the input stage with (say a Doug. Self type amp) ? Headroom I guess?

Thanks for any help

Kevin

[darkfenriz]

input sensivity/feedback factor
for full power, as far as I understand what you mean.

[Kevinbd]

What is the minimum headroom the LTP (differential needs) on it's emitters.

Can I feed a CCS at 12v with a battery? giving around 10v headroom for the LTP.

kev

[amplifierguru]

Hi Kevinbd,


I can answer this. Input sensitivity, as Darkfenriz has stated, is the voltage modulation on the input stage and is typically 1-2 Vrms for full output so +/- 3V of modulation about zero. Now for your tail supplies, if you want some introduced even harmonic distortion, you can use tail resistors from as low as 6.1V zener supplies for simplicity and best decoupling from the main supply (LOW Z zener). And you could expect around 0.2% THD.

However, if you don't want distortion due to current or voltage modulation and wish to use current sources, 12V should be plenty to ensure clean operation.


Cheers,
Greg

[Kevinbd]

Helllo amplifierguru,

Thanks for your reply.

Your answer leads me on to another question. If the LTP has very high input sensitivity (say a Zin of 150K+, maybe a couple of uA of base current) suitable for some low level sources, can the headroom be reduced ?

[amplifierguru]

Hi Kevinbd,

You can reduce the input stage supply rails to the point where -

Vs = Vin for full output (pk) + Vccs min for operation. This maybe 2-5V depending... but getting a bit compromising.

Cheers,
Greg