[sdiy] Spice simulation of diff amp

[Rutger Vlek]

Hi Achim,

Thanks for the shortcuts on simulating Vbe matching in Spice. I had a really big "duh" moment upon reading your message, especially when I remembered how lack of Vbe matching is commonly compensated with a trimmer between + and - inputs and ground of OTA based circuits. Thanks, I'm one step closer with my diff amp circuit now!

@Andrew: wow modeling that sort of thing makes my artificial intelligence trained heart beat a little faster, but it's overkill for the thing I'm trying to achieve now. Sounds really awesome for future digital emulations of analog though!

Best,

Rutger

On 10 feb 2014, at 20:05, ASSI wrote:

> On Thursday 06 February 2014, 19:36:30, Rutger Vlek wrote:
>> I'm trying to simulate a differential amplifier circuit in Spice and hope
>> to be able to run a Monte Carlo test that allows me to see how various
>> circuit properties respond to component tolerances. I have an
>> differential pair of transistors that I figured out how to simulate
>> current gain variations (within a certain matching accuracy) for, but I
>> don't know how to do this for Vbe matching. Does the Spice BJT model have
>> a parameter that corresponds directly to Vbe, such that I can use this to
>> simulate matching of Vbe within a certain range? Or is this a case of:
>> move to reality instead of the virtual domain? :).
> 
> In a nutshell, there isn't a single variable in a BJT model that directly 
> changes Vbe.  But, depending on what you want to simulate you can fake it 
> with low to moderate effort: if the bias point is almost constant, just add 
> a voltage source that models your mismatch to one of the transistor bases.  
> A slightly different approach is to put one of the transistors at a 
> different temperature (delta Vbe ~ 2mV/K).  This works well for small 
> differences and is better for large changes in biasing, but of course it 
> changes quite a bit more than just the Vbe.
> 
> 
> Regards,
> Achim.
> -- 
> +<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk Blofeld]>+
> 
> Factory and User Sound Singles for Waldorf Q+, Q and microQ:
> http://Synth.Stromeko.net/Downloads.html#WaldorfSounds
> 
> _______________________________________________
> Synth-diy mailing list
> Synth-diy at dropmix.xs4all.nl
> http://dropmix.xs4all.nl/mailman/listinfo/synth-diy