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Brokaw's Circuit and SPICE simulation

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Extra Crispy

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Hey people! New guy here...

I'm trying to understand Brokaw's Circuit but I fail...

First of all you will find an attachment of the circuit below. My first problem is to simulate this circuit in SPICE. Specifically, how am I going to simulate the current densities J1 and J2?

Secondly I don't understand the theory behind it. If someone could explain to me in order what it does, or point me to site that does explain, I'd appreciate it. But my main problem is that,since Vc1 = Vc2, then the differential amplifier should always have an output of zero, correct?

The image is from a file with notes from my professor, who I can't reach right now

Thanks!
 

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saiaditya

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Hi....vc1=vc2 doesnt mean that ouput will be zero here. The amp is in a closed loop and if the closed loop gain is high enough, the amp output will be regulated in such a way that vc1 follows vc2 (the offset between vc1 and vc2 will depend on the finite closed loop gain and the random mismatches inside the amp)
and regarding J1, J2..if you ratio your BJT's, that should be enough..does that answer your question?
 

Extra Crispy

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Hi....vc1=vc2 doesnt mean that ouput will be zero here. The amp is in a closed loop and if the closed loop gain is high enough, the amp output will be regulated in such a way that vc1 follows vc2 (the offset between vc1 and vc2 will depend on the finite closed loop gain and the random mismatches inside the amp)
and regarding J1, J2..if you ratio your BJT's, that should be enough..does that answer your question?

Short of... How am I going to ratio the BJTs in SPICE? The model I am using is Qbreakn. The only model parameter related is, I think, the saturation current IS. Is this the parameter to be changed? And if so, what is a default value? I could for example have a saturation current of 8E-5 and 1E-5. Is that good?
 

saiaditya

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I don't think you need to change the Is values...you can use 1 unit (bjt) for Q2 and have 8 (as shown in the figure or any desired ratio) units in parallel for Q1..that should serve the purpose.
 

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