Mat12 PN V/I characteristics Why Measure result not Match theory?

Question

In my applaction I use 4 PNP (two dobule match pair Mat12 and other DI4-0992 ) logarithm Amp Make an circute to make mass dynamic scale. 
 We measure V/I datas and check what it is, to get sum input output relationship equrion. like this 
 Change R5 10G 1G 100M ... and record Amp Output Voltage . Use Q1 as Mat12 and Other kind DI4-0992 .After I polyfit these data I/V I get this 
 I have two question 1) in theroy PN heat voltaty Vt=k*T/q so different NPN Vt should just with Temp but I get one 26mV one 30mV . 
 2) Is Mat12 or same NPN pair Datasheet have Is (反向漏电流) and Vt（热电压） parameter?

ADIApproved · Accepted Answer

Hi, 
 1) Theoretically that is correct, value of Vt is dependent of temp. Different NPN is expected to have different Vbe and Is instead. 
 Do you have the datasheet for DI4-0992? 
 2) For Mat12, Vt is presented in page9 of the datasheet. When computed at T=25degC, Vt = 25.696134mV or approximately 26mV. 
 Also, Is can be computed, for example, look at Fig. 7 for value of Vbe at a certain Ic, Vbe = 0.39 at Ic = 1uA and given already is Vt = 26mV. 
 Is = Ic/[e^(Vbe/Vt)-1] = 1e-6/[e^(0.39/26e-3)-1] = 3.06e-13)

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Mat12 PN V/I characteristics Why Measure result not Match theory?