LT1012 vs LT1001 input resistance

Question

LT1001AC has Rin (Input Resistance Differential Mode) typical of 100Meg on its datasheet. 
 I'd like to compare this parameter to LT1012AI as it doesn't specify this on the datasheet. Any typical value?

ADIApproved · Accepted Answer

Hi gpablo: yes and no. The bias current is usually cancelled so is NOT the true base current. In a bipolar diff pair, the differential input resistance is 2*beta*re. re is 26mV/Ie. Unfortunately LT1012 data sheet does not give the running current, but LT1097 does: 15uA tail. That gives 7.5uA each transistor, so re=3.47k. Beta we do not know... but I would guess it's about 3000 (superbeta). So 2*beta*re gives about 20Meg Ohms for LT1012. 
 For the LT1001, the d.s. does not give a number. The opamp runs 1.5mA total, and i would guess 1/4 of that is allocated to the diff pair. That's about 380uA so re=.026/380u = 68 Ohms. Beta is about 200 so 2*beta*re = 27k Ohms for LT1001.)

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LT1012 vs LT1001 input resistance