Read-head-preamplifier

  
Choosing de rather than ac coupling can reduce much of the noise associated with preamplifiers for a magnetic reading head, particularly in the low frequencies. The LM1897 eliminates the need for the capacitor that usually ac couples the read head to the preamplifier input. The read head itself has a small resistance, typically 50 0, and so is less prone to noise pickup. Moreover, theLM1897 has a low-bias current; merely 2 p.A as a worst case.
Read-head-preamplifier - schematic

Such a low-bias current flowing through the head"s low resistance generates very little noise. Accordingly, even with a gain of 25, the first stage of the preamplifier circuit produces little noise.




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