Browse Prior Art Database

Magnetic Head Preamplifier

IP.com Disclosure Number: IPCOM000080096D
Original Publication Date: 1973-Oct-01
Included in the Prior Art Database: 2005-Feb-26
Document File: 2 page(s) / 31K

Publishing Venue

IBM

Related People

Commander, RD: AUTHOR

Abstract

Magnetic head preamplifiers used in current disk files are required to have a flat response up to about 10 MHz, with a well-defined input impedance equal to the read damping required by the head. Thus while a conventional cascode amplifier can provide the necessary gain, tolerances in current gain of present integrated circuit transistors do not permit the differential input impedance to be held within the required limits. The problem can be solved by the addition of emitter-followers but these introduce a voltage offset which is undesirable in an integrated circuit amplifier, since offset usually requires a DC blocking capacitor to eliminate its effect.

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Magnetic Head Preamplifier

Magnetic head preamplifiers used in current disk files are required to have a flat response up to about 10 MHz, with a well-defined input impedance equal to the read damping required by the head. Thus while a conventional cascode amplifier can provide the necessary gain, tolerances in current gain of present integrated circuit transistors do not permit the differential input impedance to be held within the required limits. The problem can be solved by the addition of emitter-followers but these introduce a voltage offset which is undesirable in an integrated circuit amplifier, since offset usually requires a DC blocking capacitor to eliminate its effect.

The circuit shown takes advantage of the fact that transistors on an integrated circuit chip have their current gains matched to about =/-20%. For small signals T1 and T2 see antiphase emitter currents to T3 and T4. Hence T2 base current is equal and opposite to T4 base current and similarly for T3 and T1. Thus the majority of T2 base current signal flows not from the input but via diode T6 from T4 base, and similarly via diode 7 for T3 from base T1. Hence, the only signal current flowing through the input terminals A, B is that due to the possible 20% current gain mismatch, i.e., the current could be at maximum 20% of what it might be without using this cross-coupling technique. This signal current may be in phase or out-of-phase with the input voltage; hence, the differential impedanc...