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Floating Input Preamplifier

IP.com Disclosure Number: IPCOM000093350D
Original Publication Date: 1967-Aug-01
Included in the Prior Art Database: 2005-Mar-06
Document File: 2 page(s) / 51K

Publishing Venue

IBM

Related People

Walton, CA: AUTHOR

Abstract

This floating preamplifier arrangement has a high input impedance and a relatively low output impedance. The preamplifier is used for driving a step-up isolating transformer.

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Floating Input Preamplifier

This floating preamplifier arrangement has a high input impedance and a relatively low output impedance. The preamplifier is used for driving a step-up isolating transformer.

A varying level direct voltage is applied via input terminals 10 and 12 across input capacitor 14. Terminal 10 is at a floating reference level. A modulator comprising field effect transistors FET's 16 and 18 converts the voltage across capacitor 14 to a square wave with excursions equal to the amplitude of the input voltage. FET's 16 and 18 are alternately operated in response to a square voltage wave applied at terminals 20 and 22 driving an isolation power supply transformer 24.

The input square-wave modulated voltage is applied through capacitor 26 to the gate electrode of FET 30 in a source-follower configuration. FET 30 has a high input impedance. Junction transistor 32 is arranged in a constant current source configuration. Transistors 24 and 36 are connected in a compound transister configuration and coupled to FET 30 as an emitter-follower sharing the constant current source including transistor 32. The amplification factor of this stage is close to unity. The output of this source-emitter-follower is applied by capacitor 38 to primary winding 39 of step-up isolation transformer 40.

The source-emitter follower circuit is energized by a floating power supply including isolating transformer 24. Suitable filtering and voltage dividing networks adapt the supp...