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Electrical Amplifier With Input Bias Network

IP.com Disclosure Number: IPCOM000075291D
Original Publication Date: 1971-Aug-01
Included in the Prior Art Database: 2005-Feb-24
Document File: 3 page(s) / 35K

Publishing Venue

IBM

Related People

Hellwarth, GA: AUTHOR

Abstract

A direct-coupled amplifier provides a high-input impedance by using a biasing network for a full differential-cascode stage, along with a circuit arrangement for driving both the bias network and the reference potential for the common base of the second transistor of the cascode pairs.

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Electrical Amplifier With Input Bias Network

A direct-coupled amplifier provides a high-input impedance by using a biasing network for a full differential-cascode stage, along with a circuit arrangement for driving both the bias network and the reference potential for the common base of the second transistor of the cascode pairs.

Transistors T1 and T2 are emitter coupled to form the basic differential- cascode amplifier stage with T1 - T3 and T2 - T4 connected as cascode- connected pairs of bipolar transistors. The input is primarily responsive to the differential input voltage E1 - E2 or current I1 - I2, and the impedance from either input lead to the common potential of the two power supplies E8-E9 is quite high. The collector currents Ic1 and Ic2 of differential input transistors T1 and T2 are conducted through the corresponding cascode devices T3 and T4, with relatively small loss, to the two load circuits represented for the single-stage by resistors R1 and R2. The output signal of the stage may be taken single-ended as either of the two collector currents Ic3 and Ic4, either of the collector voltages E3 and E4, or differentially as the difference between the voltages or currents. A current source I30 provides emitter current bias to the differential amplifier stage.

A floating or boot-strapped supply would be required to drive the adjustable bias potentiometers R3 and R4 and to reference the common-base potential of T3 and T4, without depending on the amplifier output fed back to one of the amplifier inputs. Instead, an auxiliary amplifier with a high-input impedance and unity-voltage gain is created from emitter-follower transistor T5 to provide a voltage E6, which follows the common-emitter potential E5 of T1 and T2. The emitter follower T5 is biased by a second constant-current source 120, and could be a more complex amplifier than a single transistor stage.

A bias voltage E7 - E6 is created between the constant-current source 120 and the emitter of T5 by insertion of a voltage reference device such as diode- connected transistor T6, a zener diode, not shown, or resistors such as potentiometers R3...