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DUAL DIFFERENTIAL AMPLIFIER

IP.com Disclosure Number: IPCOM000005409D
Original Publication Date: 1980-Aug-01
Included in the Prior Art Database: 2001-Oct-10
Document File: 1 page(s) / 53K

Publishing Venue

Motorola

Related People

Paul F. Smith: AUTHOR

Abstract

This dual differential amplifier allows an extra degree of freedom in choosing closed loop characteristics when designing differential amplifiers. The technique also allows the input and output to be biased independently and provides a convenient differential input to single-ended output conversion, useful when integrating balanced differential designs such as the balanced modulator.

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MOTOROLA Technical Disclosure Bulletin vol. 1 No. 1 August 1980

DUAL DIFFERENTIAL AMPLIFIER By Paul F. Smith

  This dual differential amplifier allows an extra degree of freedom in choosing closed loop characteristics when designing differential amplifiers. The technique also allows the input and output to be biased independently and provides a convenient differential input to single-ended output conversion, useful when integrating balanced differential designs such as the balanced modulator.

   Figure 1 is a simplified schematic of a conventional differential amplifier. The gain-bandwidth pro- duct (GBP) is gm/C. Gm Is the transconductance of the input differential pair and is a function of I. Of critical importance to the design of closed loop amplifiers is the unity gain stability frequency WC. Thus, the GBP is fixed due to the requirement that GBP = WC.

Figure 1.

   Figure 2 is a schematic diagram of the novel dual differential amplifier. As can be seen! the GBP is gml/C. For this circuit, the amplifier will remain stable if gmz/C = WC. Thus, the stability critena fixes gmz, but gml can still be freely chosen to obtain the desired GBP.

Figure2

   Besides the extra degree of freedom obtained by this circuit as described above, there are several other distinct advantages. For large gains, the conventional circuit requires a large DC gain. The propos- ed circuit can obtain a large gain while requiring only a moderate DC gain and gm ratio. Thus, large gain amplifiers are mo...