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Active Load for Differential Amplifiers

IP.com Disclosure Number: IPCOM000080901D
Original Publication Date: 1974-Mar-01
Included in the Prior Art Database: 2005-Feb-27
Document File: 2 page(s) / 38K

Publishing Venue

IBM

Related People

Jaeger, RC: AUTHOR

Abstract

Frequency peaking and phase shifting in differential cascode amplifiers is minimized by use of an active load. Semiconductor devices 11. 12, 13 and 14 are arranged as a differential cascode amplifier 10, which provides high-performance amplification of inputs 15 and 16 as a differential output at 17 and 18. In the past, amplifier 10 is typically coupled to a positive source 20 by a pair of resistance elements connected to terminals 21 and 22. The frequency response of the common-mode gain Ac exhibits severe peaking and excessive phase shift, caused by a high-Q common-mode complex pole pair for such configurations. The active loads of Fig. 1 and 2 provide a replacement for the simple load resistors of the prior art. Note that Fig. 2 can be used as a direct replacement of the active load shown in Fig. 1 for amplifier 10.

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Active Load for Differential Amplifiers

Frequency peaking and phase shifting in differential cascode amplifiers is minimized by use of an active load. Semiconductor devices 11. 12, 13 and 14 are arranged as a differential cascode amplifier 10, which provides high- performance amplification of inputs 15 and 16 as a differential output at 17 and
18. In the past, amplifier 10 is typically coupled to a positive source 20 by a pair of resistance elements connected to terminals 21 and 22. The frequency response of the common-mode gain Ac exhibits severe peaking and excessive phase shift, caused by a high-Q common-mode complex pole pair for such configurations. The active loads of Fig. 1 and 2 provide a replacement for the simple load resistors of the prior art. Note that Fig. 2 can be used as a direct replacement of the active load shown in Fig. 1 for amplifier 10.

The active loads of Figs. 1 and 2 present approximately the same differential mode load resistance to the differential cascode amplifier as the load resistors of the prior art. Thus, the differential mode gain is equivalent. However, the common-mode output resistance of the active loads is low. The common-mode output resistance of the active load of Fig. 1 is approximately 2/gm, where gm is the transconductance of transistors 35 and 37. For large values of differential load resistances 31 and 32, the output resistance of transistors 35 and 37 may limit the differential mode gain.

Emitter degereration may be used to eliminate this problem, as shown in Fig.
2. The common-mode output resistance remains low and is approximately equal to 1/RE,...