Browse Prior Art Database

Differential Active Load Circuit

IP.com Disclosure Number: IPCOM000102426D
Original Publication Date: 1990-Nov-01
Included in the Prior Art Database: 2005-Mar-17
Document File: 2 page(s) / 58K

Publishing Venue

IBM

Related People

Bonaccio, AR: AUTHOR [+2]

Abstract

An active load device is used in a differential amplifier circuit to achieve high gain with good common mode rejection. A PNP bipolar implementation is described. Conceptual behavior of the circuit is the same when constructed with P-channel field-effect transistors (PFETs).

This text was extracted from an ASCII text file.
This is the abbreviated version, containing approximately 61% of the total text.

Differential Active Load Circuit

       An active load device is used in a differential amplifier
circuit to achieve high gain with good common mode rejection.  A PNP
bipolar implementation is described. Conceptual behavior of the
circuit is the same when constructed with P-channel field-effect
transistors (PFETs).

      The circuit, which is shown in the figure, operates by
switching current at input control terminals Vin+ and Vin- between
input devices T1 and T2, depending on the differential input voltage.
For example, when Vin+ is significantly positive relative to Vin-,
T1, T3, T4, and T5 conduct while T2 and T6 are off.  The converse is
true when Vin- is significantly positive relative to Vin+.  Both
Vout- and Vout+, which are connected to collectors of T1 and T2,
therefore swing between VH and VH - 2Vbe.  VH is the high voltage
supply and Vbe is the base to emitter turn-on voltage of the PNP
devices.  Thus, a truly symmetric differential output is provided.

      Current source Ib is used to properly bias the amplifier above
the low voltage supply level VL.

      When the differential input voltage is zero and the circuit is
at its balance point, a small differential input signal does not
cause significant change in the sum of currents in T5 and T6.
Current simply changes symmetrically as the input voltage alternates.
Current levels in T3 and T4 remain constant and impedance into these
device collectors is very high.  The net impedance is th...