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Operational Amplifier Using Multiplied Miller Capacitance for Compensation

IP.com Disclosure Number: IPCOM000121817D
Original Publication Date: 1991-Sep-01
Included in the Prior Art Database: 2005-Apr-03
Document File: 1 page(s) / 42K

Publishing Venue

IBM

Related People

Braden, JJ: AUTHOR [+5]

Abstract

A method of decreasing the bandwidth of an operational amplifier (OPAMP) is disclosed. The bandwidth is decreased using a Miller multiplication factor on the roll-off capacitor. The OPAMP also incorporates the use of a single power supply and functions with input signals near ground.

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Operational Amplifier Using Multiplied Miller Capacitance for Compensation

      A method of decreasing the bandwidth of an operational
amplifier (OPAMP) is disclosed.  The bandwidth is decreased using a
Miller multiplication factor on the roll-off capacitor.  The OPAMP
also incorporates the use of a single power supply and functions with
input signals near ground.

      The figure is the schematic of this amplifier.  The POS and NEG
inputs are emitter-followed with PNPs Q1 and Q2.  Q1 and Q2 are
biased with P-channel enhancement mode MOSFETs T1 and T2,
respectively.  The emitters of Q1 and Q2 drive the bases of the
differential pair Q3 and Q4.  Q3 and Q4 are biased up with T0, an
N-channel enhancement mode MOSFET. The emitter-followers Q1 and Q2
allow the POS and NEG inputs to operate near ground.

      The low bandwidth characteristic of this OPAMP comes from
capacitor C0 being connected from the OUT pin to the base of Q2.  By
connecting the low side of capacitor C0 to the base of Q2, its
capacitance will be multiplied by the Miller effect.  This
multiplication is approximately the beta of Q2 times the beta of Q4.

      Also note that it is important that the values of the resistors
R1 and R2 be large (approx. 10K).  Making R4 large allows any AC
current in C0 to be driven into the base of Q2 for Miller
multiplication.  Otherwise, if R4 is small, the AC current in C0
would also couple to the NEG pin.

      Disclosed anonymously.