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Capacitive Differential Signal Injection Circuit

IP.com Disclosure Number: IPCOM000101782D
Original Publication Date: 1990-Sep-01
Included in the Prior Art Database: 2005-Mar-16
Document File: 2 page(s) / 41K

Publishing Venue

IBM

Related People

Heitman, AR: AUTHOR [+5]

Abstract

A circuit, shown in the figure, can apply an AC test signal to the inputs of a differential amplifier and can define a DC bias point for the amplifier inputs. These functions can be enabled or disabled under digital control.

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Capacitive Differential Signal Injection Circuit

       A circuit, shown in the figure, can apply an AC test
signal to the inputs of a differential amplifier and can define a DC
bias point for the amplifier inputs.  These functions can be enabled
or disabled under digital control.

      When disabled, the circuit has minimal effect on the inputs of
the amplifier under test:
  Q1 is ON, pulling the common end of diodes D1 and D2 low. They are
back-biased, and effectively disconnected from the amplifier inputs.
The DC voltage at these points may be measured by a suitable
high-impedance device.
  Q2 and Q3 are ON, shunting the differential SINEWAVE (from an
external source) signal away from capacitors C1 and C3. These block
DC interaction between the transistors and the amplifier inputs.  C2
likewise charges to a steady-state value and places no load on the
inputs.

      For AC tests, the bias circuit and the injector circuit are
enabled:
  Q1 is OFF, allowing its collector to rise to a  DC  level defined
by the voltage divider, R1 and R2, to set a DC bias level on the
amplifier inputs.
  Q2  and Q3 are OFF, allowing the differential SINEWAVE to drive the
three series-connected capacitors, C1 - C3.  These form a capacitive
voltage divider for the AC signal; the voltage across C2 is applied
to the amplifier inputs as  a  differential signal.