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Transconductance Output Driver

IP.com Disclosure Number: IPCOM000121461D
Original Publication Date: 1991-Sep-01
Included in the Prior Art Database: 2005-Apr-03
Document File: 3 page(s) / 90K

Publishing Venue

IBM

Related People

Buckley III, F: AUTHOR [+2]

Abstract

This circuitry provides a gated transconductance amplifier useful in sample and track hold functions. When gated on, its output current (at I in Fig. 1) is linearly related to its differential input voltage V. When gated off, the output goes to a zero current high impedance state unrelated to input voltage V. Capacitively coupled current impulses to the output are minimized by cancellation.

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This is the abbreviated version, containing approximately 52% of the total text.

Transconductance Output Driver

      This circuitry provides a gated transconductance
amplifier useful in sample and track hold functions.  When gated on,
its output current (at I in Fig. 1) is linearly related to its
differential input voltage V.  When gated off, the output goes to a
zero current high impedance state unrelated to input voltage V.
Capacitively coupled current impulses to the output are minimized by
cancellation.

      The following summarizes the primary requirements of the gated
transconductance amplifier.
      A binary gate voltage G changes the state of the amplifier from
on to off.  The switching function of G must be fast and must not
introduce error. The amplifier must be accurate.  The total error
introduced by offset, drift, droop and transient response must be
small (on the order of millivolts.)

      Fig. 1 represents a common, ungated transconductance amplifier.
An analog input voltage V controls the analog output current at I.
Resistors E and M control the gain. Static current sources B and E
provide the bidirectional output current and must be well matched to
avoid amplifier offset error.  P and N are +5 volts and -5 volts,
with respect to ground 0.  In normal use, the output node is
constrained by external feedback to a voltage near zero volts.  When
the large signal input voltage is 0 volts, the output current is 0
milliamps.  Fig. 2 shows a schematic of the new circuit.  The
transconductance amplifier has an analog input V, the binary gate
input G to switch the amplifier into its high impedance "off" state,
and an analog output current at I.  The devices in Fig. 1 are present
in Fig. 2 and serve the same function (i.e., A, B, D, L, E, M,
I, and J).  There are two additional functions present in Fig. 2 that
relate to the circuitry.
1.   The new amplifier has a means for switching the Ic of I (the
collector current of transistor I) into nodes A and B, thereby
reverse biasing diodes T and U and effectively setting output current
at I to zero.
2...