Miscellaneous active electrical nonlinear devices – circuits – and – Specific signal discriminating without subsequent control – By amplitude
Reexamination Certificate
2002-06-26
2003-09-09
Tran, Toan (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific signal discriminating without subsequent control
By amplitude
C327S077000
Reexamination Certificate
active
06617886
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to data converters, and more particularly to a buffer circuit to control the differential pair output switch of a current steering digital-to-analog converter (DAC).
2. Description of the Prior Art
Implementing a current steering digital-to-analog converter with BiCMOS technology allows use of NPN bipolar junction transistors. NPN devices are very desirable in the design of output switches that direct current to either the true or complement output. Output switches are generally configured as differential pair circuits, one for each controlled current in the DAC.
A differential pair output switch
10
with current source
12
is illustrated in FIG.
1
. The output switch
10
requires a differential voltage between the bases of the differential pair transistors
14
,
16
in order to completely switch the current to either the true or complement output of the DAC. It is necessary for this current to completely switch to only one output of the DAC at a time.
FIG. 2
is a simplified schematic diagram illustrating a well known buffer circuit
20
typically used in the art to control the output switch differential pair
14
,
16
. It can be seen that the prior art buffer circuit
20
includes a differential pair common emitter amplifier circuit with a resistive load
22
,
24
. Buffer circuit
20
functions by creating a differential output voltage from a differential input voltage.
The output voltage of the differential pair
14
,
16
as a function of input voltage is also well known and is illustrated in FIG.
3
. With reference now to
FIG. 3
, it can be seen that the emitter coupled pair
14
,
16
creates a differential output voltage of zero for a differential input voltage of zero. Further, it can be seen that the common mode output voltage
30
of the buffer
20
is constant and equal to one-half the single-sided output swing of the differential pair
14
,
16
.
Looking again at
FIG. 2
, the common emitter node
22
of the differential output switch
10
acts as a rectifier and “follows” the higher output of the differential buffer
20
. This rectifier action results in the common emitter node
22
of the output switch
10
moving by one-half the single-sided output swing of the buffer
20
. This voltage change on the common emitter node
22
during a switching event will lead to a transient error current
I
c
=Cdv/dt,
where C is the capacitance on the common emitter node
22
. This error current will ultimately result in distortion of the output signal of the digital-to-analog converter.
In view of the foregoing, there is a need for a system and method for reducing the error current at the common emitter node of the output switches associated with a current steering digital-to-analog converter (DAC).
SUMMARY OF THE INVENTION
The present invention provides a circuit and method for reducing the error current at the common emitter node of the output switches associated with a current steering digital-to-analog converter (DAC).
In one aspect of the invention, a circuit is provided that controls the differential pair output switch of a current steering DAC.
In another aspect of the invention, a circuit is provided that acts to reduce the movement of the common emitter node of the output switch during a switching event in a current steering DAC.
In still another aspect of the invention, a circuit is provided that acts to increase the common mode level as the magnitude of the differential mode level decreases for a differential pair output switch of a current steering DAC. This increase in common mode voltage of the output for a decrease in the magnitude of the differential output results in less movement at the common emitter node of the output switch differential pair during a switching event.
According to one embodiment of the present invention, a buffer circuit functions as a differential pair having a high output signal crossover. The buffer circuit includes a traditional common emitter differential pair having a resistive load, but also includes an additional NPN device and resistor. The base node of the NPN device is connected to the highest input voltage level to the buffer circuit, i.e. the positive supply if it is shared. The additional NPN device and resistor act to divert an increasing amount of current away from either differential pair load resistor as the magnitude of the differential pair input to the buffer approaches zero. As the magnitude of the differential input increases, the common mode level will decrease back to one-half the single sided output swing. The common mode output voltage therefore does not remain constant during a switching event.
REFERENCES:
patent: 3938055 (1976-02-01), Buhler
patent: 5239210 (1993-08-01), Scott
patent: 5475323 (1995-12-01), Harris et al.
patent: 5933056 (1999-08-01), Rothenberg
Bright William J.
Gharpurey Ranjit
Brady W. James
Swayze, Jr. W. Daniel
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
Tran Toan
LandOfFree
Differential pair with high output signal crossover does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Differential pair with high output signal crossover, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Differential pair with high output signal crossover will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3072826