Miscellaneous active electrical nonlinear devices – circuits – and – Specific signal discriminating without subsequent control – By phase
Reexamination Certificate
1999-01-21
2001-02-27
Callahan, Timothy P. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific signal discriminating without subsequent control
By phase
C327S147000, C327S156000, C326S055000, C326S115000, C331S025000
Reexamination Certificate
active
06194917
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to delay-locked loop circuits. In particular, the invention relates to reducing the body effect of circuit elements in delay-locked loop circuits.
2. Description of the Related Art
In the design of delay-locked or phase-locked loops, the phase detector is an important component. The basic idea of the loop is to measure the phase difference between two clock signals, or more specifically, to measure the timing difference between two rising edges of two clock signals, and then to feed back this timing difference information to a component such as a voltage controlled delay chain. The delay chain adjusts the timing delay of one (or both) of the clock signals, thereby bringing the timing difference to zero. A timing difference of zero is also called a phase alignment of zero, which means that the clock signals transition at the same time. In such a case the output of the phase detector should indicate that no phase adjustment is necessary.
However, there are two situations in which poor phase detector design can lead to phase alignment errors. First, a poor design may cause the phase detector to feed back adjustment information even when the two clock edges are already in alignment. Second, a poor design may cause the phase detector to misdetect zero phase alignment and not feed back adjustment information, even when the two clock signals are not in alignment. These errors are called static phase alignment errors.
One way in which the design of the phase detector contributes to static phase alignment error is when the clock signals must drive different numbers of transistors, some of which have body effect and some of which do not. For example,
FIG. 1
shows an XOR phase detector circuit. Node
17
supplies power and node
19
is connected to ground. Node
18
connects the substrates of the transistors to ground. Clock signal
11
drives only one gate of transistor
13
but clock signal
12
drives two gates of transistors
14
and
15
. So even if the designer sizes the transistors
13
-
15
to get equal loading for clock signals
11
-
12
, clock signal
11
drives transistor
13
without body effect but clock signal
12
drives transistors
14
-
15
which do have body effect.
This body effect results from a voltage difference between the substrate and the source of each transistor. Transistor
13
has no body effect because its substrate is connected to node
18
and its source is connected to node
19
, both of which are ground nodes. On the other hand, transistor
14
has body effect because, although its substrate is connected to node
18
, its source is connected to the drain of transistor
13
. Similarly, transistor
15
has body effect because its source is connected to the drain of transistor
16
. Thus, it is easier for clock signal
11
to turn on its gate than for clock signal
12
to turn on its gates. The phase detector will then sense clock signal
11
differently than clock signal
12
. This difference will contribute to the system static phase alignment error.
Another contribution to the error is the design of the charge pump. The output of the charge pump drives an adjustment circuit such as a voltage controlled oscillator (VCO). For generation of a good, low jitter VCO output, a small charge pump output ripple is needed. However, for many existing charge pumps, the output node is connected to rapidly switching PMOS and NMOS transistors, which will generate noise at the output node and lead to a large ripple.
A third contribution to the error is the design of the transconductance stage.
FIG. 2
illustrates a typical transconductance circuit. The difference between currents I
1
and I
2
is proportional to the difference between the gate-to-source voltages of M
1
and M
2
. However, this assumes that the gate-source voltages of M
5
and M
7
are both V
B
. This is not correct if transistor body effect is considered, even when M
5
and M
7
are the same size and source the same current. This is because the threshold voltage of M
5
is larger than that of M
7
because the source-substrate voltage of M
5
is not zero. This means the gate-source voltages of M
5
and M
7
cannot both be equal to V
B
. The same is true for M
6
and M
8
.
Body effect occurs when the potential of the substrate of a MOSFET is different from the source potential. The body effect increases the threshold voltage of the MOSFET. The body effect contributes to nonlinearity. A way is needed to overcome the body effect in both the phase detector and the transconductance circuit in a delay-locked loop circuit.
SUMMARY OF THE INVENTION
The present invention addresses these and other problems of the prior art by providing an apparatus for and method of reducing transistor body effect when detecting and correcting a phase error.
According to one embodiment, an apparatus according to the present invention includes a circuit for reducing transistor body effect when generating phase signals resulting from input signals, and includes a phase detector circuit having a plurality of circuit elements. The phase detector circuit is configured to receive a first signal, a first complementary signal being complementary to the first signal, a second signal, and a second complementary signal being complementary to the second signal. The phase detector circuit is configured to generate a first XOR signal being an XOR of the first signal and the second signal, and to generate a second XOR signal being a complementary XOR of the first signal and the second signal. The first signal, the first complementary signal, the second signal, and the second complementary signal are each received by an equal portion of the circuit elements, reducing transistor body effect.
According to another embodiment, an apparatus according to the present invention includes a circuit for reducing transistor body effect when generating an output signal proportional to a difference between two input signals, and includes a transconductance circuit. The transconductance circuit is configured to receive a first signal and a second signal. The transconductance circuit is configured to generate a transconductance signal proportional to a difference between the first signal and the second signal. The transconductance circuit has two PMOS transistors each including a substrate and a source. The substrate and source of each are coupled together, reducing transistor body effect.
According to yet another embodiment, a method according to the present invention reduces transistor body effect when generating an output signal proportional to a difference between two input signals, and includes the steps of receiving a first signal, a first complementary signal being complementary to said first signal, a second signal, and a second complementary signal being complementary to said second signal, and providing each signal to an equal portion of a plurality of circuit elements. The method further includes the steps of generating a first phase signal based on a first XOR signal being an XOR of the first signal and the second signal, and generating a second phase signal based on a second XOR signal being a complementary XOR of the first signal and the second signal. The method further includes the step of coupling a source to a substrate of two PMOS transistors. The method finally includes the step of generating a transconductance signal proportional to a difference between the first phase signal and the second phase signal. Transistor body effect is reduced by generating the transconductance signal using the coupled transistors.
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Wang, et al. “A Volt
Callahan Timothy P.
Englund Terry L.
Limbach & Limbach LLP
National Semiconductor Corporation
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