Full-wave rectifier

Details Full-wave rectifier Full-wave rectifier

2001-11-01

2002-11-12

Nguyen, Matthew (Department: 2838)

Electric power conversion systems

Current conversion

Using semiconductor-type converter

Reexamination Certificate

active

06480405

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to full-wave rectifier circuits, and, more particularly, to a full-wave rectifier having a unity magnitude slope close to the origin.
BACKGROUND OF THE INVENTION
Rectifiers are the fundamental building blocks in DC power supplies of all types and in DC power transmission used by some electric utilities. Specifically, full-wave rectifiers are often used in analog circuits for power detection of a received or transmitted signal. A single-phase full-wave rectifier circuit, shown in
FIG. 1
a
, with the accompanying input and output voltage waveforms (
FIGS. 1
b
and
1
c
, respectively) includes a center tapped transformer T
1
coupled to a pair of diodes D
1
and D
2
, wherein each diode conducts on opposite half-cycles of the input voltage.
As shown in
FIG. 1
c
, while diode D
1
conducts the first half-cycle of the input signal shown in
FIG. 1
b
, diode D
2
is off. During the second half-cycle, diode D
2
conducts while diode D
1
is off. The circuit changes a sinusoidal waveform with no dc component (zero average value) to one with a dc component of 2V
peak
/&pgr;, where the root mean square (rms) value of the output is 0.707V
peak
. This implementation is not preferred in an integrated circuit (IC) form since it is difficult to implement transformers in an IC. Further the use of diodes as shown has an electrical problem since the stage that drives the diodes can get severely loaded by the diodes and may need to provide high amounts of current.
Another implementation of the single-phase full-wave rectifier circuit, shown in
FIG. 2
a
, may include a differential amplifier pair of transistors in lieu of the diode pair. Differential signals V
B
+V
i
and V
B
−V
i
are applied at the base of the two transistors Q
1
and Q
2
, where V
B
is the bias voltage and V
i
is the input voltage. The full-wave rectified voltage signal V
o
is observed at the common emitter nodes of the two devices Q
1
and Q
2
. An approximate transfer characteristic is shown in
FIG. 2
b
. For bipolar devices that follow an exponential I
c
vs. V
gs
relationship, the output voltage V
o
is represented by:
V
o
&agr;In(sech(&thgr;
i
/2V
T
)
where V
T
is the thermal voltage which is equivalent to the Boltzmann constant, k, multiplied by the temperature, T, divided by the charge, q (kT/q).
The current approach suffers from reduced accuracy for small amplitudes of the signal. Specifically, this circuit has a dead zone close to its zero crossing. An ideal transfer function of the full-wave rectifier circuit is shown in
FIG. 4
a
. A practical realizable transfer function of the circuit of
FIG. 2
a
is shown in
FIG. 4
b
. The dead zone near the zero crossing leads to the appearance of an error voltage e
i
, in response to a sinusoidal input as shown in FIG.
3
. The effect of the dead zone is that the DC voltage output, for small amplitude inputs is much smaller compared to the ideal case. Mathematically, the unity magnitude slope for the implementation of
FIG. 2
a
is approached only when &thgr;
i
>>V
T
, in which case sech(&thgr;
i
/2V
T
)&agr; exp(−|&thgr;
i
|/2V
T
). Thus, V
o
&agr;−|&thgr;
i
|, which has a slope of unity magnitude.
The non-unity slope near the zero-crossing causes problems in the rectification of very small signals, where &thgr;
i
<2V
T
, as shown in
FIGS. 4
a
,
4
b
and
4
c
. The output voltage of the rectifier is very much smaller than the ideal case.
One approach to solve this problem is to use amplification before the rectifier, but this requires increased power dissipation and reduces the upper limit of the dynamic range. The dynamic range is reduced by a factor of the reciprocal of the amplification. Further, the pre-amplifier needs to be linear over the range of input signals applied.
For example, where the amplification is 10 and the signals to be rectified have peak to peak excursions of 0.3 volt, the full-wave rectifier circuit would require 3 volts to operate. This presently is difficult in an IC implementation. Thus, there is a dynamic range tradeoff in which it is possible to rectify a signal from a smaller voltage input but it is not possible for larger voltages.
Thus, a need exists for an accurate full-wave rectification circuit having a unity magnitude slope close to the origin.
SUMMARY OF THE INVENTION
To address the above-discussed deficiencies of the biasing circuitry for single-ended circuits, the present invention teaches a full-wave rectifier having a unity magnitude slope close to the origin. In particular, a full-wave rectifier in accordance with the present invention includes an emitter coupled pair circuit coupled to a bias circuit. At least one constant current source couples to the base of each transistor in the emitter coupled pair circuit. A pair of transistors cross-couple across the emitter coupled pair circuit. These cross-coupled devices are used as positive feedback to increase gain for small amplitude signals and to degenerate the devices of the full-wave rectifier. Using this design very precise rectification can be achieved even for &thgr;
i
<V
T
.
Specifically, the bias circuit includes a current source which supplies &agr; multiplied by the current supplied by the current source connected to the base of the transistors in the emitter coupled pair circuit. By choosing an appropriate value of &agr;, a unity magnitude slope close to the origin is achieved.


REFERENCES:
patent: 4053796 (1977-10-01), Van De Plassche
patent: 4158882 (1979-06-01), Citta
patent: 4228429 (1980-10-01), Tsuchiya et al.
patent: 4605901 (1986-08-01), Kobori et al.
patent: 4708146 (1987-11-01), Lane

Affiliated with

Also associated with

Rating

Full-wave rectifier does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Full-wave rectifier, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Full-wave rectifier will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2946195