High bandwidth, low power, single stage cascode...

Details High bandwidth, low power, single stage cascode... High bandwidth, low power, single stage cascode...

2003-01-14

2004-02-10

Nguyen, Khanh Van (Department: 2817)

Amplifiers

With semiconductor amplifying device

Including atomic particle or radiant energy impinging on a...

C330S311000

Reexamination Certificate

active

06690239

ABSTRACT:

FIELD
The present invention relates to electronic analog circuits, and more particularly, to transimpedance amplifiers.
BACKGROUND
With increasing data rates in electronic systems, it is expected that optical interconnects (optical fibers) may in the near future replace wire interconnects at the board-to-board and chip-to-chip level. For example, a computer system such as that illustrated in
FIG. 1
may comprise one or more boards
102
and memory hierarchy
104
that exchange data packets over optical interconnects
106
. These packets may be routed via switch
108
, or perhaps the various integrated circuits may be directly connected to one another. Each board
102
may comprise one or more microprocessors.
In many applications, a photo-detector provides an electrical signal indicative of a received optical signal. A simplified small-signal model for a photo-detector is a small-signal current source, where the small-signal current is representative of the received optical signal. Transimpedance amplifiers provide a small-signal output voltage signal in response to a small-signal input current signal. Many transimpedance amplifiers used in optoelectronic telecommunication applications employ the two popular designs shown in
FIGS. 2
a
and
2
b.
The photo-detector in
FIGS. 2
a
and
2
b
is modeled as small-signal current source
202
and small-signal parasitic capacitor
204
. The transimpedance amplifier of
FIG. 2
a
is a two stage, common-source, common-drain amplifier. The first stage comprises common-source nMOSFET (n-Metal-Oxide-Semiconductor-Field-Effect-Transistor)
206
and load pMOSFET
208
. The second stage comprises common-drain nMOSFET
210
and load pMOSFET
212
. Resistor
214
provides negative feedback. The transimpedance amplifier of
FIG. 2
b
is a single stage, common-gate amplifier, where the single stage comprises common-gate nMOSFET
218
, with nMOSFET
216
and pMOSFET
220
providing bias current and active loads to common-gate nMOSFET
218
.
In telecommunication applications, the received optical signals are typically very small due to attenuation in optical fibers, which may be hundreds of kilometers long. Consequently, a primary goal for transimpedance amplifiers for long haul communications is to provide high transimpedance with low noise amplification, while attaining as large a bandwidth as practical.
However, at the board-to-board and chip-to-chip level, such as the computer system of
FIG. 1
, attenuation is relatively negligible, and the received signals are typically orders of magnitude larger than for the case of long haul optical communication systems. In such short haul optical applications, transimpedance amplifiers may be integrated with other circuits on an integrated circuit die, such as input-output chips on boards
102
or switch
108
, and perhaps on a microprocessor itself. Accordingly, for such applications, designing for large bandwidth while minimizing power plays a critical role in the design of transimpedance amplifiers. Under these criteria, the amplifiers in
FIGS. 2
a
and
2
b
may contain several drawbacks.
One drawback is that each stage in the amplifier of
FIG. 2
a
requires a relatively large DC bias current. Furthermore, the two load transistors
208
and
212
are each biased at bias voltages V
B1
and V
B2
, respectively. To avoid bias voltage coupling, two bias circuits may be needed to bias the load transistors. Consequently, power consumption for the circuit of
FIG. 2
a
may be too high. Another drawback is that common-drain nMOSFET
210
may cause a 30% to 50% drop in transimpedance from the first stage output to the second stage output.
The amplifier of
FIG. 2
b
contains only one stage, but transistors
216
,
218
, and
220
require three bias voltages V
B1
, V
B2
, and V
B3
, respectively. Again, to avoid bias voltage coupling, three separate bias circuits may be needed for the amplifier of
FIG. 2
b
, thus contributing to power consumption. Furthermore, a problem shared by amplifiers requiring one or more constant bias voltages is that generating a constant bias voltage with good power supply noise rejection is considered a difficult problem.
Consequently, transimpedance amplifiers that are commonly used for long haul communications may not be suitable for short haul optical communications, such as computer systems, where power consumption may be an issue.


REFERENCES:
patent: 6037841 (2000-03-01), Tanji et al.
patent: 6509799 (2003-01-01), Franca-Neto
patent: 6556085 (2003-04-01), Kwon et al.

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