Opto-electronic transconductors

Details Opto-electronic transconductors Opto-electronic transconductors

1995-08-24

1997-10-28

Mottola, Steven

Amplifiers

With semiconductor amplifying device

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

330303, H03F 308, H03F 3191

Patent

active

056821220

DESCRIPTION:

BRIEF SUMMARY
The present invention relates to transconductance devices employing optical elements, particularly, although not exclusively, for use in high quality, high frequency active filter circuits.
Classical high frequency RLC passive filters can be synthesised by integrated transconductance-C approaches. However, in doing so the filter characteristics are now totally determined by those of the Operational Transconductance Amplifiers (OTAs) used, both in terms of frequency performance and filter quality factor. However, the low sensitivity properties of the passive equivalent are still preserved.
FET technology has been the most popular for realising these integrated filters because of its natural transconductor features, although bipolar technology has been used successfully for 200 MHz integrated filters (1). These filters are usually realised by integrator loops comprising transconductors driving a grounded load capacitance. Integrator time constants are determined by C/G.sub.m where G.sub.m is the transconductance gain of the OTA, and can be tuned via G.sub.m tuning. The filter Quality factor Q is determined by the unloaded DC gain of the OTA, G.sub.m /G.sub.o, where G.sub.o is transconductor output conductance. Assuming very high frequency poles for the transconductor then for ideally infinite unloaded filter Q the integrators must have a phase of ninety degrees, requiring infinite DC gain, hence zero G.sub.o. For high frequency filtering the OTAs must be kept as simple as possible.
Unfortunately, for high frequency filter design employing FET technology the output conductance of short channel MOSFETS and GaAs MESFETs is undesirable high (2) even at relatively low frequency and so high Q filters are very difficult to realise. The conflict usually arises between suitable DC gain magnitude and circuit complexity leading to high frequency phase shift, both of which effect filter Q (2). Cascoding (3) is a well-known technique to improve the output resistance of the OTA, but due to low intrinsic voltage gain (g.sub.m /g.sub.o) of the short channel MOSFET (4) and the GaAs MESFET, single cascode (3) techniques can only achieve OTA voltage gain figures of the order of a few hundred. Double cascode (5) and regulated cascode (4,6,7) techniques can achieve 60 dB gain figures but at the expense of increased circuit complexity. Furthermore, the lack of P-type devices in GaAs MESFET technology further complicates the design, in particular the difficulty in realising a good negative current-source (3,8) which is essential to most transconductor topologies.
The emergence of fibre-optic communications has led to the development of high performance laser diodes and p-i-n photodiodes at very high speed. P-i-n photodiodes convert a light signal into an electrical current which acts as a current source in parallel with very high output resistance and capacitance. The quantum efficiency of p-i-n photodiodes depends on the material and wavelength, fortunately the quantum efficiency of Si and GaAs can be as high as 80% or more (9). Laser diodes convert electrical current into laser light, the ac efficiency is quite high (9) (10-50%) and input resistance is very low.
Accordingly, a first aspect of the present invention provides a transconductance device comprising an input stage including a field effect transistor arranged to drive a laser diode, and an output stage comprising a first photodiode optically coupled to the laser diode, the said photodiode being provided with a high impedance bias current supply circuit. Such a device is referred to below as "a transconductance device of the type herein described".
Preferably, the said high impedance bias current supply circuit comprises a second photodiode connected in series with the said first photodiode and optically coupled to a further light source having a constant output.
Preferably the said further light source comprises a second laser diode, or an LED, driven by a constant current source.
According to a second aspect of the present invention there is provided an opto-electronic

REFERENCES:
patent: 4295225 (1981-10-01), Pan
patent: 5021361 (1991-06-01), Kinoshita et al.
patent: 5089787 (1992-02-01), Wang et al.
patent: 5343177 (1994-08-01), Williams

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