Miscellaneous active electrical nonlinear devices – circuits – and – Specific input to output function – Exponential
Reexamination Certificate
1999-02-24
2001-01-09
Cunningham, Terry D. (Department: 2816)
Miscellaneous active electrical nonlinear devices, circuits, and
Specific input to output function
Exponential
C327S361000
Reexamination Certificate
active
06172549
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates generally to RMS-to-DC converters, and more particularly, to RMS-to-DC converters that are capable of measuring true power at high frequencies and low supply currents.
SUMMARY OF THE INVENTION
The present invention utilizes two balanced squaring cells operating in opposition to implement the difference of squares function, thereby achieving true RMS-to-DC conversion. By implementing the squaring cells as simple three transistor cells, each having a grounded base transistor and a two-transistor current mirror, an RMS-to-DC converter in accordance with the present invention can operate at microwave frequencies while dissipating as little as 1 mW of quiescent power in these cells.
One of the squaring cells receives the high frequency (HF) input signal and generates a first current which represents the square of the HF input signal. The other squaring cell generates a second current that represents the square of a DC feedback current which is input to the cell.
Used as a measurement device, a nulling circuit closes a feedback loop around the DC squaring cell so as to balance the output currents from the squaring cells. This path includes a filter capacitor for low-pass filtering the output signal from the HF squaring cell, an error amplifier for sensing the difference between the output currents from the squaring cells, and a circuit for converting the output from the error amplifier to a feedback current for driving the DC squaring cell. The error amplifier includes a balanced resistive load for converting the currents from the squaring cells to voltages, and an op-amp for sensing the resulting voltage difference. In a preferred embodiment, a nonlinear load is used to extend the dynamic range of the squaring cells.
Each of the squaring cells includes a grounded base transistor and a current mirror. The grounded base transistor has its base anchored at a suitable bias voltage. The emitter of the grounded base transistor and the input terminal of the current mirror are connected together at the input terminal of the squaring cell. The collector of the grounded base transistor and the output of the current mirror are connected together at the output terminal of the squaring cell to generate the output current which approximates the square of the input signal.
The squaring cell provides a good square-law approximation over an input signal range that is largely determined by the thermal voltage V
T
=kT/q. The input node of a squaring cell according to the present invention appears as a broad-band matching network to an external signal source, thereby terminating the generator without the need for an external termination resistor. The bias current through the squaring cells determines this input impedance. The two squaring cells are balanced by careful device matching and layout techniques. In a preferred embodiment, the HF cell is implemented as two parallel-connected cells which are physically located on opposite sides of the DC squaring cell to cancel effects from doping and thermal gradients. Using a single bias voltage for all of the cells further insures a high degree of balance between the two cells.
The output signal is obtained by replicating the current flowing into the input cell through a feedback interface; this current is unidirectional, that is, its sign is independent of the sign of the input current presented to the first squaring cell. The replicated current is converted to a voltage and buffered to provide substantial load driving capability even though quiescent current consumption is low.
REFERENCES:
patent: 4250457 (1981-02-01), Hofmann
patent: 5909136 (1999-06-01), Kimura
Gilbert, Barrie; Novel Technique for R.M.S.—D.C. Conversion Based on the Difference of Squares;Electronics Letters; vol. 11, No. 8; Apr. 17, 1975; pp. 181-182.
Gilbert, Barrie; Current-mode Circuits From A Translinear Viewpoint: A Tutorial;Analogue IC design: the current mode approach; 1990; pp. 33-53.
Analog Devices Inc.
Cunningham Terry D.
Marger & Johnson & McCollom, P.C.
Tra Quan
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