Browse Prior Art Database

Active Impedance Circuit

IP.com Disclosure Number: IPCOM000084359D
Original Publication Date: 1975-Oct-01
Included in the Prior Art Database: 2005-Mar-02
Document File: 4 page(s) / 74K

Publishing Venue

IBM

Related People

Stuckert, PE: AUTHOR

Abstract

A transistor circuit is described which is functionally similar to a resistance and an inductance connected in series.

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Active Impedance Circuit

A transistor circuit is described which is functionally similar to a resistance and an inductance connected in series.

In electrical practice a resistance and an inductance connected in series are used when the DC characteristic shown in Fig. 1 and the AC impedance characteristic shown in Fig. 2 are desired. In this circuit the DC and AC characteristics shown are invariant, regardless of the DC polarity and the relative magnitudes of the AC and DC potentials applied. While highly useful, the circuit is limited in its application under certain conditions. For example, with reference to Fig. 2, when a transition from a low DC resistance to a high AC impedance at low frequencies (such as audio) is desired, the inductance required is frequently large and costly.

Again with reference to Fig. 2, when a transition from a low DC resistance to a high AC impedance at higher frequencies, (such as RF), is desired, difficulties due to core losses and winding capacitance frequently limit operation, or require the use of more costly core material and/or more exotic winding techniques.

Again with reference to Fig. 2, the AC impedance of the series connected resistance and inductance increases substantially linearly with frequency. In many applications an AC impedance characteristic similar to that shown in Fig. 3 is more desirable.

The series connected resistance and inductance can rarely be fabricated, either in whole or in part, by integrated circuit techniques, particularly for low- frequency operation.

In some applications a DC resistance which departs from linearity through.the origin, say for one polarity of DC, as illustrated in curves A and B of Fig. 4 is desired. The series connected resistance and inductance cannot provide this feature without additional circuitry.

The circuit of Fig. 5 described requires no inductance and circumvents the limitations listed above.

All DC characteristics of the circuit of Fig. 5 were taken on a Tektronix 575 curve tracer with the capacitor C removed and +48 volts provided by an external power supply. All DC characteristics described below are the DC resistance between points A and B of Fig. 5.

Fig. 6 shows the DC characteristics of the circuit at 2 volts/ division horizontal and 10 ma/division vertical when R(1) = 80 K ohms, R(2) = 40 K ohms, R(3) = 100 ohms, and R(4) per Table 1 below. Note that curve B of Fig. 6, except for the 0 volt to approximately 1.0 volt region, is equivalent to a 300 ohm resistor.

Fig. 7 shows the DC characteristic with the polarity applied at point A of Fig. 5 negative, for the resistor magnitudes used for Fig. 6, 2 volts/division horizontal, and 0.1 ma/division vertical. This illu...