US3503000A - Means for d.c. biasing a device for operation at high frequency - Google Patents

Description

March 24, 1970 E. L. lvr-:Y 3,503,000

MEANS FOR D.C4. BIASING A DEVICE FOR OPERATION AT HIGH FREQUENCY Filed May 5, 1966 INVENTOR EDWARD L. IVEY ATT RNEY United States Patent O 3,503,000 MEANS FOR D.C. BIASING A DEVICE FOR OPERATIN AT HIGH FREQUENCY Edward L. Ivey, Garland, Tex., assignor to Texas Instruments Incorporated, Dallas, Tex., a corporation of Delaware Filed May 5, 1966, Ser. No. 547,806 Int. Cl. H03i` 3/14 U.S. Cl. 330--38 5 Claims ABSTRACT F THE DISCLOSURE Disclosed is a microwave circuit having two sections, one having an input for A C. signals and one having a conductor for D.C. bias voltage. The conductor is A.C. coupled to the A C. signal section from a point of low impedance to the point where the bias A.C. signals are applied to a device.

This invention relates generally to high frequency circuits, and more particularly, but not by way of limitation, relates to method and apparatus for biasing an active component, such as a transistor, with a D.C. voltage or current Without interfering with a microwave signal associated with the component.

In high frequency circuits, it often becomes necessary to divide an A C. circuit into two or more D.C. isolated sections so that a D.C. bias can be applied to one of the sections. The conventional method for achieving this is t0 capacitively couple the two sections to complete the A.C. path while thus maintaining D.C. isolation. Then the D.C. bias is applied to one of the sections by a meandering line or one-quarter wave-length choke in order to minimize the effect of the D.C. connection on the A.C. characteristics of the circuit. Several problems are associated with this approach. The coupling capacitor, due to its size, often produces an impedance discontinuity at microwave frequencies. Further, the D.C. meander line or choke is often quite lossy at microwave frequencies.

An important object of this invention is to provide a means for applying a D.C. voltage or current bias to an A.C. circuit without adversely interfering with the A C. characteristics of the circuit.

Another object is to provide such a means that is particularly suitable for use in thin film microwave circuits.

Another object is to provide means for introducing a D.C. bias to a microwave circuit which will not create impedance discontinuities and which produces virtually no energy loss.

These and other objects are accomplished by A C. coupling a first conductor for the D.C. bias to a second conductor carrying the A.C. signal from a point of low impedance on the second conductor to the point where the bias is to be applied to the circuit. By tightly coupling the rst conductor to the second conductor between the low impedance point and the point where the bias is to be injected, the D.C. bias is actually introduced to the A.C. system at a low impedance point and therefore does not affect the impedance cha-racteristics of the A.C. signal path. As a result of the close coupling between the two conductors, the two conductors appear as a single conductor at high frequency so that energy losses are virtually eliminated, yet the two conductors are D.C. isolated.

In accordance with a more specific aspect of the invention, a high frequency thin film circuit is provided wherein a strip transmission line conductor carrying an A.CA signal is terminated at ground through a tuning stub or the like. The D.C. bias conductor is closely coupled to, but is D.C. isolated from, the A.C. signal strip line conductor from a point adjacent the ground potential to the point on 3,503,000 Patented Mar. 24, 1970 the A C. conductor Where the A.C. signal is to be coupled to the section of the circuit to be biased. The D.C. conductor is then D.C. coupled to the section of the circuit to 'be D.C. biased. In accordance with a more specific aspect of the invention, an `active component, such as a transistor, may be biased using this technique wherein one terminal, such as the collector, is grounded, and a D.C. bias is applied to one or more of the other terminals using the method just described.

The novel features believed characteristic of this invention are set forth in the appended claims. The invention itself, however, as Well as other objects and advantages thereof, may best be understood by reference to the following detailed description of an illustrative embodiment, when read in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a plan view of a thin film microwave circuit constructed in accordance with the present invention;

FIGURE 2 is an enlarged view of the central section of the circuit illustrated in FIGURE l;

FIGURE 3 is a sectional view taken substantially on lines 3-3 of FIGURE 1;

FIGURE 4 is a sectional view taken substantially on lines 4--4 of FIGURE 1;

FIGURE 5 is a sectional view taken substantially on lines 5-5 of FIGURE 1; and

FIGURE 6 is a schematic equivalent circuit diagram of the circuit illustrated in FIGURE 1.

Referring now to the drawing, a circuit constructed in accordance with the present invention is indicated generally by the reference numeral 10. The circuit 10 is a microwave amplifier stage comprised of a transistor 12 and input and output tuning means to achieve input and output characteristic impedances of 50 ohms. The tuning circuitry associated with the transistor 12 is fabricated using known thin film techniques on a ceramic substrate 14 having a uniform thickness. A metallized film 16 is deposited on the underside of the substrate 14 and forms a ground plane so that all strip conductors form strip transmission lines. An input conductor 18 is, therefore, a strip transmission line by reason of its association with the ground plane 16. The input transmission line 18 is connected to the input 20 of a quarterwave transformer section 22 which may be considered as terminating generally at an output point 24. However, the strip conductor continues through a tuning stub section 26 to a ground strap 28 which is electrically connected to the ground plane 16 by means not illustrated and which is, therefore, at ground potential.

A D.C. biasing conductor 30 extends from an expanded contact pad 32 to an enlarged capacitor plate section 34 and finally a bonding tab 36 as seen in FIGURE 3. The conductor 30 is electrically insulated from the conductor 26 by a thin insulating layer 38. The expanded contact pad 32 is preferably formed directly on the substrate 14 within an opening 40 formed in the ground strap 28. It will be noted that the insulating layer 38 extends partially into the opening 40 to prevent the conductor 30 from contacting the edge of the ground strap 28. Similarly, the insulating layer 38 has an expanded portion 42 which extends over the edges of the strip conductor forming the transformer 22 and tuning stub 26 so that the contact tab 36 may extend onto the ceramic substrate 14 as best seen in FIGURE 3. Placing the bonding tab 36 and the expanded contact 32 directly on the ceramic substrate mechanically facilitates subsequent electrical contact with these metal films without damage to the circuit. The insulating layer 38 may be a very thi-n layer of a suitable insulating oxide deposited using the standard thin film techniques so as to have maximum A C. coupling between the line 26 and line 30 while still maintaining D.C. isolation for purposes which will presently be described.

An expanded collector contact pad 44 is formed on the ceramic substrate 14 and is connected through a meandering tuning line 46 to an output strip line conductor 48. The meandering tuning line continues through a section 50 and terminates at a ground strap 52 which is integral with the ground strap 28. A second D.C. biasing conductor 54 extends from an expanded contact pad 56 formed on the substrate in an opening in the ground strap 52 to a bonding tab 58, and is D.C. isolated from the underlying ground strap 52 by a layer of insulating material 59, which may also be a suitable conventional oxide film. The conductor 54 has an enlarged portion 60 for increasing the efficiency of the A.C. coupling between the conductor 54 and the ground strap 52. The transistor 12 is mounted directly upon the expanded pad 44 so that the collector of the transistor will be in good electrical contact with the pad. A bonded lead wire 62 interconnects the base contact of the transistor and bonding tab 36, and a bonded lead wire 64 interconnects the emitter contact of the transistor and the bonding tab 58. A D.C. bias voltage may be applied to pads 32 and 56 by any suitable means such as by a solder or pressure connection.

The biasing circuit illustrated in FIGURE 1 may be roughly represented by the equivalent circuit illustrated in FIGURE 6 wherein corresponding parts are represented by corresponding reference numerals. The strip transmission line sections 22 and 26 may be represented by coils. The strip conductor 30, including the expanded portion 34, may be represented collectively by a coil, it being understood that the two coils 26 and 30 are very tightly coupled inductively by reason of their close proximity. Also, the strip conductor 30 is capacitively coupled to the conductor 26 throughout its entire length, and the capacitive coupling is enhanced by the expanded portion 34. This capacitive coupling is lumped and represented by capacitor 30-34 in FIGURE 6. It is important to note that although the conductor 30 has an expanded portion 34, the capacitive coupling does occur for the entire length of the conductor 30 and that the enlarged portion 34 is provided merely to increase the coupling eticiency. If desired, the conductor 30 may also extend along the conductor 22 to enhance the A.C. coupling. Capacitors 70 and 72 represent the capacitive coupling between the conductors 26 and 30 and the ground plane 16, respectively, although this capacitance is actually present over the length of the respective conductor. D.C. bias for the base of transistor 1`2 is applied to the terminal pad 32.

At microwave frequencies, the conductor 30 and the conductor 26 are equivalent to a single conductor as a result of the very tight A.C. coupling between the two conductors. Since the D.C. Ibias is applied to expanded contact 32 at a point of essentially zero impedance in the microwave circuit, the application of the D.C. bias does not disturb the microwave characteristics of the circuit. The function of the transformer 22 and the tuning stub 26 is well known and does not, per se, constitute a part of the present invention. The transformer 22 is used to establish a characteristic input impedance of ohms in the instant case, and the tuning stub 26 is used to cancel the imaginary components of the microwave energy. Since the D.C. bias applied to expanded contact terminal 32 is D.C. isolated from the strip conductors 26 and 22, the base of transistor 12 may be D.C. biased so that the transistor 12 will be operated under optimum conditions to amplify the microwave signal applied through input 18 and A.C. coupled to the base of the transistor 12 through the D.C. bias line 30. Since the collector of the transistor is already at ground, the meandering strip transmission tuning line 46 is provided merely to establish the desired 50 ohm characteristic output impedance, and the meandering tuning stub 50 is provided to cancel out the imaginary components of the microwave energy, The emitter of the transistor 12 is biased by the potential applied to expanded contact terminal 56. The capacitive coupling between the D.C. bias conductor 54, as enhanced by the expanded portion 60, and the underlying ground strap 52 provides an emitter bypass which short circuits microwave energy to ground.

Thus, it will be noted that in accordance with the method of the present invention, a D.C. bias is introduced at a low impedance point in an A.C. circuit path, preferably a point of essentially zero impedance, as represented by a point at essentially ground potential. A conductor carrying the D.C. bias potential is then extended along the A.C. circuit path from the low impedance point to the point at which an a D.C. signal is to be transferred between D.C. isolated sections of the A.C. circuit and the conductor is tightly coupled, capacitively and inductively, to the A.C. circuit over substantially its entire length.

Although in the particular embodiment selected to illustrate the invention the D.C. bias is applied to the base of a transistor 12, the same technique may be used to `bias the emitter, or the collector of a transistor, or to bias any other component or portion of the circuit desired.

Although a preferred embodiment of the invention has been described in detail, it is to be understood that various changes, substitutions and alterations can be made therein without departing from the spirit and scope of the invention.

What is claimed is:

1. In a microwave circuit comprising in combination:

(a) a semiconductor device;

(b) a microwave transmission line for coupling a microwave signal to the input of said device, said line includes a tuning stub that terminates remote from the input of said device;

(c) a D.C. biasing conductor for connecting a D.C.

voltage to the input of said device; wherein (d) said D.C. biasing conductor overlies but is insulated from said tuning stub of said line so as to be D.C. isolated from said line but A.C. coupled to said line through the distributed capacitance between said tuning stub of said line and substantially the entire length of said conductor;

(e) a D.C. voltage connected to said conductor; and

(f) an A.C. signal coupled to the other end of said line; wherein (g) said D.C. voltage does not interfere with the A.C.

characteristics of said circuit, and said line and conductor operate as a single conductor at microwave frequencies so as to substantially minimize energy losses in said circuit.

2. The microwave circuit of claim 1 wherein (a) said microwave transmission line is a thin conductive film formed on an insulating substrate; and wherein (b) said D.C. biasing conductor is a thin conductive lm overlying said line; and wherein (c) said conductor is insulated from said line by a thin layer of insulating material.

3. The microwave circuit of claim 1 wherein said semiconductor device is a transistor having one of its electrodes D.C. connected to said D.C. biasing conductor and A.C. coupled to said microwave transmission line.

4. In a microwave thin film circuit comprising in combination:

(a) a semiconductor device mounted on an insulating substrate;

(b) a thin film microwave transmission line formed on said substrate, said line including a transformer section connected to a tuning stub that terminate at a reference potential with the junction of said section and stub being A.C. coupled to the input of said device for coupling a microwave signal thereto; and

(c) a thin film D.C. biasing conductor ymounted on said substrate in overlapping relationship to but insulated from at least Said .Stub of said line, said conductor having one end D.C. connected to the input of said device for connecting a D.C. voltage thereto; wherein (d)4 said conductor is D.C. isolated from said line but A C. coupled to said line through the distributed capacitance between at least said stub of said line and substantially the entire length of said conductor; and wherein (e) said D.C. voltage is connected to the unconnected end of said conductor so as to connect said D.C. voltage to the input of said device at a relatively low impedance point of the circuit; and wherein (f) said A C. signal is coupled to the unconnected end of said line so as to couple said signal to the input of said device from said junction of said section and stub; whereby (g) said D.C. voltage does not interfere with the` A.C.

characteristics of said circuit, and said line and conductor operate as a single conductor at microwave frequencies so as to substantially minimize energy losses in said circuit.

5. In a microwave circuit comprising in combination:

(a) a semiconductor device;

(b) a microwave transmission line for coupling a microwave signal to the input of said device said line includes a quarterwave transformer section with one of its ends connected to a quarterwave tuning stub that terminates to ground; and

(c) a D.C. biasing conductor for connecting a D.C. voltage to the input of said device said D.C. biasing conductor substantially overlies but is insulated from the entire length of said tuning stub with one end D.C. connected to the input of said device so as to be D.C. isolated from said line but A.C. coupled to said line through through the distributed capacitance .between said line and substantially the entire length of said conductor; wherein (d) said D.C. voltage is connected to the unconnected end of said conductor so as to connect said D.C. voltage to said circuit at relatively low impedance point; and wherein (e) said A.C. signal is coupled to the unconnected end of said quarterwave section; and wherein (f) said A.C. signal is injected into said input of said device from the junction of said transformer section and tuning stub; whereby (g) said D.C. voltage does not interfere with the A.C.

characteristics of said circuit, and said line and conductor operate as a single conductor at microwave frequencies so as to substantially minimize energy losses in said circuit.

References Cited UNITED STATES PATENTS 2,926,308 2/1960 Thanos 330-40 3,189,823 6/ 1965 Mitchell. 3,246,256 4/ 1966 Sommers 331-107 3,155,881 11/1964 St. Jean.

ROY LAKE, Primary Examiner J. B. MULLINS, Assistant Examiner U.S. C1. X.R.

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