RF power transistor with internal bias feed

Details RF power transistor with internal bias feed RF power transistor with internal bias feed

2002-12-19

2004-05-11

Shingleton, Michael B. (Department: 2817)

Amplifiers

Involving structure other than that of transformers per se

With printed circuits

C330S302000, C330S307000, C257S401000

Reexamination Certificate

active

06734728

ABSTRACT:

FIELD OF THE INVENTION
The present invention pertains generally to the field of radio frequency (RF) amplifiers and more specifically to high frequency, high power transistors used in wireless communication applications.
BACKGROUND
The use of RF power amplifiers in wireless communication applications is well known. With the recent growth in the demand for wireless services, such as personal communication services, the operating frequency for wireless networks has increased dramatically and is now in excess of two gigahertz. RF power amplifier stages are commonly used in wireless communication network radio base station amplifiers. Such power amplifiers are also widely used in other RF-related applications, such as cellular telephones, paging systems, navigation systems, television, avionics, and military applications. At the, high frequencies that such circuits must operate, impedance matching and biasing of the active elements is an important factor for efficient operation of the power amplifier. The input and output circuits used to match power transistors to external devices are typically implemented with a combination of bondwire inductance, stripline or microstrip structures on a printed circuit board, and discrete capacitors.
A typical common source power amplifier stage, as illustrated in
FIGS. 2 and 3
, has an RF feed, a power transistor
200
, and an RF output. The power transistor
200
is a three terminal device, having an input terminal
210
, an output terminal
220
, and a common terminal that is the flange
205
which is grounded. The power transistor
200
amplifies the low power signal coming from the RF feed, into a high power signal delivered from the RF output to a load. An input bias network provides a DC voltage, called the input bias feed, to the power transistor
200
establishing an input operating point for the transistor
200
. An output bias network provides a DC voltage, called the output bias feed, to the power transistor
200
establishing an output operating point for the transistor
200
.
An input impedance transformer
231
transforms the impedance of the RF feed (typically 50 ohms) into the impedance at input terminal
210
(typically 8-10 ohms) at the frequency and power level of operation. The input impedance transformer
231
is typically a microstrip transmission line of ¼ wavelength (lambda) at the operating frequency.
Similarly, an output impedance transformer
241
transforms the load impedance at the output terminal
220
(typically 1 to 10 ohms) into the impedance at the RF output (typically 50 ohms) at the frequency and power level of operation. The output impedance transformer
241
is also preferably a microstrip transmission line of ¼ lambda at the operating frequency.
Input blocking capacitor
232
prevents DC voltages from entering the wrong amplifier stage. Output blocking capacitor
242
prevents loading by the RF output circuits by blocking DC voltages from the RF output.
In addition, it is important to prevent high frequency signals generated inside the power amplifier stage from escaping along unwanted transmission paths. In order to prevent the high frequency signals in the power amplifier from contaminating the sources of DC voltage which bias the amplifier, designers typically use a ¼ lambda transmission line, implemented with a microstrip structure. Transmission line theory predicts that a ¼ lambda transmission line terminated at its distal end with a short circuit has an input impedance, at the proximal end, that is equal to an open circuit. As a practical matter, a one-quarter wavelength transmission line terminated with a relatively low impedance presents a high impedance to the driving source. This approach prevents RF power directed toward the input terminal
210
from leaking into the input bias network, and provides a method of coupling a DC voltage into the power transistor
200
, without disturbing the impedance matching structures.
For instance, on the input bias circuit illustrated in
FIGS. 2 and 3
, an input bias transmission line
233
is a ¼ lambda transmission line which has its distal end coupled to the DC voltage source of input bias feed. The proximal end is coupled to the power transistor input terminal
210
. The combination of the DC voltage source of input bias feed, and decoupling capacitors
234
and
235
approaches a short circuit over a broad range of frequencies at the distal end of line
233
.
Capacitor
234
has a small capacitance value and is selected to have series resonance at or near the operating frequency. Typical values for capacitor
234
are 5 to 50 pF with ceramic dielectric. Capacitor
235
has a large capacitance value and is selected to have high capacitive reactance and moderate inductance for lower intermediate frequencies. Typical values for capacitor
235
are 0.05 to 0.5 uF with tantalum dielectric. Should the amplifier be operated as a Continuous Wave (CW) amplifier, capacitor
235
is not required for adequate decoupling.
The DC voltage source of input bias feed voltage forms a short circuit for low frequency AC signals and DC. Since the distal end of the line
233
is terminated with a short circuit, the input impedance of the line
233
at the proximal end appears to be an open circuit to the high frequency signals near the input terminal
210
. This open circuit blocks RF signals from escaping along unwanted paths, and in particular from contaminating the DC voltage source of input bias feed.
Similarly, a ¼ lambda transmission line is used to prevent RF signals from the output terminal
220
from flowing back into the DC voltage source of the output bias feed. An output bias transmission line
243
is a ¼ lambda transmission line which has its distal end coupled to the DC voltage source of output bias feed. The proximal end is coupled to the power transistor output terminal
220
. The combination of the DC voltage source of output bias feed, and decoupling capacitors
244
and
245
form s a short circuit over a broad range of frequencies at the distal end of line
243
. Capacitor
244
has a small capacitance value and is selected to have series resonance at or near the operating frequency. Typical values for capacitor
244
are 5-50 pF with ceramic dielectric. Typical values for capacitor
245
are 0.05 to 0.5 uF with tantalum dielectric. Since the distal end of the line
243
is terminated with a short circuit, the input impedance of the line
243
at the proximal end appears to be an open circuit to the signals near the output terminal
220
which blocks RF signals from contaminating the DC voltage source of output bias feed.
Although using a ¼ lambda transmission line for providing input and output bias to transistor
200
has been found to be a practical biasing solution, there are several factors that make its use less than optimal. Considerable area on the printed circuit board is required for its implementation, reducing the packaging density for the amplifier. In addition, the ¼ lambda transmission tends to radiate RF energy, reducing the overall amplifier efficiency. Further, coupling the ¼ lambda transmission line to the power transistor input is difficult to model due to unequal distributed element effects that complicate the design process.
The physical configuration of a typical power transistor
200
is illustrated, in more detail, in
FIG. 1A
, and an equivalent circuit for transistor
200
appears in FIG.
3
. The power transistor
200
has a transistor die
219
, a gate tuning network, and a drain tuning network. The transistor die
219
is preferably a field effect transistor die and particularly a lateral diffused metal-oxide-silicon device (LDMOS) with a gate and drain region formed on the upper surface. A high conductivity sinker region is formed to provide a low resistance conduction path between a source region and the lower surface of the die
219
. The die
219
is bonded to the flange
205
, thereby thermally and mechanically coupling the die to the flange and electr

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