# Patent

## Title
Cascode power amplifier stage using HBT and FET

## Abstract

A power amplifier comprising a bipolar transistor connected in cascode with a field effect transistor (FET) such as a pseudomorphic high electron mobility transistor (PHEMT) device. The bipolar transistor has a common emitter and the FET a common gate. Advantageously, the bipolar transistor is a heterojunction bipolar transistor (HBT); and the HBT and the FET may be integrated on a single die. Illustrative materials for the HBT and FET are Gallium Nitride, Indium Phosphide, or Gallium Arsenide/Indium Gallium Phosphide.

## Background
CROSS REFERENCE TO RELATED APPLICATION
This application claims benefit of provisional application Ser. No. 62/087,157, filed Dec. 3, 2014, which application is incorporated by reference herein in its entirety.


BACKGROUND
The use of GaAs InGaP heterojunction bipolar transistor (HBT) devices for RF power amplifiers above 2.5 GHz becomes more challenging due to the lower input impedance of the common emitter stage as well as lower gain. The use of field effect transistor (FET) devices such as high electron mobility transistor (HEMT) and pseudomorphic high electron mobility transistor (PHEMT) devices at higher frequencies is common due to the higher gain and ease of impedance matching. However the FET device needs a relatively high quiescent current compared to the HBT when operated in class AB mode. Depletion mode PHEMT are normally on devices and also require a negative gate voltage with power supply sequencing to prevent damage. HBT devices are also susceptible to overdrive conditions that will cause failure at higher current densities.
SUMMARY
The use of a cascode configuration with a common emitter HBT followed by a common gate FET solves these problems. The turnoff and negative voltage problem is gone since the HBT controls the current. The quiescent current in class AB bias and the current versus power behavior of the cascode is essentially that of the HBT. Also the FET device will limit the current and limit the HBT current density to safe levels. The cascode will also have higher gain and bandwidth than either the HBT or FET due to the reduced Miller capacitance provided by the low impedance of the common gate input. This also results in a higher impedance at the input of the HBT. Use of the FET in the cascade also allows for utilization of higher supply voltage since the overall breakdown voltage of the cascode would be the sum of the two devices.
Advantageously, the HBT and the FET may be integrated on a single die, a feature that becomes more important at the higher frequencies. Alternatively a silicon bipolar transistor can be used in place of the HBT.



BRIEF DESCRIPTION OF DRAWING
These and other objects, features and advantages of the invention will be more readily apparent from the following detailed description in which:
FIG. 1 is a schematic diagram depicting a first illustrative embodiment of the present invention; and
FIG. 2 is a plot depicting gain in dB (on the left-hand vertical axis) and efficiency (on the right-hand vertical axis) versus output power in dBm (on the horizontal axis) for a simulation of a cascode circuit such as that of FIG. 1.



DETAILED DESCRIPTION
FIG. 1 is a schematic diagram depicting an illustrative embodiment of a HBT-FET cascode such as might be used at 5.5 GHz with a supply voltage of 15 Volts. In this example, L1 and C1 form an input impedance matching network. R1 is the base resistance/ballast feed. X1 is an HBT common emitter device. C3 and L3 provide a second harmonic trap. C2 and L2 and C5 and L4 provide additional harmonic traps. TL1 is a small inductance for band shaping and stability. X2 is a PHEMT common gate device. C4 is RF by pass for the gate. R2 is the gate bias injection for the PHEMT device. The output load was set at 120 Ohms and the source impedance is 50 Ohms.
Other types of FET devices may be substituted for the PHEMT in the cascode of FIG. 1. These include any type of HEMT device and any type of metal-semiconductor field effect transistor (MESFET) device.
Illustratively, the HBT and FET devices are formed in a III-V semiconductor material such as Gallium Nitride, Indium Phosphide, or Gallium Arsenide/Indium Gallium Phosphide. In some applications, it may be advantageous to integrate the HBT and the FET device in a single semiconductor crystal by epitaxially growing the HBT device on an epitaxially grown FET device. Such a device and the process for making it in a GaAs/InGaP epitaxial growth process is described in U.S. Pat. No. 7,015,519, which is incorporated herein by reference. Other materials may also be used. For example Silicon Germanium (SiGe) may be used as the HBT. Alternatively a common emitter Silicon bipolar transistor may be used in place of the HBT.
FIG. 2 is a plot depicting gain in dB (on the left-hand vertical axis) and efficiency (on the right-hand vertical axis) versus output power in dBm (on the horizontal axis) for a simulation of a cascode circuit such as that of FIG. 1.
As will be apparent to those skilled in the art, numerous variations may be practiced within the spirit and scope of the present invention. Further details concerning HBT and HEMT devices may be found in F. Ali et al., HEMTs & HBTs: Devices, Fabrication, and Circuits (Artech House, 1991) which is incorporated by reference herein.

## Claims

1. A power amplifier comprising:
a heterojunction bipolar transistor;
a field effect transistor connected in cascode; and
a harmonic trap between the heterojunction bipolar transistor and the field effect transistor.

2. The power amplifier of claim 1 wherein the heterojunction bipolar transistor and the field effect transistor are formed in a Gallium Nitride semiconductor crystal.

3. The power amplifier of claim 1 wherein the heterojunction bipolar transistor and the field effect transistor are formed in an Indium Phosphide semiconductor crystal.

4. The power amplifier of claim 1 wherein the heterojunction bipolar transistor is formed in a Silicon Germanium semiconductor crystal.

5. The power amplifier of claim 1 wherein the heterojunction bipolar transistor has a common emitter and the field effect transistor has a common gate.

6. The power amplifier of claim 1 wherein the field effect transistor is a high electron mobility transistor device.

7. The power amplifier of claim 1 wherein the field effect transistor is a pseudomorphic high electron mobility transistor device.

8. The power amplifier of claim 1 wherein the field effect transistor is a metal-semiconductor field effect transistor device.

9. The power amplifier of claim 1 wherein the heterojunction bipolar transistor and the field effect transistor are integrated on a single die.

10. The power amplifier of claim 9 wherein the single die is formed by an epitaxial growth process.

11. A power amplifier comprising:
a bipolar transistor;
a field effect transistor connected in cascode with the bipolar transmitter having a common emitter and the field effect transistor a common gate; and
a harmonic trap between the bipolar transistor and the field effect transistor.

12. The power amplifier of claim 11 wherein the bipolar transistor is a heterojunction bipolar transistor formed in a Gallium Nitride or Indium Phosphide semiconductor crystal.

13. The power amplifier of claim 11 wherein the bipolar transistor is a heterojunction bipolar transistor formed in a Silicon Germanium semiconductor crystal.

14. The power amplifier of claim 11 wherein the bipolar transistor is a Silicon bipolar transistor.

15. The power amplifier of claim 11 wherein the field effect transistor is formed in a Gallium Nitride or Indium Phosphide semiconductor crystal.

16. The power amplifier of claim 11 wherein the field effect transistor is a high electron mobility transistor (HEMT) device.

17. The power amplifier of claim 11 wherein the field effect transistor is a pseudomorphic high electron mobility transistor device.

18. The power amplifier of claim 11 wherein the field effect transistor is a metal-semiconductor field effect transistor device.

19. The power amplifier of claim 11 wherein the bipolar transistor is a heterojunction bipolar transistor and the heterojunction bipolar transistor and the field effect transistor are integrated on a single die.

20. The power amplifier of claim 19 wherein the single die is formed by an epitaxial growth process.

21. A power amplifier comprising:
a heterojunction bipolar transistor;
a pseudomorphic high electron mobility transistor connected in cascode; and
a harmonic trap between the heterojunction bipolar transistor and the pseudomorphic high electron mobility transistor.

22. The power amplifier of claim 21 wherein the heterojunction bipolar transistor has a common emitter and the pseudomorphic high electron mobility transistor has a common gate.

23. The power amplifier of claim 1 further comprising an additional harmonic trap between an input impedance matching network and the heterojunction bipolar transistor.

24. The power amplifier of claim 1 further comprising an additional harmonic trap between the field effect transistor and an output port.

25. The power amplifier of claim 11 further comprising an additional harmonic trap between an input impedance matching network and the bipolar transistor.

26. The power amplifier of claim 11 further comprising an additional harmonic trap between the field effect transistor and an output port.

27. The power amplifier of claim 21 further comprising an additional harmonic trap between an input impedance matching network and the heterojunction bipolar transistor.

28. The power amplifier of claim 21 further comprising an additional harmonic trap between the pseudomorphic high electron mobility transistor and an output port.