This invention relates to power amplifiers and, more specifically, to a circuit and method for adjusting the voltage at the commonly connected ballasted ends of a plurality of base ballasted HBTs forming a power amplifier when the voltage appearing at one of the bases of the plurality of HBTs drops below a threshold.
Hetero-junction bipolar transistor (HBT) power amplifiers are becoming the standard for cellular applications due to their high power density and reduction in die size. HBT""s can draw substantial base current during high power operation. Since multi-finger power devices suffer from thermally related current collapse they cannot be operated without ballast resistors on either the emitter or base. Emitter ballasting is not practical for power amplifiers because of the high emitter currents and small resistor values, so base ballasting must be used. Because an individual HBT is a very small device, it is required to be paralleled with multiple HBTs to achieve high power operation required for most power amplifier applications.
Accordingly, HBT power amplifiers such as those used in radio frequency (RF) applications employ multiple small devices connected in parallel. As noted above, each of these small HBT devices require a ballast resistor to be connected to its base before being connected to the other HBTs forming the power amplifier (PA). The resistive ballasting of individual cells keeps parallel HBT fingers from entering thermal collapse. Additionally, a capacitor may be used to bypass the base resistor to preserve high frequency gain or the RF signal may be fed to the base connections through a separate capacitor. For purposes of illustration, the figures contained herein will illustrate the principal using capacitor bypassed ballast resistors although those skilled in the art will realize that this embodiment of the invention will work the same regardless of the connection of the RF capacitors feeding the base connection.
FIG. 1 shows a typical multi-fingered base ballasted Power Amplifier (PA) circuit. A plurality of HBTs 110, each ballasted with a resistor 130/capacitor 120 are connected in parallel. For each small HBT device, first ends 130a, 120a of a resistor 130 and a capacitor 120 are connected to the base 185 of the individual HBT devices and the other ends 130b, 120b of the resistor and the capacitor become the input 180 of each base ballasted HBT device 190. For purposes of simplicity, when two or more base ballasted HBT devices 190 are xe2x80x9cconnected in parallelxe2x80x9d, their collectors 160 share a first common node, their emitters 170 share a second common node connected to ground, and the inputs 180 share a third common node. A radio frequency signal is received at the input 140 and connected to the commonly connected inputs 180 of the base ballasted HBT devices 190. The commonly connected collectors 160 that are connected to a voltage source 155 produce an amplified RF output 150.
Due to the base current requirements, a biasing circuit 195 is usually included. Typical biasing circuits with RF decoupling components neglected for simplicity are shown in FIGS. 2 and 3. FIG. 2 shows a diode biasing circuit 200. The base of an HBT device 210 is connected to the collector and the collective inputs 180 of the of the base ballasted HBT devices 190 of FIG. 1. The emitter is connected to ground. The first end 220b of a reference resistor 220 is connected to the collector and base while the second end 220a of the reference resistor 220 is connected to a reference voltage 230.
FIG. 3 shows a preferred current mirror biasing circuit 300. A current mirror is formed by HBT devices 310 and 320. The collector of the first HBT device 310 is connected to a voltage source 350, its emitter is connected to the base of the second HBT device 320 and its base is connected to the collector of the second HBT device 320. The emitter of the second device is connected to ground. And finally, the first end 330b of a reference resistor 330 is connected to the base of the first HBT device 310 and the collector of the second HBT device 320 while the second end 330a of the reference resistor is connected to a reference voltage 340.
Although not exhaustive, these biasing circuits are typical of those employed in the industry although other types of biasing circuits are contemplated and may be used with the invention. The biasing circuits try to keep the current through the power device constant with variations in temperature and reference voltages. Although either of these biasing circuits or others could be used, current mirror biasing is typically preferred and will be used in the discussion.
Using the current mirror of FIG. 3 as the Bias of FIG. 1, the reference voltage 340 and the reference resistor 330 form a constant current source which is mirrored by the first HBT device 310 and the second HBT device 320. If no ballast resistors 130 were required, the current mirror would be adequate up to the limits of the HBT devices 310 and 320. However, with ballast resistors and during high power operation, the current mirror is unable to keep the voltage on the bases 185 of the individual HBT power device cells 110 constant because of the drop on the ballast resistor. In power operation when more HBTs are connected in parallel, increased base current is required from the current mirror. This strain on the current mirror results in increased voltage drops across the ballasting resistors 130 resulting in the voltage at the base of the individual HBT devices 110 to droop, limiting linearity and maximum output power.
What is required is an improved HBT power amplifier circuit that doesn""t effect the quiescent point at lower output powers, but comes into play when higher powers are being generated that effectively prevents this drooping from occurring.