Electronic Design

Grounding Technique Lets Bipolar Process Create Wireless RF PAs

A new grounding method developed by Ericsson Microelectronics makes feasible mass-production of wireless integrated power amplifiers for portable wireless applications. The double-polysilicon bipolar 0.5-µm process achieves a transit frequency (fT) of 25 GHz. It improves wireless performance and permits operation up to 5 V. Also, it includes npn and pnp transistors for use in analog and digital designs (see the figure).

To design an RF circuit that performs well for modern wireless applications, use this popular rule of thumb often employed by radio circuit designers: An fT of at least 10 times the operating frequency is needed for the active device. Yet this may only be true for the elementary radio building blocks. The design of the output power amplifier places additional re-quirements on the semiconductor process (and according to some designers, a touch of black magic), which has precluded the use of silicon processes for the power amplifier. For this reason, GaAs MESFETs and other more advanced technologies have been widely used in this area up until now.

The Ericsson trench-isolated process uses on-chip capacitors and inductors for the integrated internal matching network. Four layers of metallization are employed with a thick top layer used for improved performance of the integrated inductors.

One of the most important factors in achieving high performance in power amplifiers lies in providing a good conducting path from the active devices to ground. The straightforward solution is to use many bond wires from the chip's surface to the package. Preferably, this includes a grounded lead frame, possibly even with a number of the pins permanently connected (fused) to outside ground connections on the circuit board. Another option would be to use a package where the backside of the lead frame is exposed and is soldered directly to the circuit board.

At these high frequencies, however, the bond wire from the chip to the package will severely limit the performance of the circuit. To achieve low resistance and low impedance to ground, the number of ground bond wires is increased. But since a power amplifier is usually a very small circuit, the extra bonding pads needed for this increases the size of the chip considerably. Therefore, this technique can become very costly. Furthermore, packages with exposed lead frames are considerably more expensive. Ericsson's new grounding method solves all of these problems.

RF IC technology may make it easier for a greater number of functions to be added to the power amplifier. In particular, power control and active linearization are two important features that will be needed for the coming 2.5G and 3G (UMTS) mobile phone systems.

For more information, go to www.ericsson.com.

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