Darlington Gain Blocks Hurdle Reliability Problems

Jan. 26, 1998
Based on a Darlington-pair architecture, these new gain blocks overcome the reliability issues of previous designs with usable performance from near DC to 6 GHz.

Gain blocks are used by the millions in commercial and military applications ranging from cable-television (CATV) systems to test instruments. Typically, these broadband amplifiers are designed with a pair of transistors in a Darlington configuration which, although providing high broadband gain, can suffer from reliability problems. But by using a mature in-house InGaP semiconductor process, and designing in safeguards, the engineering team at Celeritek (Santa Clara, CA) has developed a new line of Darlington-based gain blocks in the CGB7000 series that promises high reliability and excellent immunity to electrostatic discharge (ESD) over a frequency range of 01 to 6.0 GHz.

Fifty years after US Patent No. 2663806 was issued to Sidney Darlington (December 22, 1953) for a variety of transistor and resistor combinations, the circuit configuration is still widely used in millions of Darlington-pair gain blocks. Unfortunately, such devices have several disadvantages, including poor ESD resistance, poor thermal resistance, and a difficult-to-test architecture. In one of the best-known Darlington configurations, resistors are connected in parallel with the base-collector and base-emitter junctions of the transistors. While an essential part of the biasing scheme, the resistors prevent thorough testing of the transistors, allowing some defects to go undetected until they become the cause of in-field failures. Although Darlington-pair gain blocks have suffered from reliability issues, they are still widely used today in test equipment, cellular base stations, and military systems.

The CGB7000 series of gain blocks was developed to transcend the shortcomings of traditional Darlington-pair gain blocks. Based on reliable InGaP heterojunction-bipolar-transistor (HBT) technology, the 11 devices exhibit thermal resistances ranging from 75°C/W in the high-output model CGB70012-SC to 110°C/W in the high-gain/low-noise-figure model CGB7001-SC. Based on maximum device ratings, the projected mean time to failure (MTTF) for the two devices are 2500 years for the CGB7012-SC and 28000 years for the CGB7001-SC (with all other models in between).

To further enhance reliability, the gain blocks include protection circuits for immunity to ESD pulses as high as 1000 V based on the human-body model (HBM) and pulses as high as 2000 V using the charged-device model (CDM). Also, extra pads are included on each die to facilitate testing. This feature, which is not available on other Darlington-pair gain blocks, allows greatly enhanced testing of each die, permitting Celeritek's automated testers to detect and reject parts with subtle defects that might lead to field failures. Some of the tests enabled by the inclusion of extra pads include current-leakage tests (with failure limits set in the low microampere range); breakdown voltage tests (since low breakdown voltages are an indication of possible transistor reliability issues); DC current gain (beta) tests at low current for each of the transistors in the Darlington pair; measurements of base-collector and base-emitter diode forward-voltage drops at low current levels (deviations from typical results indicate latent defects); and measurements of the functionality of the ESD protection circuitry.

The CGB7000 series (see table) offers a variety of choices depending upon a specifier's needs for output power, gain, noise figure, and linearity . Not surprisingly, the lowest-power device, model CGB7001-SC, is the most reliable in terms of projected MTTF. The gain block provides 20.8 dB gain at 850 MHz with +14.2 dBm output power at 1-dB compression as well as a noise figure of 3.3 dB and OIP3 of +28 dBm. The CGB70011-SC delivers the highest output power, at +21 dBm, and also the highest gain, at 22 dB, with outstanding OIP3 of +36 dBm and still respectable noise figure of 3.4 dB. The gain blocks are useful in a variety of broadband and narrowband applications, as driver amplifiers for larger amplifiers, buffer amplifiers for mixers and local oscillators, and as intermediate-frequency (IF) amplifiers. The devices, which are supplied in SOT-89 packages, and in bare die form, with SOT-86 and ceramic micro-X packages available soon, can be used with a single bypass capacitor, optional RF choke, and two DC blocking capacitors. Celeritek, Inc., 3236 Scott Blvd., Santa Clara, CA 95054; (408) 986-5060, FAX: (408) 986-5095, Internet: www.celeritek.com.

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