Darlington Gain Blocks Eliminate Bias Resistor

Jan. 17, 2006
A 5-V-supply-operated Darlington amplifier conquers overhead-voltage issues to deliver temperature and supply stability while maintaining efficient operation.

Wideband Darlington RF gain blocks are commonly used in wireless and wire-line applications. Their strong broadband linearity makes them suitable for base-station predrivers, repeater-pole transceivers, fiber-optic transceivers, cable-television (CATV), and instrumentation systems. Unfortunately, conventional Darlington gain blocks need an external-dropping resistor to set and stabilize the bias current. To ensure good temperature stability, an overhead voltage of about 2 to 3 V is applied. This overhead voltage leads to inefficient amplifier operation from an 8-V supply, however. It also presents a design challenge for newer system applications running on 3- and 5-V supplies. A possible solution may be a new line of InGaP gain blocks from Sirenza Microdevices (Broomfield, CO) that is designed for 5-V supplies.

InGaP technology is a popular solution for high-linearity and wideband-amplifier applications. Combined with the Darlington feedback topology, it can provide high third-order-intercept (IP3) performance and wide bandwidth in a small, low-pin-count package. It offers repeatable DC and RF characteristics, employing a vertical device structure grown by molecular organic chemical vapor deposition (MOCVD). The RF yield for this process is in the high 90 percent. The process also delivers transistors with high breakdown characteristics, high output power, and good linearity. In addition, InGaP technology offers the superior reliability performance that is needed for high-volume wireless and infrastructure applications.1

Figure 1a shows the common Darlington-feedback-amplifier topology with a design application in Fig. 1b. The Darlington feedback amplifier has evolved from Sidney Darlington's original-US Patent #4,236,119 on a high-current-gain transistor pair into what is now one of the most commonly used topologies in RF-amplifier design.2 The use of the Darlington pair in an RF feedback-amplifier configuration was first employed in silicon-bipolar technology by several commercial companies. It provided enhanced gain, cutoff frequency, and higher input impedance, which are attractive features for application-in feedback designs. In the late 1980s, microwave performance extended beyond the X-band by using a faster GaAs heterojunction-bipolar-transistor (HBT) technology.3 Eventually, these advances led to the popular GaAsbased HBT gain-block product family that was first introduced by Sirenza Microdevices (formerly known as Stanford Microdevices).

Design Application
Figure 1b illustrates the design application of a conventional InGaP-GaAs-based Darlington amplifier. An external resistor, Rbias, is integrated outside the package in order to set the bias current. To obtain an output 1-dB compression point of ~ +20 dBm, a device voltage Vd of ~ 5 V and bias current of ~ 80 mA is typically required. To set up a robust bias current, which is insensitive to temperature and supply variations, a voltage drop of ~ 3 V is required about the Rbias resistor. It requires a supply voltage of roughly 8 V.

To obtain lower supply operation, the resistive voltage drop may be reduced at the expense of bias stability over the temperature and supply variation. This point is depicted in Fig. 2, which shows bias current versus voltage for various supply-voltage conditions. As the supply voltage and corresponding voltage drop across Rbias is reduced, a steeper slope results in the current-voltage (I-V) curve. The steeper the slope, the more sensitive the bias is to voltage-supply variations and temperature. Thus, there is a trade-off between supply voltage and bias stability. The higher the supply and voltage drop across Rbias, the more robust the design. Yet a higher voltage drop means that there will be more wasted dc power dissipated in the Rbias resistor, which could otherwise be converted to RF power.

It is therefore desirable to employ a bias solution that does not require the Rbias resistor, but can maintain performance robustness. Moreover, it would be attractive if this solution could be employed in a simple three-terminal package, such as the SOT-89. In order to operate at a lower supply voltage while providing equivalent RF performance and bias stability, a new Darlington feedback design has been developed by Sirenza (Fig. 3). This topology has recently been approved for a US patent.4

The new design employs a bias topology, which allows the Darlington feedback amplifier to operate directly from a lower 5-V supply without the need for the Rbias resistor. The design achieves this functionality without requiring additional package leads to control or set the bias externally. As a result, it can be implemented in the widely accepted, low-cost, three-terminal SOT-89, SOT-86, and SOT-363 packages. In addition, similar RF performance can be obtained with 35-percent less DC power consumption while operating from a lower, standard 5-V supply voltage.

Bias Robustness
With this technique, bias robustness is maintained over temperature-and voltage-supply variations. Figure 4 compares the I-V characteristics of the new Sirenza design with respect to the conventional Darlington approach. This figure illustrates that a smaller temperature variation is achieved with Sirenza's 5-V design. Furthermore, it maintains reasonable supply-variation sensitivity, which is characteristic of the higher-voltage resistive-bias designs.

The 5-V design also has the capability to realize Class AB action due to the absence of the Rbias resistor. Typically, that resistor restricts the amplifier to Class A operation. Figure 5 gives the output characteristics of an early prototype design, which demonstrates bias-current ramp up with increasing input power. The slight Class AB behavior is evident from the increase in bias current, Icc, as the amplifier moves into compression. The amplifier's quiescent current is 67 mA. It ramps up to 82 mA near the 1-dB compression point.

The corresponding 1-dB compression point is a little over +18 dBm from a 5-V supply. When properly designed, it is possible to obtain lower quiescent current while delivering high linear output power on demand. This feature is required for many wireless-communications systems.

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Design Topology
This new amplifier design topology was applied to Sirenza's line of 5-V-supply InGaP gain blocks, called the SBB series. These gain blocks all operate from a 5-V supply with no dropping resistor, Rbias. Currently, they are offered in the SOT89 package. These amplifiers are designed to address RF-and IF-amplifier applications in which high gain and IP3 are required. Figure 6 gives a specification summary of the SBBseries products. The SBB 1089 and 2089 are designed for IF and CATV amplifier applications in the 50-to-500-MHz range. There, a high IP3 of ~ +42 dBm and a low noise figure of 2.8 to 3.3 dB are needed. The SBB 4089 and 5089 are general-purpose wideband RF gain blocks designed for applications between 50 MHz and 8 GHz. The SBB-5089 amplifier has a gain of 20 dB. It maintains a gain of 15 dB at 6 GHz. The bias current is regulated to within 2 percent over a ?40 C to +85 C ambient temperature. It maintains good supply insensitivity that is similar to the conventional, resistive, self-bias Darlington gain blocks, which operate from the higher 8-V supply. At 2 GHz, the gain varies by less than 1 dB across the ?40 C to +85 C temperature range. The table offers a direct performance comparison between the 5-V SBB-5089 and the conventional 8-V SBW-5089 and SBA-5089 designs. Generally, the 5-V SBB-5089 obtains similar or better performance to these 8-V supply designs while consuming 37-percent less total DC power consumption. In particular, the 5-V SBB-5089 outperforms the 8-V SBW and SBA series designs in IP3 by over 2.5 dB at 850 MHz and 0.8 dB at 1950 MHz. The 5-V design also achieves a 0.4-to-1-dB improvement in the 1-dB output compression while operating from a much lower 5-V supply.

In addition to the 5-V-supply SBB InGaP solutions, which address various medium-power (+20-dBm), high-IP3 applications, Sirenza is developing lower 3- and 3.3-V-supply Darlington gain blocks. Those blocks will cover a variety of standard low-voltage wireless applications. The new design will allow the devices to fully utilize the supply without wasting power in an external bias resistor. The impact on efficiency should be even greater with lower supply voltages than with the 5-V standard supply.

Using this design approach, a family-of wider-bandwidth Darlington gain blocks operating as high as 20 GHz also has been in development. These higher-microwave-frequency gain blocks are based on a higher-frequency process with a transistor-cutoff frequency (fT) of 70 GHz. These gain blocks will be targeted for military and defense, instrumentation, and fiber-optic applications that require multi-decade and microwave performance. This product family will include both standard 3- and 5-V-supply products. It will deliver medium power up to 20 dBm with gains of ~ 15 dB. The products are expected to be available in the near future.


The authors would like to acknowledge Jeff Yee, Dave Seymour, Kin Tan, B. Pinato, and Greg Baker for their technical discussions and support.


  1. Kevin Feng, Yuefei Yang, Chanh Nguyen, "Reliability of commercial InGaP/GaAs HBTs under high voltage operation," 2003 IEEE Gallium Arsenide Integrated Circuit (GaAs IC) Symposium, pp. 71-73.
  2. S. Darlington, "Semiconductor Signal Translating Device," US Patent #2,663,806, Dec. 22, 1953.
  3. K.W. Kobayashi, R. Esfandiari, A.K. Oki, D.K. Umemoto, J.B. Camou, and M.E. Kim, 1989 IEEE Gallium Arsenide Integrated Circuit (GaAs IC) Symposium, pp. 87-90.
  4. K.W. Kobayashi, US Patent No. 6,927,634 "Self-Biased Darlington Amplifier," August 9, 2005, assignee Sirenza Microdevices.

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