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 R_bias, the more robust the design. Yet a higher voltage drop means that there will be more wasted dc power dissipated in the R_bias 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.