Wideband Darlington amplifiers-offer versatile gain for a variety of wireless and wire-line applications, including in base stations, fiber-optic transceivers, cable-television (CATV) systems, and measurement systems. A Darlington feedback amplifier features multi-decade coverage, can be realizedin a small size, and is simple to use with few additional external components. What follows is a report on a Darlington amplifier that makes use an innovative bias topology and enhancement-mode pseudomorphic highelectronmobility-transistor (E-PHEMT) device process to achieve new levels of dynamic range for both 3.3- and 5.0-V designs.

Until now, silicon-bipolar junction transistors, silicon-germanium (SiGe) heterojunction bipolar transistors (HBTs), and indium-gallium-phosphide (InGaP) HBTs have been the predominant technologies used for RF Darlington amplifiers. Traditional bipolar and HBT Darlington amplifiers offer high linearity; their positive turn-on voltages allow ease of use through positive-single-supply operation. InGaP HBT-based Darlington amplifiers have gained in popularity for their high third-order-intercept (IP3) performance through cellular bands (about 2 GHz).¹ Above 2 GHz, however, the IP3 performance rolls off fairly rapidly for these InGaP solutions, creating opportunities for other advanced semiconductor solutions, such as PHEMTs. Compared to InGaP HBTs, PHEMTS offer lower noise figures and more gain at higher frequencies.

High-linearity PHEMTs are usually depletion-mode devices that require negative gate bias voltage. However, the negative gate bias voltage has precluded the use of these devices in RF Darlington feedback amplifier topologies. Because of the need for a single positive supply in cellular handsets, enhancementmode PHEMTS are used in that application. But even with the availability of e-mode PHEMTs and e-mode/d-mode PHEMTs,² stable monolithic biasing using PHEMT active topologies is still challenging due to threshold variations over process and temperature which is an order of magnitude greater than bipolar HBT technologies.

By combining enhancement-mode PHEMT devices with a new Darlington active bias topology, it has been possible to realize self-biased Darlington amplifiers. The approach provides amplifiers with robust performance over temperature, power supply and process variations, and achieves a higher IP3-bandwidth product than for InGaP Darlington solutions.

Figure 1a illustrates the conventional Darlington feedback amplifier topology with an enhancement-mode PHEMT implementation. The positive turn-on voltage of an enhancement-mode PHEMT, which can be as high as 0.5 to 0.6 V at moderate current densities, enables resistive biasing of the Darlington amplifier. An external resistor (Rdc) in combination with resistive feedback sets the quiescent bias current of the amplifier. For robust biasing that is insensitive to supply variations, a voltage drop of about 2 to 3 V is required across R_dc, forcing an inefficient use of a higher supply voltage. To compound matters, the threshold voltage of PHEMTs can vary much greater than their bipolar/HBT counterparts due to process and temperature variations. To achieve the best performance from a Darlington PHEMT implementation, a less process-, temperature-, and supply-sensitive bias scheme is needed.

Figure 1b presents the schematic diagram of the new bias scheme in which a mirror transistor, M3, is used to set the bias for the Darlington amplifier's output transistor. The current of the mirror is set through internal feedback resistor Rfb. This eliminates the need for the external set resistor of the conventional case (Fig.1a) and enables the Darlington amplifier to operate directly from the supply voltage. For example, a conventional 8- or 5-V Darlington may be operated at a reduced supply of 5 or 3.3 V, respectively. Without the 2- to 3-V overhead voltage across an external Rdc resistor, an amplifier's overall efficiency may be improved by as much as 40 percent. A filter is embedded in the amplifier to isolate and reduce the effects of bias circuit on RF performance. This new technique, for which a patent has been obtained, ³ helps maintain robust bias over temperature and voltage supply variations. The current-temperature sensitivity of this new bias scheme is summarized by:

where:

A₂₃ = the ratio of the area of amplifier transistor M2 to mirror transistor M3.

A similar relationship may be derived for a conventional resistively biased Darlington:

Plugging in typical values for the parameters of Eqs. 1 and 2, it is clear that the new bias approach can reduce the bias current sensitivity with respect to temperature by as much as an order of magnitude. By inspection, it can also be concluded that the sensitivity with respect to Vgs process threshold variation is also reduced by the same factor. These claims are supported by comparing the current-voltage (I-V) curves of a 5-V conventional resistive-bias Darlington amplifier to a 3.3-V activebias PHEMT Darlington amplifier (Fig. 2). The tightly spaced new 3.3-V design illustrates lower temperature sensitivity compared to the loosely spaced 5-V conventional resistive design, even while operating at a lower 3.3-V supply. The quiescent bias is maintained to within ±3 percent and ±11 percent for the new active and conventional resistive Darlington biases, respectively; this is nearly a factor of 4 improvement for the active case. Also, the similar slopes of the curves illustrate that voltage supply insensitivity can be maintained at lower supplies with the new bias scheme.