Double-diffused MOS (DMOS) devices offer some inherent advantages that make them strong competitors for bipolar devices in high-frequency and high-power applications. One particularly promising application of the DMOS transistor is in cable communications systems, an area currently dominated by bipolar hybrid ICs. A wideband (40-to-350-MHz) hybrid DMOS amplifier has been designed and built to demonstrate the feasibility of the device in this application.
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The advantages of the DMOS transistor include high input impedance, high current gain and power gain, and no thermal runway problems. Besides these intrinsic advantages, in a DMOS a very short channel length (1 μm or less) is possible without sub-micrometer lithography, and a high breakdown voltage can be obtained and tailored independently of the channel length by incorporating a lightly doped drift region between the active channel and the drain contact.
In cable communications systems, amplifiers must operate from a 24-V supply, have about 17 dB of gain and a power consumption of about 5 W, exhibit low noise over a wide bandwidth, and be very linear. The stringent distortion demands require bipolars to be overdriven (since distortion is reduced as bias increases), but overdriving causes excessive power dissipation. The potentially better distortion characteristics and the lower fabrication cost of the DMOS device may make it suitable replacement for the bipolar. Though some work has been done in this area, further investigation is needed.
The specifications chosen for the amplifier design are typical of commonly used RF amplifiers built with 2-μm-technology bipolar transistors:
- gain of 17.0 ± 0.1 dB over a bandwidth of 350 MHz,
- input and output impedances of 75 Ω with a return loss of less than -18 dB,
- 5-W maximum power dissipation, and
- second- and third-order intermodulation distortions of less than -70 and -55 dB, respectively, at signal levels of 0 dBm across the band.
The choice of amplifier configuration was based on the condition that feedback be used instead of matching networks at the input and output. This condition requires higher-performance devices, but it also ensures an economical design, particularly for a multistage amplifier, which preliminary investigation showed was needed to meet the amplifier’s AC requirements.
Three configurations were considered: (a) two-stage with multiple feedback, (b) cascode, and (c) cascade. Initial investigations concentrated on satisfying bandwidth and input/output requirements, with the objective of finding the configuration that best the specification while imposing the least strenuous device requirements.
The simplified AC configuration of the multiple feedback circuit shows that simultaneous shunt and series feedback resistors (RF1 and RF2) are incorporated across the stages to realize broadband and resistive input/output impedances (Fig. 1a). Input impedance is controlled by RF2, RS1, and RS2; output impedance is controlled by RF1, RS1, and RS2.
The cascade circuit (Fig. 1b) has several advantages, including minimum Miller capacitance, which results in better high-frequency performance, low noise, and the possibility of integrating two transistors into a single structure with two gates. Resistive series (RS) and shunt (RF) feedback is used to match input and output impedances. In the cascade circuit (Fig. 1c), resistive shunt feedback optimizes the input and output matches, and peaking inductors (microstrip lines) compensate the frequency response.