Design Articles

Monolithic wideband amplifiers are widely used in the RF and microwave systems. Many of these amplifiers use Darlington configuration. Mini-Circuits monolithic amplifiers with model prefix MAR and ERA, for example, use this configuration.
The frequency range of the Darlington amplifiers is specified down to DC, but for proper biasing conditions to occur input and output coupling capacitors need to be incorporated. These capacitors determine the low frequency end of the amplifier frequency range.

Normally the Darlington gain block amplifiers are designed to operate with an ideal current source. In practice the current source is often substituted with a voltage source and a series resistor, which converts it to a reasonably good current source. The resistor value might be few hundred ohms; therefore, the DC voltage required to operate the amplifier will be higher than the device voltage and may go up to 10 or 15V. An optional RF choke may be connected in series with the resistor to increase the total shunting impedance in order to minimize gain and power loss.