Application Note
Selecting The Proper Gain Block Bias Network Components
Skyworks gain blocks are Darlington feedback amplifiers which feature a constant gain-bandwidth product for use as an RF gain block. They use a circuit topology in which two transistors are combined to form a configuration known as a Darlington pair. This transistor pair behaves like a single transistor with a current gain equivalent to the product of the current gain of the two transistors. Darlington transistors are connected in a common emitter configuration, while sharing the same collector contact.
The emitter current of the first transistor is applied to the base of the second transistor. This gives a high current gain (written b or Hfe), and takes less space than two discrete transistors in the same configuration. As RF gain blocks, Darlington pairs are offered as integrated packaged devices, usable from very low frequency to over 10 GHz. They exhibit good broadband matching into 50 W and tight performance distributions.
Bias application of the amplifier is based on a fixed current (IS), rather than a fixed voltage (VS). The DC voltage that appears at the RF output of the amplifier, known as VD, is fixed by the base-emitter voltages of Q1 and Q2, and by the voltage divider formed by RF and RB. The values of resistors RF and RB are selected to establish the necessary feedback to set the input and output impedances of the amplifier, as well as to set the base voltages and currents of the Darlington pair.
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