LogicMicrowave offers a wide variety of broadband driver, low noise, and low phase noise amplifiers and broadband RF bias tees. Some amplifiers have on-chip supply networks, while others require a bias tee. Why can't every amplifier use on chip biasing? In this note we discuss the limitations of MMIC technology and why bias tees are sometimes necessary. 

What is amplifier biasing?

The term 'amplifier biasing' is a little misleading. An amplifier consists of a nonlinear gain device (typically a transistor) and a passive network. The passive network has two functions:

1. Impedance matching and tuning of the gain device for the desired amplifier specs at the required frequency (typically low noise or power, or a balance of both)

2. Separating the DC power supply and the bias voltages from the input and output RF signals. The circuit elements that achieve this are blocking capacitors that isolate the bias and power supplies from the system and RF chokes that connect the power supply or bias voltage source to the transistor.

Bias voltages are low current/low power voltages that must be applied to the transistor so that it is operating at an appropriate point on the transimpedance curve. Amplifiers designed for high linearity will be biased in the middle of the linear region. Amplifiers designed for high efficiency will be biased at a lower point, consuming much less current but also sacrificing linearity. 

The important thing to note about these bias voltages is that they are low current, and therefore low power. The RF choke that supplies the bias voltage could be an inductor or a resistor, or it could be integrated into the matching network of the amplifier. Either way it is easy to integrate this circuitry on-chip. Typically the only off-chip components necessary for bias voltages are bypass capacitors and sometimes resistive networks to set the voltages from a single supply. 

Power supply voltages, in contrast, are the source of the DC power that a transistor uses to amplify a signal. More specifically, the transistor modulates the power supply current between the output and the ground, leading to amplification. The greater the power supply current, the greater the output power (subject to impedance matching and other considerations). If this current is applied through a resistive choke, it will generally lead to undesired power losses and poor power added efficiency. With few exceptions for very broadband distributed amplifiers (discussed below), power is supplied through an inductor.