Active Biasing in RF Amplifiers

Key Takeaways

• Active biasing in RF amplifiers is implemented using active components such as BJTs, FETs, etc. 

• Employing active biasing in RF amplifiers is the best practice for achieving stable, flexible, and adjustable bias conditions. 

• Traditional active biasing in RF circuits allows one to choose a value for Vc and Ic so that a balanced performance is achieved in terms of power output and harmonics.

Bias circuitry can be either an active biasing or passive biasing circuit

Increasing bandwidth is required to keep up with the high data rates in modern RF communication systems. However, in RF circuits (especially in RF amplifiers), bias circuits pose a serious problem to the instantaneous bandwidth of the signal. The bias circuit generates variations in the baseband voltage of the transistor in the amplifier, destabilizing bias conditions.

Employing active biasing in RF amplifiers is the best practice for achieving stable, flexible, and adjustable bias conditions. In this article, we will explore different techniques for active biasing in RF circuits and compare them to passive biasing.

Active Biasing in RF Amplifiers

Equation of the bias condition

The above circuit shows a basic RF amplifier consisting of an NPN transistor Q₀. The resistor R_f, capacitor C_f, and inductor L_f form the feedback stabilizing network. The RF choke L_c connects the collector of the transistor Q₀ to the voltage supply and also helps to minimize losses. The resistor R_e connects the emitter of the transistor Q₀ to the ground. The input RF signal denoted by RF_in is applied to the base of the transistor Q₀ and the output RF signal RF_out is measured across the collector terminal and ground.

The above-described RF amplifier is biased using an active biasing circuit illustrated inside the red box. Active biasing uses the active component – a PNP transistor Q_B. The resistor R_b connects the collector terminal of the transistor Q_B to the base of the amplifier transistor Q₀. The bias resistor R_c connects the supply voltage V_CC to the emitter terminal of Q_B and inductor L_c.

The base of the bias transistor Q_B is supplied with a reference voltage V_ref. The bias condition of the transistor V_c and I_c can be given by the following equations:

The Influence of Active Biasing on RF Amplifier Performance

RF amplifier operation requires either moderate output power with excellent harmonic performance or high output power with moderate harmonic performance. The output power and harmonic performance of RF amplifiers are dependent on the values of  V_c and I_c.

Increasing V_c enhances the harmonic performance without much improvement in the output power. Similarly, increasing I_c improves the output power of the RF amplifier, with degraded harmonic performance.

Traditional active biasing in RF circuits allows one to choose a value for V_c and I_c so that a balanced performance is achieved in terms of power output and harmonics. A feedback control system is implemented along with the active biasing circuit to maintain the chosen V_c and I_c values. 

Passive vs. Active Biasing in RF Amplifiers

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